Patent Publication Number: US-6336067-B1

Title: Electronic control system and control device for construction machine

Description:
TECHNICAL FIELD 
     The present invention relates to a control unit for a construction machine, and more particularly to an electronic control system for a construction machine which includes a plurality of control units for controlling a prime mover, hydraulic equipment, a working device, a display unit, etc., and which interconnects the plurality of control units via a common communication line for transmission and reception of data. 
     BACKGROUND ART 
     Recently, with the advancement of electronic control incorporated in construction machines, particularly in a hydraulic excavator as a typical example thereof, the amount of computation to be processed by a control unit has increased more and more because of various kinds of computation required for the electronic control. The increased amount of computation necessitates the use of a high-performance microcomputer, and hence pushes up a cost. Also, the number of input/output signals is increased, which results in an increased number of wire harnesses. To overcome such a problem, dispersion of control units has been studied. In dispersion of control units, control functions of a hydraulic excavator are divided in units of function, control units are provided one for each of the unit functions, and the control units are interconnected via a network for performing control. 
     For example, JP, B, 7-113854 discloses an electronic control system for a hydraulic excavator wherein control units are provided one for each device, the control units for the respective devices are connected to a master controller via a common communication line, and integration of the overall system is maintained by the master controller. 
     Also, JP, B, 8-28911 discloses an electronic control system for a construction machine wherein control units are provided one for each device, and the control units are interconnected via a multi-transfer serial communication circuit to construct a network capable of two-way communication. This system is easily expandable. 
     Further, SAE Paper 941796 Development of Intelligent Hydraulic Excavator—HYPER GX Series (published in 1994) discloses an electronic control system for a hydraulic excavator wherein control units are provided one for each device, and the control units are interconnected via a network. The network is divided into a low-speed network and a high-speed network to ensure reliability of high-speed communication data and realize a cost reduction of the overall system. 
     DISCLOSURE OF THE INVENTION 
     In a hydraulic excavator as a typical example of construction machines, as mentioned above, electronic control has been advanced and various improvements have been achieved in points of control performance and production cost. 
     On the other hand, customer needs for hydraulic excavators have become diversified, for example, ranging from a demand for high-performance functions to a demand for an inexpensive machine. 
     In the electronic control system disclosed in JP, B, 7-113854, to satisfy the above customer needs, software of the master controller must be developed and changed, and the master controller must be replaced for each need. Moreover, with replacement of the master controller, software of the control unit provided for each device and connected to the master controller must be developed and changed, and the resultant software must be substituted for that of the existing control unit. Particularly, when high-performance functions are demanded, e.g., when automatic control functions such as operating area limitation and locus control are to be added to an excavating device of a hydraulic excavator, the kinds of software installed in the control units are increased. As a result, the number of steps for development and the development cost or management cost are increased. 
     On the contrary, when high-performance functions of a hydraulic excavator are not needed, software of the master controller and the control units must also be developed and changed correspondingly, and the resultant software must be substituted for the existing software. Therefore, the number of steps required for software development and the development cost are increased. 
     In the electronic control systems disclosed in JP, B, 8-28911 and SAE Paper 941796, the control units for the respective devices are interconnected via the network, and all signals to be transmitted and received among the control units can be transferred using the network. Accordingly, even when a number of signals are transmitted and received, it is not required to increase the number of signal lines. Also, even when a control unit having a new function is added to the system, there is no need of increasing the number of signal lines. Those electronic control systems are thus flexibly adaptable for system expansion. 
     In the case of adding a control unit for system expansion, there is no need of increasing the number of signal lines, but each control unit must be itself adapted for an increase in the number of signals (data) to be transmitted and received. In other words, the existing control units must be replaced and hence the cost is pushed up. Further, in the case of requiring not so high-performance functions and desiring to reduce the number of control units, each control unit must also be adapted for a decrease in the number of signals (data) to be transmitted and received. Such a case therefore similarly requires replacement of the existing control units. Further, in the case of replacing one of a plurality of control units by another unit upon change of a hydraulic system or a control system, if a particular one of the plurality of control units executes processing using a received signal and a control unit transmitting the received signal is replaced and excluded from the system, the particular control unit must also be replaced. 
     Additionally, when a multiplicity of control units transfer data among them via a network, the network would be too crowded to utilize necessary data on demand unless communication frequency is held at an optimum level. This results in a deterioration of the control performance of the control units. 
     A first object of the present invention is to provide an electronic control system and a control unit for a construction machine including a plurality of control units interconnected via a common communication line, wherein additional connection of a control unit to the common communication line and disconnection of any control unit from the common communication line can be performed without changing software in the existing control units and replacing the control units themselves, and system change including addition, exclusion and replacement of one ore more control units can be easily realized. 
     A second object of the present invention is to provide an electronic control system for a construction machine including a plurality of control units interconnected via a common communication line, wherein data communication frequency over a network is held at an optimum level and the control performance of the control units is avoided from deteriorating. 
     (1) To achieve the above first object, the present invention provides an electronic control system for a construction machine comprising a prime mover, a working device, and a hydraulic system for generating liquid pressure power by the prime mover and driving the working device, the construction machine further comprising a plurality of control units, the plurality of control units being interconnected via a common communication line to transmit and receive data, wherein at least one of the plurality of control units includes minimum processing means capable of executing least necessary processing by itself when no data is transmitted via the communication line. 
     With that feature of providing the minimum processing means in at least one control unit, in the case of reducing the number of control units, the least necessary processing can be performed by the minimum processing means even when no data is transmitted via the communication line. There is hence no need of changing software and replacing the control units. This also similarly applies to the case where a control unit transmitting the relevant data is excluded as a result of partial replacement of a plurality of control units. Further, by designing software of the control units in anticipation of expansion of the system beforehand, need of changing software and replacing the control units can also be eliminated even for addition of a control unit. Therefore, one or more control units can be additionally connected to the common communication line or can be disconnected from the common communication line without changing software in the existing control units or replacing the control units themselves. It is hence possible to easily realize system change including addition, exclusion and replacement of one or more control units, and to hold down an increase of the development cost. 
     (2) Also, to achieve the above first object, the present invention provides an electronic control system for a construction machine comprising a prime mover, a working device, a hydraulic pump rotationally driven by the prime mover, actuators for driving the working device, control valves for controlling a hydraulic fluid supplied from the hydraulic pump to the actuators, and operating means for operating the control valves, the construction machine further comprising a plurality of control units, the plurality of control units being interconnected via a common communication line to transmit and receive data, wherein at least one of the plurality of control units includes minimum processing means capable of executing least necessary processing by itself when no data is transmitted via the communication line. 
     With that feature of dividing a control unit for a construction machine into a plurality of control units and interconnecting the plurality of control units via a common communication line to transmit and receive data, it is only required to modify, add or exclude the control unit at the least necessary level when manufacturing machines having different customer-demanded functions. Therefore, a system can be changed with the least necessary development cost and the least necessary number of development steps. Also, since the control units are divided for each function, the system is more convenient from the viewpoint of management and the management cost can be reduced. 
     In addition, with the feature of providing the minimum processing means in at least one control unit, as described in above (1), when one or more control units are added, excluded or replaced, there is no need of changing software and replacing the other control units. Thus, system change including addition, exclusion and replacement of one or more control units can be easily realized. 
     (3) In above (2), preferably, the plurality of control units include at least two of a control unit for controlling the prime mover, a control unit for controlling the working device, a control unit for controlling the hydraulic pump, a control unit for operating the control valves through the operating means, and a control unit for performing display and/or input in relation to the control units. 
     With that feature, system change including addition, exclusion and replacement of one or more control units can be easily realized, as described in above (2), for the control unit for controlling the prime mover, the control unit for controlling the working device, the control unit for controlling the hydraulic pump, the control unit for operating the control valves through the operating means, and the control unit for performing display and/or input in relation to the control units. 
     (4) In above (1) or (2), preferably, the minimum processing means has an initial value set therein for each data to be received via the communication line for fulfilling the least necessary function of each control unit, and performs computational processing by using the initial value when no data is transmitted via the communication line. 
     With that feature, when no data is transmitted via the communication line, the minimum processing means performs computational processing by using the initial value, and therefore the control unit can execute the least necessary processing by itself. 
     (5) Further, to achieve the above second object, in the electronic control system of above (1) or (2) according to the present invention, the plurality of control units each have an optimum transmission time interval set therein for each data to be transmitted to another control unit via the communication line, and transmits the data at the set time interval. 
     With that feature of setting an optimum transmission time interval for each data to be transmitted on the transmitting side and transmitting the data at the set time interval, the control unit can transmit data depending on a varying speed of the data or the cycle required for the control unit on the receiving side, and the amount of data flowing over the common communication line can be minimized within the necessary range. As a result, the common communication line can be utilized efficiently and the control performance is avoided from being affected by a lowering of the communication efficiency. In addition, even with an increase in the number of control units, the system is less susceptible to such a trouble as disabling communication due to excessive traffic on the common communication line. 
     (6) In above (5), preferably, the transmission time interval preset for each data to be transmitted is set depending a varying speed of the data or the cycle required for the control unit receiving the data. 
     With that feature, as described in above (5), the amount of data flowing over the common communication line can be minimized within the necessary range, and the common communication line can be utilized efficiently. 
     (7) In above (1) or (2) or (5), preferably, the plurality of control units each set a specific ID for each data to be transmitted or received via the communication line, and each have include communication means for transmitting data, which is to be transmitted via the communication line, with a specific ID assigned thereto, and for receiving only necessary item of data received via the communication line by identifying the necessary data based on a specific ID assigned thereto. 
     With that feature, in spite of various data flowing over the common communication line, each control unit can receive only necessary data. Further, each control unit can receive necessary information at a cycle required from the control point of view in a combination with the above feature (5), and hence the control performance is avoided from being affected by a lowering of the communication efficiency. 
     (8) Also, to achieve the above first object, the present invention provides a control unit for a construction machine comprising a prime mover, a working device, and a hydraulic system for generating liquid pressure power by the prime mover and driving the working device, the control unit being provided in the construction machine and connected to another control unit via a common communication line to transmit and receive data, wherein the control unit includes minimum processing means capable of executing least necessary processing by itself when no data is transmitted via the communication line. 
     With that feature, as described in above (1), when one or more of the control units provided in the construction machine are added, excluded or replaced, there is no need of changing software and replacing the other control units. Thus, system change including addition, exclusion and replacement of one or more control units can be easily realized. 
     (9) In above (8), preferably, the minimum processing means has an initial value set therein for each data to be received via the communication line, and performs computational processing by using the initial value when no data is transmitted via the communication line. 
     With that feature, as described in above (4), the minimum processing means performs computational processing by using the initial value, and therefore the control unit can execute the least necessary processing by itself. 
     (10) Further, to achieve the above first object, the present invention provides an electronic control system for a construction machine comprising a prime mover, a working device, and a hydraulic system for generating liquid pressure power by the prime mover and driving the working device, the construction machine further comprising a plurality of control units, the plurality of control units being interconnected via a common communication line to transmit and receive data, wherein at least one of the plurality of control units comprises first processing means for performing computational processing without using data transmitted from another control unit, second processing means for performing computational processing by using data transmitted from the another control unit, detecting means for detecting whether or not the another control unit is connected to the common communication line, and processing changeover means for executing the computational processing in the first processing means when connection of the another control unit is not detected by the detecting means, and for executing the computational processing in the second processing means when connection of the another control unit is detected by the detecting means. 
     With that feature of providing the first processing means, the second processing means, the detecting means and the processing changeover means in at least one control unit, when another control unit is connected to the common communication line, this fact is detected by the detecting means and the processing changeover means executes the computational processing in the second processing means. Also, when another control unit is disconnected from the common communication line, this fact is detected by the detecting means and the processing changeover means executes the computational processing in the first processing means. Therefore, one or more control units can be disconnected from the common communication line or displaced without changing software in the existing control units or replacing the control units themselves. 
     On the other hand, when another control unit is not connected to the common communication line, this fact is detected by the detecting means and the processing changeover means executes the computational processing in the first processing means. Also, when another control unit is additionally connected to the common communication line to increase the number of control units, this fact is detected by the detecting means and the processing changeover means executes the computational processing in the second processing means. Therefore, one or more control units can be additionally connected to the common communication line without changing software in the existing control units or replacing the control units themselves. 
     Thus, additional connection of a control unit to the common communication line and disconnection of any control unit from the common communication line can be performed without changing software in the existing control units and replacing the control units themselves. As a result, system change including addition, exclusion and replacement of one or more control units can be easily realized, and an increase of the development cost can be held down. 
     (11) Still further, to achieve the above first object, the present invention provides an electronic control system for a construction machine comprising a prime mover, a working device, a hydraulic pump, actuators for driving the working device, control valves for controlling a hydraulic fluid supplied from the hydraulic pump to the actuators, and operating means for operating the control valves, the construction machine further comprising a plurality of control units, the plurality of control units being interconnected via a common communication line to transmit and receive data, wherein at least one of the plurality of control units comprises first processing means for performing computational processing without using data transmitted from another control unit, second processing means for performing computational processing by using data transmitted from the another control unit, detecting means for detecting whether or not the another control unit is connected to the common communication line, and processing changeover means for executing the computational processing in the first processing means when connection of the another control unit is not detected by the detecting means, and for executing the computational processing in the second processing means when connection of the another control unit is detected by the detecting means. 
     With that feature of dividing a control unit for a construction machine into a plurality of control units and interconnecting the plurality of control units via a common communication line to transmit and receive data, it is only required to modify, add or exclude the control unit at the least necessary level when manufacturing machines having different customer-demanded functions. Therefore, a system can be changed with the least necessary development cost and the least necessary number of development steps. Also, since the control units are divided for each function, the system is more convenient from the viewpoint of management and the management cost can be reduced. 
     In addition, with the feature of providing the first processing means, the second processing means, the detecting means and the processing changeover means in at least one control unit, as described in above (10), system change including addition, exclusion and replacement of one or more control units can be easily realized. 
     (12) In above (11), preferably, the plurality of control units include at least two of a control unit for controlling the prime mover, a control unit for controlling the working device, a control unit for controlling the hydraulic pump, a control unit for operating the control valves through the operating means, and a control unit for performing display and/or input in relation to the control units. 
     With that feature, system change including addition, exclusion and replacement of one or more control units can be easily realized, as described in above (11), for the control unit for controlling the prime mover, the control unit for controlling the working device, the control unit for controlling the hydraulic pump, the control unit for operating the control valves through the operating means, and the control unit for performing display and/or input in relation to the control units. 
     (13) In above (10) or (11), preferably, the detecting means detects whether or not the another control unit is connected, depending on whether or not data is received from the another control unit. 
     With that feature, whether or not another control unit is connected can be detected by software processing. 
     (14) In above (10) or (11), preferably, the detecting means changes the status of a flag depending on whether for not data is received from the another control unit, and the processing changeover means determines based on the status of the flag whether or not the another control unit is connected, and changes over the computational processing to be executed. 
     With that feature, whether or not another control unit is connected can be detected by software processing, and the computational processing to be executed can be changed over. 
     (15) In above (10) or (11), preferably, the another control unit exists in plural number, each data transmitted from the plurality of other control units is assigned with a specific identifiers, and the detecting means detects whether or not the plurality of other control units are connected, depending on whether or not data is received from the plurality of other control units, and also detects based on the identifier of received data which one of the plurality of other control units is connected. 
     With that feature, even in the case where the another control unit exists in plural number and the second processing means performs computational processing by using data transmitted from the plurality of other control units, the detecting means can detect whether the plurality of control units are connected for each control unit, the processing changeover means can appropriately change over the computational processing, and the second processing means can execute the appropriate computational processing. 
     (16) Still further, to achieve the above first object, the present invention provides a control unit for a construction machine comprising a prime mover, a working device, and a hydraulic system for generating liquid pressure power by the prime mover and driving the working device, the control unit being provided in the construction machine and connected to another control unit via a common communication line to transmit and receive data, wherein the control unit comprises first processing means for performing computational processing without using the transmitted data, second processing means for performing computational processing by using the transmitted data, detecting means for detecting whether or not the another control unit is connected to the common communication line, and processing changeover means for executing the computational processing in the first processing means when connection of the another control unit is not detected by the detecting means, and for executing the computational processing in the second processing means when connection of the another control unit is detected by the detecting means. 
     With that feature, as described in above (10), when one or more of the control units provided in the construction machine are added, excluded or replaced, there is no need of changing software and replacing the other control units. Thus, system change including addition, exclusion and replacement of one or more control units can be easily realized. 
     (17) In above (16), preferably, the detecting means detects whether or not the another control unit is connected, depending on whether or not data is received from the another control unit. 
     With that feature, as described in above (13), whether or not another control unit is connected can be detected by software processing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing an electronic control system for a hydraulic excavator according to a first embodiment of the present invention, along with the hydraulic excavator and a hydraulic system thereof. 
     FIG. 2 is a diagram showing the configuration of a first control unit shown in FIG.  1 . 
     FIG. 3 is a diagram showing the configuration of a second control unit shown in FIG.  1 . 
     FIG. 4 is a diagram showing the configuration of a third control unit shown in FIG.  1 . 
     FIG. 5 is a diagram showing the configuration of a fourth control unit shown in FIG.  1 . 
     FIG. 6 is a diagram showing the configuration of a display unit shown in FIG.  1 . 
     FIG. 7 is a diagram showing a display section of the display unit shown in FIG.  1 . 
     FIG. 8 is a diagram showing the configuration of a communication device used in each of the first to fourth control units and the display unit shown in FIG.  1 . 
     FIG. 9 is a timing chart showing the relationship between main processing and both timer interrupt processing and reception interrupt processing which are performed by a single-chip microcomputer in each of the control units and the display unit shown in FIGS. 2-7. 
     FIG. 10 shows a message definition table used in the first control unit. 
     FIG. 11 shows a message transmission management table created during initialization after start-up of the first control unit. 
     FIG. 12 shows a message definition table used in the second control unit. 
     FIG. 13 shows a message transmission management table created during initialization after start-up of the second control unit. 
     FIG. 14 shows a message definition table used in the third control unit. 
     FIG. 15 shows a message transmission management table created during initialization after start-up of the third control unit. 
     FIG. 16 shows a message definition table used in the fourth control unit. 
     FIG. 17 shows a message transmission management table created during initialization after start-up of the fourth control unit. 
     FIG. 18 shows a message definition table used in the display unit. 
     FIG. 19 shows a message transmission management table created during initialization after start-up of the display unit. 
     FIG. 20 is a flowchart for explaining the timer interrupt processing (transmission processing) performed by the single-chip microcomputer in each of the first to fourth control units and the display unit. 
     FIG. 21 is a flowchart for explaining transmission processing performed by the communication device in each of the first to fourth control units and the display unit. 
     FIG. 22 is a flowchart for explaining reception processing performed by the communication device in each of the first to fourth control units and the display unit. 
     FIG. 23 is a flowchart for explaining the reception interrupt processing performed by the single-chip microcomputer in each of the first to fourth control units and the display unit. 
     FIG. 24 is a flowchart for explaining the main processing (control operation) in the first control unit. 
     FIG. 25 is a flowchart for explaining the main processing (control operation) in the second control unit. 
     FIG. 26 is a flowchart for explaining the main processing (control operation) in the third control unit. 
     FIG. 27 is a flowchart for explaining the main processing (control operation) in the fourth control unit. 
     FIG. 28 is a flowchart for explaining the main processing (control operation) in the first control unit. 
     FIG. 29 is a time chart showing communication data flowing over a common communication line and status of control computation in the respective control units during a period in which control computational processing is performed by the first to fourth control units and the display unit. 
     FIG. 30 is a table listing the relationships in transmission and reception of the communication data among the first to fourth control units and the display unit, including transmission cycles. 
     FIG. 31 is a diagram, similar to FIG. 1, showing a modified system in which control units are excluded from the electronic control system shown in FIG.  1 . 
     FIG. 32 is a diagram, similar to FIG. 1, showing a modified system in which more control units are excluded from the electronic control system shown in FIG.  1 . 
     FIG. 33 is a diagram showing an electronic control system for a hydraulic excavator according to a second embodiment of the present invention, along with the hydraulic excavator and a hydraulic system thereof. 
     FIG. 34 is a diagram showing an electronic control system for a hydraulic excavator according to a third embodiment of the present Invention, along with the hydraulic excavator and a hydraulic system thereof. 
     FIG. 35 is a diagram showing an electronic control system for a hydraulic excavator according to a fourth embodiment of the present invention, along with the hydraulic excavator and a hydraulic system thereof. 
     FIG. 36 is a diagram showing an electronic control system for a hydraulic excavator according to a fifth embodiment of the present invention, along with the hydraulic excavator and a hydraulic system thereof. 
     FIG. 37 is a diagram showing an electronic control system for a hydraulic excavator according to a sixth embodiment of the present invention, along with the hydraulic excavator and a hydraulic system thereof. 
     FIG. 38 is a flowchart showing a processing sequence in the third control unit, along with a processing sequence in the fourth control unit, when the fourth control unit is connected to the common communication line. 
     FIG. 39 is a flowchart showing processing to manage whether a predetermined transmission cycle is reached or not in data transmission processing as a part of the processing function of a communication management section. 
     FIG. 40 is a flowchart showing an essential processing sequence in the third control unit when the fourth control unit is not connected to the common communication line. 
     FIG. 41 is a functional block diagram showing overall processing functions of software in the third control unit and the fourth control unit. 
     FIG. 42 shows one example of a data definition table used in common to the respective control units. 
     FIG. 43 shows a message definition table used in the fourth control unit. 
     FIG. 44 shows a transmission time management table created in the fourth control unit. 
     FIG. 45 shows a message definition table used in the third control unit. 
     FIG. 46 shows a transmission time management table created in the third control unit. 
     FIG. 47 is a flowchart showing reception interrupt processing performed by the communication management section in each of the third and fourth control units. 
     FIG. 48 is a flowchart showing selection and execution processing performed by a processing selecting and executing section of the third control unit. 
     FIG. 49 is a flowchart showing selection and execution processing performed by a processing selecting and executing section of the fourth control unit. 
     FIG. 50 is a flowchart showing status of the processing function of the third control unit when only the third control unit is connected to the common communication line. 
     FIG. 51 is a diagram showing an electronic control system for a hydraulic excavator according to a seventh embodiment of the present invention, along with the hydraulic excavator and a hydraulic system thereof. 
     FIG. 52 is a functional block diagram showing overall processing functions of software in the second control unit, the third control unit, the fourth control unit and the display/setting unit. 
     FIG. 53 shows a message definition table used in the fourth control unit. 
     FIG. 54 shows a transmission time management table created in the fourth control unit. 
     FIG. 55 shows a message definition table used in the second control unit. 
     FIG. 56 shows a transmission time management table created in the second control unit. 
     FIG. 57 shows a message definition table used in the display/setting unit. 
     FIG. 58 shows a transmission time management table created in the display/setting unit. 
     FIG. 59A shows the flag configuration in a flag setting section of the second control unit. 
     FIG. 59B shows the flag configuration in a flag setting section of the display/setting unit. 
     FIG. 60 is a flowchart showing reception interrupt processing performed by the communication management section of the second control unit. 
     FIG. 61 is a flowchart showing selection and execution processing performed by a processing selecting and executing section of the second control unit. 
     FIG. 62 is a flowchart showing reception interrupt processing performed by the communication management section of the display/setting unit. 
     FIG. 63 is a flowchart showing selection and execution processing performed by a processing selecting and executing section of the display/setting unit. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiments of the present invention will be described below with reference to the drawings. 
     FIG. 1 is a diagram showing an electronic control system for a hydraulic excavator according to a first embodiment of the present invention, along with the hydraulic excavator and a hydraulic system thereof. In FIG. 1, numeral  1  denotes a hydraulic excavator. The hydraulic excavator  1  comprises a track body  2 , a swing body  3  swingably mounted on the track body  2 , an accommodating room  4  formed on the swing body  3  to accommodate a prime mover  14  and hydraulic equipment such as a hydraulic pump  18  (described later), a counterweight  5  provided at the back of the swing body  3 , a cab  6  provided in a front portion of the swing body  3  on the left side, and an excavating device  7  provided in the front portion of the swing body  3  at the center thereof. 
     The excavating device  7  is made up of a boom  8  provided on the swing body  3  to be able to turn upward and downward, an arm  9  rotatably provided to a fore end of the boom  8 , a bucket  10  rotatably provided to a fore end of the arm  9 , a boom operating hydraulic cylinder  11  for turning the boom  8  upward and downward, an arm operating cylinder  12  for rotating the arm  9 , and a bucket operating cylinder  13  for rotating the bucket  10 . 
     The prime mover  14  is disposed in the accommodating room  4  as described above, and includes an electronic governor device  15  for maintaining the revolution speed of the prime mover  14  within a certain range. A target revolution speed Nr of the prime mover  14  is set by a target revolution speed setting unit  16 . 
     Numeral  17  denotes a first control unit provided in a control system for the prime mover  14  and controlling the prime mover  14 . The first control unit  17  performs the predetermined computation based on the target revolution speed Nr from the target revolution speed setting unit  16  and an actual revolution speed Ne detected by the governor device  15 , and outputs a control signal R to the governor device  15  so that the actual revolution speed Ne coincides with the target revolution speed Nr. Details of the first control unit  17  will be described later. 
     The hydraulic pump  18  is disposed in the accommodating room  4  as described above, and is rotatably driven by the prime mover  14 . Also, the hydraulic pump  18  is a variable displacement pump and includes a swash plate  19  for changing a delivery rate of the pump. A delivery rate regulator  20  is coupled to the swash plate  19 . Additionally, there are provided a swash-plate position sensor  21  for detecting a tilting position of the swash plate  19  and a pressure sensor  22  for detecting a delivery pressure of the hydraulic pump 
     Numeral  23  denotes a second control unit provided in a control system for the hydraulic pump  18  and controlling the hydraulic pump  18 . The second control unit  23  performs the predetermined computation based on a delivery pressure Pd of the hydraulic pump  18  detected by the pressure sensor  22  and a tilting position θ of the swash plate  19  detected by the swash-plate position sensor  21 , and outputs a control signal for the swash plate  19  to the delivery rate regulator  20  for the hydraulic pump  18 . Details of the second control unit  23  will be described later. 
     The boom operating hydraulic cylinder  11 , the arm operating cylinder  12  and the bucket operating cylinder  13  constitute a hydraulic system  55  in cooperation with the hydraulic pump  18 , control valves  24 ,  25 ,  26 , etc. The hydraulic cylinders  11 ,  12 ,  13  are connected to the hydraulic pump  18  through the control valves  24 ,  25 ,  26 , respectively. Flow rates and directions of a hydraulic fluid supplied from the hydraulic pump  18  to the respective cylinders  11 ,  12 ,  13  are adjusted by the control valves  24 ,  25 ,  26 . The control valves  24 ,  25 ,  26  are disposed in the accommodating room  4 . Control levers  27 ,  28 ,  29  are provided in association with the control valves  24 ,  25 ,  26 , and lever operating units  30 ,  31 ,  32  are coupled to the control levers  27 ,  28 ,  29 , respectively. The lever operating units  30 ,  31 ,  32  output electrical signals corresponding to respective input amounts by which the control levers  27 ,  28 ,  29  are operated. 
     Numeral  33  denotes a third control unit provided in an operating system for shifting the control valves  24 ,  25 ,  26  with the control levers  27 ,  28 ,  29 . The third control unit  33  performs the predetermined computational processing based on the electrical signals from the lever operating units  30 ,  31 ,  32 , and outputs control signals to shifting sectors  24 L,  24 R,  25 L,  25 R,  26 L,  26 R of the control valves  24 ,  25 ,  26 . Details of the third control unit  33  will be described later. 
     The excavating device  7  is provided with a boom rotational angle sensor  34  for detecting a rotational angle of the boom  8 , an arm rotational angle sensor  35  for detecting a rotational angle of the arm  9 , and a bucket rotational angle sensor  36  for detecting a rotational angle of the bucket  10 . 
     Numeral  37  denotes a fourth control unit provided in a control system for the excavating device  7  and controlling the excavating device  7 . The fourth control unit  37  performs the predetermined computational processing based on respective rotational angle signals α, βγ from the rotational angle sensors  34 ,  35 ,  36 , and provides control driving commands Yα, Yβ, Yγ to the control unit  33 . Details of the fourth control unit  37  will be described later. 
     Numeral  38  denotes a display unit constituting a fifth control unit. The display unit  38  instructs a target working locus of the excavating device  7  to the fourth control unit  37 , and obtains information from the other control units  17 ,  23 ,  33 ,  37  to display the information. 
     The first to fourth control units  17 ,  23 ,  33 ,  37  and the display unit  38  are interconnected by a common communication line  39 , and transmit and receive data among them via the communication line  39 . 
     In this way, the prime mover  14  is controlled by the first control unit  17  through the governor device  15  so that the actual revolution speed Ne coincides with the target revolution speed Nr from the target revolution speed setting unit  16 . 
     The delivery rate of the hydraulic pump  18  is controlled through the delivery rate regulator  20  in accordance with the control signal created by the second control unit  23  based on the signals from the pressure sensor  22  and the swash-plate position sensor  21 . 
     The shift positions of the control valves  24 ,  25 ,  26  are controlled by the third control unit  33  in accordance with respective operation signals X 1 , X 2 , X 3  from the control levers  27 ,  28 ,  29 , thereby controlling the flow rates and directions of the hydraulic fluid supplied through the respective control valves. 
     The working locus of the excavating device  7  is controlled through the third control unit  33  in accordance with the control driving commands (working locus signals) Yα, Yβ, Yγ which are outputted from the fourth control unit  37  based on the rotational angle signals α, β, γ from the rotational angle sensors  34 ,  35 ,  36 . On that occasion, the third control unit  33  modifies the operation signals X 1 , X 2 , X 3  from the control levers  27 ,  28 ,  29  in accordance with the control driving signals from the fourth control unit  37 , and controls the control valves  24 ,  25 ,  26  for controlling the operation of the excavating device  7 . 
     The display unit  38  instructs the target working locus of the excavating device  7  to the fourth control unit  37 , and obtains information from the first to fourth control units  17 ,  23 ,  33 ,  37  to display the information. 
     FIG. 2 shows the configuration of the first control unit  17 . In FIG. 2, the same numerals as those in FIG. 1 denote the same components. The first control unit  17  comprises a single-chip microcomputer  176  including an A/D converter  170  for receiving a target revolution speed signal Nr from the target revolution speed setting unit  16  and converting it into a digital signal; a counter  175  for receiving, in the form of pulse signals, the actual revolution speed Ne of the prome mover  14  provided from the governor device  15 ; a central processing unit (CPU)  171 ; a Read Only Memory (ROM)  172  for storing a program of control procedures and constants necessary for the control; a Random Access Memory (RAM)  173  for temporarily storing computation results and numerical values in the course of computation; and a D/A converter  174  for converting a digital signal into an analog signal. The first control unit  17  further comprises an amplifier  177  for outputting a signal from the D/A converter  174  to the governor device  15 , a communication device  178  connected to the common communication line  39  for controlling communication to and from the other control units  23 ,  33 ,  37  and the display unit  38 , and a nonvolatile memory (EEPROM (Electrically Erasable Programmable Read Only Memory))  179  for storing control parameters, etc. 
     FIG. 3 shows the configuration of the second control unit  23 . In FIG. 3, the same numerals as those in FIG. 1 denote the same components. The second control unit  23  comprises a single-chip microcomputer  235  including an A/D converter  230  for receiving a pressure signal Pd from the pressure sensor  22  and a swash-plate position signal θ from the swash-plate position sensor  21 , and converting them into digital signals; a central processing unit (CPU)  231 ; a Read Only Memory (ROM)  232  for storing a program of control procedures and constants necessary for the control; a Random Access Memory (RAM)  233  for temporarily storing computation results and numerical values in the course of computation; and an output interface (I/O)  234 . The second control unit  23  further comprises an amplifier  236  for outputting a signal for driving the swash plate  19  of the hydraulic pump  18  to the swash-plate position regulator  20 , a communication device  237  connected to the common communication line  39  for controlling communication to and from the other control units  17 ,  33 ,  37  and the display unit  38 , and a nonvolatile memory (EEPROM)  238  for storing control parameters, etc. 
     FIG. 4 shows the configuration of the third control unit  33 . In FIG. 4, the same numerals as those in FIG. 1 denote the same components. The third control unit  33  comprises a single-chip microcomputer  335  including an A/D converter  330  for converting the operation signals X 1 , X 2 , X 3  from the lever operating units  30 ,  31 ,  32  associated with the control levers  27 ,  28 ,  29  into digital signals; a central processing unit (CPU)  331 ; a Read Only Memory (ROM)  332  for storing a program of control procedures and constants necessary for the control; a Random Access Memory (RAM)  333  for temporarily storing computation results and numerical values in the course of computation; and a D/A converter  334  for converting a digital signal into an analog signal. The third control unit  33  further comprises amplifiers  336   a - 337   f  for outputting signals from the D/A converter  334  to the shifting sectors  24 L,  24 R,  25 L,  25 R,  26 L,  26 R of the control valves  24 ,  25 ,  26 , a communication device  337  connected to the common communication line  39  for controlling communication to and from the other control units  17 ,  23 ,  37  and the display unit  38 , and a nonvolatile memory (EEPROM)  338  for storing control parameters, etc. 
     FIG. 5 shows the configuration of the fourth control unit  37 . In FIG. 5, the same numerals as those in FIG. 1 denote the same components. The fourth control unit  37  comprises a single-chip microcomputer  374  including an A/D converter  370  for receiving the angle signal α from the boom rotational angle sensor  34 , the angle signal β from the arm rotational angle sensors  35  and the angle signal γ from the bucket rotational angle sensor  36 , and converting them into digital signals; a central processing unit (CPU)  371 ; a Read Only Memory (ROM)  372  for storing a program of control procedures and constants necessary for the control; a Random Access Memory (RAM)  373  for temporarily storing computation results and numerical values in the course of computation. The fourth control unit  37  further comprises a communication device  375  connected to the common communication line  39  for controlling communication to and from the other control units  17 ,  23 ,  33  and the display unit  38 , and a nonvolatile memory (EEPROM)  376  for storing control parameters, etc. 
     FIG. 6 shows the configuration of the display unit  38  constituting the fifth control unit. In FIG. 6, the same numerals as those in FIG. 1 denote the same components. The display unit  38  comprises input devices  380 A,  380 B,  380 C, e.g., switches or keys, for changing over particulars displayed, and a single-chip microcomputer  386  including an interface (I/O)  381  for receiving signals from the input devices  380 A,  380 B,  380 C; a central processing unit (CPU)  382 ; a Read Only Memory (ROM)  383  for storing a program of control procedures and constants necessary for the control; a Random Access Memory (RAM)  384  for temporarily storing computation results and numerical values in the course of computation; and an output interface (I/O)  385 . The display unit  38  further comprises a display section  387  constituted by an LCD, for example, and displaying information thereon, a screen image display controller  388  for receiving a display command supplied from the CPU  382  to the display section  387  and sending data to the display section  387 , and a communication device  389  connected to the common communication line  39  for controlling communication to and from the other control units  17 ,  23 ,  33 ,  37 . 
     FIG. 7 shows the construction of the display unit  38  shown in FIG.  6 . In FIG. 7, the same numerals as those in FIG. 6 denote the same components. The input device  380 A is to change over an ON/OFF state of automatic operation, and is constituted by a switch. The input device  380 C is to set a target locus of the excavating device  7 , and is constituted by a switch. 
     Where an excavation depth, for example, is instructed as the target locus, the input device  380 C serves as an UP/DOWN switch for setting the target excavation depth. The switch  380 C may be implemented in various forms depending on how the target locus is instructed. In that exemplary case, the switch  380 B is changed over to display, on a display screen  387   a  of the display section  387 , the target excavation depth when the target locus is set, and an actual excavation depth of the excavating device  7  otherwise. 
     FIG. 8 shows one example of the configuration of each of the communication devices  178 ,  237 ,  337 ,  375 ,  389 . In FIG. 8, the same numerals as those in FIGS. 1-6 denote the same components. Each of the communication devices  178 ,  237 ,  337 ,  375 ,  389  is made up of a memory  80  having a storage location to manage data using the same number as an ID number (described later) added to the data; a communication controller  81 ; a data line  82  connected to the single-chip microcomputer  176 ,  235 ,  335 ,  374 ,  386  of the control unit  17 ,  23 ,  33 ,  37  and the display unit  38 ; an interrupt signal line  83  for sending a reception interrupt signal from the communication controller  81  to the single-chip microcomputer  176 ,  235 ,  335 ,  374 ,  386 ; and a reception line  84  and a transmission line  85  for connecting the communication controller  81  to the common communication line  39 . 
     Details of the data transmitting and receiving function of the single-chip microcomputers and the communication devices in the first to fourth control units  17 ,  23 ,  33 ,  37  and the display unit  38  will be described with reference to FIGS. 9-23. 
     FIG. 9 shows a software timing chart of computational processing performed by each of the single-chip microcomputers  176 ,  235 ,  335 ,  374 ,  386  in the first to fourth control units  17 ,  23 ,  33 ,  37  and the display unit  38 . Each of the single-chip microcomputers  176 ,  235 ,  335 ,  374 ,  386  generates a timer interrupt at intervals of a certain time, e.g., 1 ms, using a timer (not shown) to interrupt main processing, and starts up a timer interrupt processing program. Also, when any of the communication devices  178 ,  237 ,  337 ,  375 ,  389  generates a reception interrupt signal, the corresponding single-chip microcomputer interrupts the main processing and starts up a reception interrupt processing program. 
     FIGS. 10-19 show a message definition table and a message transmission management table prepared in the first to fourth control units  17 ,  23 ,  33 ,  37  and the display unit  38  for transmitting a message to the other control units at intervals of a certain time and receiving a necessary message when messages are transmitted from the other control units. In these tables, a group of data to be transmitted or received is referred to as a “message”. 
     First, in the first control unit  17 , the ROM  172  stores a message definition table in which whether to transmit or receive the message and a transmission cycle are described as shown in FIG.  10 . Also, based on the message definition table, a message transmission management table, in which the transmission cycle of the transmitted message and the counter are described as shown in FIG. 11, is created during initialization after start-up. 
     Then, in the second control unit  23 , the ROM  232  stores a message definition table in which whether to transmit or receive the message and a transmission cycle are described as shown in FIG.  12 . Also, based on the message definition table, a message transmission management table, in which the transmission cycle of the transmitted message and the counter are described as shown in FIG. 13, is created during initialization after start-up. 
     Further, in the third control unit  33 , the ROM  332  stores a message definition table in which whether to transmit or receive the message and a transmission cycle are described as shown in FIG.  14 . Also, based on the message definition table, a message transmission management table, in which the transmission cycle of the transmitted message and the counter are described as shown in FIG. 15, is created during initialization after start-up. 
     Further, in the fourth control unit  37 , the ROM  372  stores a message definition table in which whether to transmit or receive the message and a transmission cycle are described as shown in FIG.  16 . Also, based on the message definition table, a message transmission management table, in which the transmission cycle of the transmitted message and the counter are described as shown in FIG. 17, is created during initialization after start-up. 
     Further, in the display unit  38 , the ROM  383  stores a message definition table in which whether to transmit or receive the message and a transmission cycle are described as shown in FIG.  18 . Also, based on the message definition table, a message transmission management table, in which the transmission cycle of the transmitted message and the counter are described as shown in FIG. 19, is created during initialization after start-up. 
     In the above message definition tables and message transmission management tables, each transmitted or received message (data) is assigned with a specific ID number for management. The ID number is used by the control unit on the receiving side for identifying the message (data). 
     Also, in the above message definition tables, the transmission cycle (transmission time interval) of the transmitted message is set depending a varying speed of respective data or the cycle required for the control unit receiving the data (as described later). 
     The operation in the case of transmitting data will be described, taking the second control unit  23  as an example. 
     During initialization after start-up, the second control unit  23  creates the message transmission management table shown in FIG. 13 from the message definition table shown in FIG.  12 . The second control unit  23  generates a timer interrupt at intervals of a certain time using the timer as described above, and starts up a timer interrupt processing program shown in FIG.  20 . The timer interrupt processing program executes the following procedures. 
     First, the counter in the transmission management table shown in FIG. 13 is incremented upon each timer interrupt (step  221 ). It is then determined whether the counter value has become equal to the number n of timer interrupts corresponding to the transmission cycle in the transmission management table shown in FIG. 13 (step  222 ). Where the time interval for timer interrupts is set to 1 ms as described above, n=cycle is held. It is therefore just required to determine in step  222  whether the counter value has become equal to the transmission cycle. Because the cycle is “8” in the example of the transmission management table shown in FIG. 13, it is determined whether the counter value has become “8”. If the counter value has become equal to the transmission cycle, the counter is initialized (step  223 ), and data (θ and Pd) to be transmitted is transferred from the RAM  233  shown in FIG. 3 to the memory  80  in the communication device  237  and then written in the storage location corresponding to the ID number of the data in the memory  80  (step  224 ). Next, a transmission request flag in the communication controller  81  of the communication device  237  is turned to “1” (set) (step  225 ), thereby ending the processing. 
     The communication device  237  converts the data, on which the transmission request flag has been set, into time-serial data and outputs the time-serial data to the common communication line  39  for supplying the data to the other control units. The operation of the communication device  238  on that occasion will be described with reference to a flowchart of FIG.  21 . 
     First, the communication controller  81  determines the state of the transmission request flag (step  231 ). If the transmission request flag is turned to “1” (set), i.e., if transmission is instructed from the microcomputer side, the communication controller  81  reads data out of a storage location at which the transmission request flag is set (step  232 ), and affixes an ID number corresponding to the storage location to the data (step  233 ). Then, the communication controller  81  determines whether the common communication line  39  is vacant. If the communication line  39  is vacant, the communication controller  81  transmits the data affixed with the ID number to the common communication line  39  via the transmission line  85  (step  235 ). Thereafter, the communication controller  81  clears the transmission request flag (to 0), thereby indicating the end of transmission (step  236 ). 
     The data transmission processing is completed through the above-described procedures. 
     While the above processing has been described in connection with the second control unit  23 , the other control units and the display unit can also transmit data at respective set cycles in a similar manner as described above. 
     The operation in the case of receiving data will be described, taking the second control unit  23  likewise as an example. 
     The controller  81  of the communication device  237  receives data from the common communication line  39  and takes in only necessary items of the received data. The details will be described with reference to the flowchart of FIG.  22 . 
     When receiving data, the communication controller  81  first receives all of data once (step  241 ). Then, the ID numbers affixed to the received data are compared with the ID numbers set by the single-chip microcomputer  235  beforehand (step  242 ), and if both the ID numbers coincide with each other, the data is written in a storage location of the memory  80  corresponding to that ID number (step  243 ). At the same time, the communication controller  81  transmits an interrupt signal to the single-chip microcomputer  235  via the interrupt signal line  83  for notifying the fact that the data has been received, and turns a reception interrupt flag to “1” (step  244 ). 
     When the reception interrupt is received from the communication device  237  and the reception interrupt flag is set, the microcomputer  235  automatically starts a reception interrupt processing program shown in FIG.  23 . The reception interrupt processing program executes the following procedures. 
     First, the microcomputer  235  transfers the received data, which has been written in the memory  80  of the communication device  237 , to the RAM  233  (step  251 ). Then, the microcomputer  235  clears the reception interrupt flag (writes “0” as a flag level) in the communication device  237  (step  252 ). 
     With the above-described processing of transferring the received data to the RAM  233 , it becomes possible to execute a control program as the main processing (described later) by utilizing the received data. 
     The above data receiving processing is also performed in the other control units  17 ,  33 ,  37  and the display unit  38  through similar procedures. 
     Incidentally, the communicating method is not limited to the above-described one, and the communication devices  178 ,  237 ,  337 ,  375 ,  389  can also be implemented with any other suitable method. Further, the function of the communication controller  81  can also be realized using software in the microcomputer in accordance with an ordinary serial communicating method. 
     In each of the message definition tables shown in FIGS. 10,  12 ,  14 ,  16  and  18 , the transmission cycle (transmission time interval) of the transmitted message is decided in consideration of the cycle required for the message from the control point of view, or a varying speed of data or a frequency of variation thereof. 
     In FIG. 10, for example, because the actual revolution speed Ne and the target revolution speed Nr of the prime mover  14  transmitted by the first control unit  17  has a relatively low frequency of variation, the transmission cycle is set to 50 ms. 
     In FIG. 12, because the delivery pressure signal Pd and the swash-plate position signal θ for the hydraulic pump  18  transmitted by the second control unit  23  has a relatively high varying speed, the transmission cycle is set to 8 ms. 
     In FIG. 14, because the operation signals X 1 , X 2 , X 3  transmitted by the third control unit  33  have a relatively high varying speed, and because a period of about 10 ms is required for control computation for the excavating device  7  performed in the fourth control unit  37  or computation of the target swash-plate tilting angle θr of the hydraulic pump  18  performed in the second control unit  23 , the transmission cycle for each of the operation signals X 1 , X 2 , X 3  is set to about 10 ms. 
     In FIG. 16, because the control driving commands Yα, Yβ, Yγ transmitted by the fourth control unit  37  has a relatively high varying speed, and because a period of about 10 ms is required for computation to modify the operation signals X 1 , X 2 , X 3  which is performed in the third control unit  33 , the transmission cycle for each of the control driving commands Yα, Yβ, Yγ is set to about 10 ms. Also, because the actual depth h is a numerical value displayed on the display unit  38  and is not required to be adapted for a so quick change, the transmission cycle for the actual depth h is set to about 100 ms. 
     In FIG. 18, because a target locus hr of the excavating device  7  transmitted from the display unit  38  to the fourth control unit  37  and employed In the fourth control unit  37  is hardly changed once set, a period of about 100 ms is sufficient for transmitting the target locus hr and therefore the transmission cycle is set to such a value. Also, because an automatic operation command C auto  transmitted from the display unit  38  to the third control unit  33  and the fourth control unit  37  is also hardly changed, the transmission cycle for the automatic operation command C auto  is set to about 100 ms. 
     In each of the message definition tables shown in FIGS. 10,  12 ,  14 ,  16  and  18 , ( ) added on the right side of a row of each received message represents an initial value for use in starting up the control unit. In the present invention, the initial value is appropriately set so that each control unit can perform computational processing using the set initial value and fulfill the least necessary function. As a result, each control unit can always have a minimum processing function to eliminate or minimize change of software when the number of control units is increased or decreased, thereby enabling the control units to be easily increased or decreased in number. A value with which the control unit can fulfill the least necessary function is varied depending on the types of control units, and hence the initial value is set to a different value for each of the control units in spite of the message (data) having the same name. 
     More specifically, in FIG. 12, initial values of the operation signals X 1 , X 2 , X 3  and the actual revolution speed Ne and the target revolution speed Nr of the prime mover  14 , which are received by the second control unit  23 , are set as follows when these message are not received, so that the hydraulic pump  18  delivers the hydraulic fluid at a maximum flow rate under horsepower control using the delivery pressure signal Pd: 
     X 1 =XFULL (maximum input amount) 
     X 2 =XFULL (maximum input amount) 
     X 3 =XFULL (maximum input amount) 
     Ne=NMAX (maximum revolution speed) 
     Nr=NMAX (maximum revolution speed) 
     Also, in FIG. 14, the automatic operation command C auto  and the control driving commands Yα, Yβ, Yγ, which are received by the third control unit  33 , are set as follows when these message are not received, so that the control valves  24 ,  25 ,  26  can be driven by manual operation of the control levers  27 ,  28 ,  29 : 
     C auto =OFF 
     Yα=0 
     Yβ=0 
     Yγ=0 
     Further, in FIG. 16, the target locus hr of the excavating device  7 , the automatic operation command C auto , the operation signals X 1 , X 2 , X 3 , the delivery pressure signal Pd and the swash-plate position signal θ for the hydraulic pump  18 , and the actual revolution speed Ne of the prime mover  14 , which are received by the fourth control unit  37 , are set as follows when these message are not received, so that working locus control of the excavating device  7  is not performed: 
     hr=h out  (numerical value outside the reachable region of the excavating device  7 ) 
     C auto =OFF 
     X 1 =0 
     X 2 =0 
     X 3 =0 
     θ=θMAX (maximum tilting angle) 
     Pd=P normal  (average delivery pressure for ordinary excavation, e.g., 200 Kg/cm 2 ) 
     Ne=NMAX (maximum revolution speed) 
     Still further, in FIG. 18, the actual depth h received by the display unit  38  is set as follows when this message is not received, so that nothing is displayed on the display screen  387   a:    
     h=h out  (numerical value outside the reachable region of the excavating device  7 ) 
     The above-described initial values are each stored in the ROM or EEPROM of the corresponding control unit. 
     The control operation of main processing in the single-chip microcomputers  176 ,  235 ,  335 ,  374 ,  386  and the first to fourth control units  17 ,  23 ,  33 ,  37  and the display unit  38  will be next described. 
     First, the control operation of the first control unit  17  performed for the prime mover  14  will be described with reference to a flowchart of FIG.  24 . 
     In FIG. 24, the first control unit  17  first reads constants required for control computation from the ROM  172  or the EEPROM  179  shown in FIG. 2 (step  301 ). Then, the first control unit  17  reads the target revolution speed Nr from the target revolution speed setting unit  16  through the A/D converter  170  (step  302 ). Then, the first control unit  17  receives the actual revolution speed Ne of the prime mover  14  from the governor device  15  through the counter  175  (step  303 ). Then, the first control unit  17  computes the control signal R so that the actual revolution speed Ne coincides with the target revolution speed Nr, and outputs the control signal R to the governor device  15  through the D/A converter  174  and the amplifier  177  shown in FIG. 2, thereby controlling the revolution speed of the prime mover  14  to be coincident with the target revolution speed Nr (step  304 ). Thereafter, the first control unit  17  returns to step  302  and repeats the above-described processing. 
     During the above-described processing, the target revolution speed Nr and the actual revolution speed Ne of the prime mover  14 , which are read by the first control unit  17 , are transmitted to the control units  23 ,  33 ,  37  and the display unit  38  via the common communication line  39  in accordance with the transmitting method described above. 
     Next, the control operation of the second control unit  23  performed for the hydraulic pump  18  will be described with reference to a flowchart of FIG.  25 . 
     In FIG. 25, the second control unit  23  first reads the initial values of the received data X 1 , X 2 , X 3 , Nr, Ne and other constants required for control computation, which are used in the second control unit  23 , from the ROM  232  or the EEPROM  238  shown in FIG. 3 (step  311 ). Then, the second control unit  23  reads the pressure signal Pd from the pressure sensor  22  and the swash-plate position signal θ from the swash-plate position sensor  21  through the A/D converter  230  (step  312 ). Then, the second control unit  23  computes a load status of the prime mover  14  using the target revolution speed Nr and the actual revolution speed Ne which are data received from the first control unit  17  (step  313 ). Then, the second control unit  23  computes a delivery rate of the hydraulic fluid demanded by the hydraulic pump  18  based on the control-lever operation signals X 1 , X 2 , X 3  which are data received from the third control unit  33  (step  314 ). 
     Then, based on the demanded delivery rate of the hydraulic pump  18  computed in step  314 , the second control unit  23  computes a deliverable rate of the hydraulic pump  18  from both the load status of the prime mover  14  computed in step  313  and the pressure signal Pd from the pressure sensor  22  read in step  312 , and calculates a target swash-plate tilting signal θr from the computed deliverable rate (step  315 ). Then, the second control unit  23  computes a control signal so that the swash-plate position signal θ coincides with the target swash-plate tilting signal θr, and outputs the control signal to the delivery rate regulator  20  through the interface (I/O)  234  and the amplifier  236  shown in FIG. 3, thereby controlling the tilting position of the swash plate  19  of the hydraulic pump  18  (step  316 ). Thereafter, the second control unit  23  returns to step  312  and repeats the above-described processing. 
     During the above-described processing, the pressure signal Pd and the swash-plate position signal θ for the hydraulic pump  18 , which are read by the second control unit  23 , are transmitted to the control units  17 ,  33 ,  37  and the display unit  38  via the common communication line  39  in accordance with the transmitting method described above. 
     Also, the target revolution speed Nr and the actual revolution speed Ne used for computing the load status of the prime mover  14  in step  313  and the control-lever operation signals X 1 , X 2 , X 3  used for computing the demanded delivery rate of the hydraulic pump  18  in step  314  are received via the common communication line  39  in accordance with the receiving method described above. 
     Here, the target revolution speed Nr and the actual revolution speed Ne are data transmitted from the first control unit  17 , and the control-lever operation signals X 1 , X 2 , X 3  are data transmitted from the third control unit  33 . If a system is constructed with exclusion of at least one of the first control unit  17  and the third control unit, the second control unit  23  performs the computation in steps  313  and  314  using the above-described initial values read in step  311  in place of data to be received from the control unit having been excluded, and therefore can fulfill the least necessary processing function. In other words, steps  311 ,  313 ,  314  function as a minimum processing means that is able to execute the least necessary processing by itself, when no data is transmitted via the transmission line  39 . 
     Next, the control operation of the third control unit  33  performed for the control levers  27 ,  28 ,  29  will be described with reference to a flowchart of FIG.  26 . 
     In FIG. 26, the third control unit  33  first reads the initial values of the received data C auto , Yα, Yβ, Yγ and other constants required for control computation, which are used in the third control unit  33 , from the ROM  332  or the EEPROM  338  shown in FIG. 4 (step  321 ). Then, the third control unit  33  reads the operation signals X 1 , X 2 , X 3  from the lever operating units  30 ,  31 ,  32  through the A/D converter  330  (step  322 ). Then, the third control unit  33  determines whether the automatic operation command C auto , which is data received from the display unit  38 , is turned on (step  323 ). If it Is determined that C auto  is turned off, the processing goes to step  324 , and if it is determined that C auto  is turned on, the processing goes to step  325 . In step  325 , because the automatic operation is instructed, the third control unit  33  modifies the operation signals X 1 , X 2 , X 3  using the control driving commands Yα, Yβ, Yγ for the excavating device  7  which are data received from the fourth control unit  37 . 
     In step  324 , the third control unit  33  computes valve shift amounts by which the control valves are to be shifted in accordance with operation signals. by using the operation signals X 1 , X 2 , X 3  read in step  322  as they are because the automatic operation is not instructed when C auto  is turned off, and by using the operation signals X 1 , X 2 , X 3  modified in step  325  when C auto  is turned on. Then, the third control unit  33  outputs signals corresponding to the valve shift amounts to the control valves  24 ,  25 ,  26  through the D/A converter  334  and the amplifier  336  shown in FIG.  4 . Thereafter, the third control unit  33  returns to step  322  and repeats the above-described processing. 
     During the above-described processing, the operation signals X 1 , X 2 , X 3  read by the third control unit  33  are transmitted to the control units  17 ,  23 ,  37  and the display unit  38  via the common communication line  39  in accordance with the transmitting method described above. 
     Also, the automatic operation command C auto  used for determining in step  323  whether the automatic operation is instructed and the control driving commands Yα, Yβ, Yγ used for modifying the operation signals X 1 , X 2 , X 3  in step  325  are received via the common communication line  39  in accordance with the receiving method described above. 
     Here, the automatic operation command C auto  is data transmitted from the display unit  38 , and the control driving commands Yα, Yβ, Yγ are data transmitted from the fourth control unit  37 . If a system is constructed with exclusion of at least one of the display unit  38  and the fourth control unit  37 , the third control unit  33  performs the determination and computation in steps  323  and  325  using the above-described initial values read in step  321  in place of data to be received from the control unit having been excluded, and therefore can fulfill the least necessary processing function. In other words, steps  321 ,  323 ,  325  function as a minimum processing means that is able to execute the least necessary processing by itself, when no data is transmitted via the transmission line  39 . 
     Next, the control operation of the fourth control unit  37  performed for locus control of the excavating device  7  will be described with reference to a flowchart of FIG.  27 . 
     In FIG. 27, the fourth control unit  37  first reads the initial values of the received data hr, C auto , X 1 , X 2 , X 3 , θ, Pd, Ne and other constants including dimension data of the excavating device  7  required for computing an attitude of the excavating device  7 , which are used in the fourth control unit  37 , from the ROM  372  or the EEPROM  376  shown in FIG. 5 (step  331 ). Then, the fourth control unit  37  reads the boom angle signal α from the boom angle sensor  34 , the arm angle signal β from the arm angle sensors  35  and the bucket angle signal γ from the bucket angle sensor  36  through the A/D converter  370  (step  332 ). Then, the fourth control unit  37  computes an attitude of the excavating device  7  using the dimensions of the components of the excavating device  7  and the angle signals α, β, γ (step  333 ). This attitude computation includes computation of the excavation depth (bucket end position) h at that time. 
     Then, the fourth control unit  37  determines whether the automatic operation command C auto , which is data received from the display unit  38 , is turned on (step  334 ). If it is determined that C auto  is turned off, the processing goes to step  335 , and if it is determined that C auto  is turned on, the processing goes to step  336 . 
     In step  336 , because the automatic operation is instructed, the fourth control unit  37  computes the control driving commands Yα, Yβ, Yγ for the excavating device components (the boom  8 , the arm  9  and the bucket  10 ) to move the excavating device  7  to the target locus hr by using the target locus hr which is data received from the display unit  38 , the operation signals X 1 , X 2 , X 3  which are data received from the third control unit  33 , and the attitude of the excavating device  7  computed in step  333 . On this occasion. more precise control can be achieved by referring to the deliverable flow rate of the hydraulic pump  18  at that time based on such data as the actual revolution speed Ne of the prime mover  14  which is data received from the first control unit  17 , and the delivery pressure signal Pd and the swash-plate position signal θ for the hydraulic pump  18  which are data received from the second control unit  23 . 
     In step  335 , because the automatic operation is not instructed, the control driving commands Yα, Yβ, Yγ used in the above process are not required, and hence the control driving commands Yα, Yβ, Yγ are each set to  0 . Thereafter, the fourth control unit  37  returns to step  332  and repeats the above-described processing. 
     During the above-described processing, the control driving commands Yα, Yβ, Yγ and the actual excavation depth h, which are computed in the fourth control unit  37 , are transmitted to the control units  17 ,  23 ,  33  and the display unit  38  via the common communication line  39  in accordance with the transmitting method described above. 
     Also, the automatic operation command C auto  used for determining in step  334  whether the automatic operation is instructed, and the target locus hr, the operation signals X 1 , X 2 , X 3 , the actual revolution speed Ne of the prime mover  14 , and the delivery pressure signal Pd and the swash-plate position signal θ for the hydraulic pump  18 , which are used for computing the control driving commands Yα, Yβ, Yγ, are received via the common communication line  39  in accordance with the receiving method described above. 
     Here, the automatic operation command C auto  and the target locus hr are data transmitted from the display unit  38 , and the operation signals X 1 , X 2 , X 3  are data transmitted from the third control unit  33 . The actual revolution speed Ne of the prime mover  14  is data transmitted from the first control unit  17 , and the delivery pressure signal Pd and the swash-plate position signal θ for the hydraulic pump  18  are data transmitted from the third control unit  23 . If a system is constructed with exclusion of at least one of the first control unit  17  and the second control unit  23 , the fourth control unit  37  performs the computation in step  336  using the above-described initial values read in step  331  in place of data to be received from the control unit having been excluded, and therefore can fulfill the least necessary processing function. In other words, steps  331 ,  336  function as a minimum processing means that is able to execute the least necessary processing by itself, when no data is transmitted via the transmission line  39 . 
     Next, the control operation of the display unit  38  will be described with reference to a flowchart of FIG.  28 . 
     In FIG. 28, the display unit  38  first reads the initial value of the received data h and other constants required for control computation, which are used in the display unit, from the ROM  383  shown in FIG. 6 (step  341 ). Then, the display unit  38  receives states of operation of the switches  380 A,  380 B,  380 C shown in FIG. 7 via the I/O  381  shown in FIG. 6 (step  342 ). Then, the display unit  38  determines based on an input from the switch  380 A whether the switch  380 A instructs an automatic or manual mode (step  343 ). If the automatic mode is instructed, the display unit  38  goes to step  344  and turns on the automatic operation command C auto . Then, the display unit  38  determines a state of operation of the switch  380 C and sets the target excavation depth hr (step  345 ), followed by going to step  346 . If the manual mode is determined in step  343 , the display unit  38  goes to step  347  and turns off the automatic operation command C auto , followed by going to step  346 . 
     Then, the display unit  38  determines the state of the display changeover switch  380 B in step  346 . If it is determined that the target excavation depth hr is instructed, the display unit  38  goes to step  348  and displays the target excavation depth hr on the display screen  397   a  of the display section  387 . If it is determined in step  346  that the actual excavation depth h is instructed, the display unit  38  goes to step  349  and displays the actual excavation depth h, which is data received from the fourth control unit, on the display screen  397   a . Thereafter, the display unit  38  returns to step  342  and repeats the above-described processing. 
     During the above-described processing, the set automatic operation command C auto  and target excavation depth hr are transmitted to the control units  17 ,  23 ,  33 ,  37  via the common communication line  39  in accordance with the transmitting method described above. 
     Also, the actual excavation depth h, which is displayed in step  349 , is received via the common communication line  39  in accordance with the receiving method described above. 
     Here, the actual excavation depth h is data transmitted from the fourth control unit  37 . If a system is constructed with exclusion of the fourth control unit  37 , the display unit  38  performs the display processing in step  349  using the above-described initial value read in step  341  in place of the actual excavation depth h, and therefore can fulfill the least necessary processing function. In other words, steps  341 ,  349  function as a minimum processing means that is able to execute the least necessary processing by itself, when no data is transmitted via the transmission line  39 . 
     FIG. 29 is a time chart showing communication data flowing over the common communication line  39  and status of control computation in the respective control units during a period in which control computational processing is performed by the first to fourth control units  17 ,  23 ,  33 ,  37  and the display unit  38  as described above. FIG. 30 is a table listing the relationships in transmission and reception of the communication data among those units, including transmission cycles. 
     In FIG. 29, (a- 1 ) to (a- 3 ) represent the timed relationship of processing in the display unit  38 . Specifically, (a- 1 ) represents the timing of the normal control computational processing, (a- 2 ) represents the timing of the transmission processing, and (a- 3 ) represents the timing of the reception processing. Likewise, (b- 1  to - 3 ) represent the timed relationship of processing in the fourth control unit, and (c- 1  to - 3 ) represent the timed relationship of processing in the third control unit. Also, (d- 1  to - 3 ) represent the timed relationship of processing in the third control unit, and (e- 1  to - 3 ) represent the timed relationship of processing in the first control unit. Further, (f) represents situations in flow of data on the common communication line  39  resulting from the transmission processing in the respective control units. 
     In FIG. 29, for example, {circumflex over (1)} in (a- 2 ) represents execution of the processing for transmitting the automatic operation command C auto  and the target locus (target excavation depth) hr both created in the display unit  38 . As a result, the data flows over the common communication line as indicated by {circumflex over (2)} in (f). Correspondingly, the fourth control unit  37  performs the reception processing as indicated by {circumflex over (3)}, and the third control unit  33  performs the reception processing as indicated by {circumflex over (4)}. Whether data is to be received or not is determined by the communication device in each control unit, which identifies the ID number affixed to the data. 
     In this connection, the transmission cycle of each data is set depending on a varying speed of the data or the cycle required for the control unit on the receiving side, as described above, and the data is transmitted at the set transmission cycle while the transmission cycle is managed on the transmitting side. On the other hand, the receiving side identifies necessary items of various data flowing over the common communication line  39  based on the ID numbers, and receives only the necessary data. Therefore, each unit can receive only necessary information at a cycle required from the control point of view, and the amount of data flowing over the common communication line  39  can be held down to the least necessary level. As a result, the common communication line  39  can be utilized efficiently. 
     Advantages of this embodiment thus constructed will be described below. 
     (I) First, according to this embodiment, control functions of a hydraulic excavator are divided in units of least necessary function, i.e., a control system for the prime mover  14 , a control system for the hydraulic pump  18 , an operating system through the control levers  27 ,  28 ,  29 , a control system for the excavating device  7 , and a display system by the display unit  38 . Control units, i.e., the first to fourth control units  17 ,  23 ,  33 ,  37  and the display unit  38 , are provided in one-to-one relation to those systems, and the first to fourth control units  17 ,  23 ,  33 ,  37  and the display unit  38  are interconnected via the common communication line  39  for transmitting and receiving data. Further, a minimum processing means is provided in each of the second to fourth control units  23 ,  33 ,  37  and the display unit  38  which employ received data, so that the least necessary processing can be performed by the minimum processing means when no data is transmitted via the communication line. Such a feature provides advantages as follows. 
     1. When manufacturing machines that require different functions, it is just needed to change, add or cut the least necessary control unit. Therefore, a system can be changed with the least necessary development cost and the least necessary number of development steps. Also, since the control units are divided for each function, the system is more convenient from the viewpoint of management and the management cost can be reduced. 
     2. Since a communication section is not so changed, system change can be made with the least necessary development cost and the least necessary number of development steps. Also, troubles accompanying with the system change can be lessened. 
     3. In relation to above 1, when a system is changed by increasing or reducing the number of control units in one construction machine, change of software and replacement of the control unit(s) are no longer needed, and the development cost and the number of development steps can be held down to a minimum. 
     4. In relation to above 1. when changing control procedures of control units in one construction machine or when applying an electronic control system to a construction machine having another hydraulic system, replacement of the control unit(s) is reduced to the least necessary level, and the cost and the number of steps required for such a modification can be held down to a minimum. 
     5. Because of not including a control unit (master controller) which supervises a plurality of control units in a centralized manner, the possibility that a failure in any one of the control units or a trouble of the common communication line may totally disable the other control units can be reduced, and the construction machine can be avoided from stopping the operation even in such an event. 
     The advantages of above 1 and 3 will be described more concretely with reference to FIGS. 31 and 32. 
     FIG. 31 shows a modification of the electronic control system in which the fourth control unit  37  and the display unit  38  are excluded from the system of FIG.  1 . In this modified system, the data C auto , Yα, Yβ, Yγ are not transmitted which are to be received by the third control unit  33  and used in computation performed therein. However, the third control unit  33  can, as described above, perform the computation using the initial values of those data and perform the least necessary processing. 
     FIG. 32 shows a modification of the electronic control system in which the first control unit  17  is further excluded from the system of FIG.  31 . In this modified system, the data Ne, Nr are also not transmitted which are to be received by the second control unit  23  and used in computation performed therein. However, the second control unit can, as described above, perform the computation using the initial values of those data and perform the least necessary processing. 
     In the case of changing (expanding) the system shown in FIG. 31 to the system shown in FIG. 1, just by connecting the fourth control unit  37  and the display unit  38  to the common communication line  39 , the third control unit can perform the processing using the data transmitted from those added control units on condition that a program (software) in the third control unit  33  is configured beforehand, as described above, in anticipation of the expansion to the system of FIG.  1 . Changing (expanding) the system shown in FIG. 32 to the system shown in FIG. 1 can also be made in a similar way. 
     Thus, according to this embodiment, since the minimum processing means is provided in each of the second to fourth control units  23 ,  33 ,  37  and the display unit  38  which employ received data, the least necessary processing can be performed by the minimum processing means when no data is transmitted via the communication line. Therefore, when a system is changed by increasing or reducing the number of control units, even change of software and replacement of the control unit(s) are no longer needed, and system change is very facilitated. 
     In relation to the advantages of above 1 and 4, when changing control procedures of control units in one construction machine, it is just needed to remove one or more of the control units (including the display unit), which are subjected to the change, from the common communication line  39  and connect one or more new control units to the communication line  39 , while the other control units can be used as they are, as described later in a second embodiment of the present invention with reference to FIG.  33 . Further, when applying the electronic control system to a hydraulic excavator having another hydraulic system, it is also just needed to remove only those control units which are associated with a changed section of the hydraulic system, while the other control units can be used as they are, as described later in third to fifth embodiments of the present invention with reference to FIGS. 34 to  36 . Especially, even where a particular one of a plurality of control units executes processing using a received signal and a control unit transmitting the signal to be received by the particular control unit is excluded from the system as a result of replacement of one or more control units, the particular control unit is not required to be replaced and the other control units including the particular control unit can be all used as they are, as described later in the third embodiment of the present invention with reference to FIG.  34 . 
     (II) Further, according to this embodiment, as described above, the transmission cycle of each data is set depending on a varying speed of the data or the cycle required for the control unit on the receiving side, and the data is transmitted at the set transmission cycle while the transmission cycle is managed on the transmitting side. Further, the receiving side identifies necessary items of various data flowing over the common communication line  39  based on the ID numbers, and receives only the necessary data. Therefore, each unit can receive only necessary information at a cycle required from the control point of view, and the amount of data flowing over the common communication line  39  can be held down to the least necessary level, thus enabling the common communication line  39  to be utilized efficiently. As a result, even with a plurality of control units connected to the common communication line  39 , the control performance is avoided from being affected by a lowering of the communication efficiency. Also, even with an increase in the number of control units, the system is less susceptible to such a trouble as disabling communication due to excessive traffic on the common communication line  39 . 
     FIG. 33 shows, as a second embodiment of the present invention, an electronic control system wherein control procedures of a part of the above-described control units is changed from those in the embodiment shown in FIG.  1 . In FIG. 33, the same numerals as those in FIG. 1 denote the same components. 
     Referring to FIG. 33, the electronic control system of this embodiment differs from the system of the embodiment shown in FIG. 1 in that a fifth control unit  40  for controlling an operating area of the excavating device  7  is provided in place of the fourth control unit  37 , and the display unit  38  shown in FIG. 1 is replaced by a display unit  41  correspondingly. In this case, the first control unit  17 , the second control unit  23  and the third control unit  33  are the same as those in the embodiment shown in FIG.  1 . 
     In this embodiment, the fifth control unit  40  computes the control driving commands Yα, Yβ, Yγ and the actual excavation depth h for the excavating device  7  in accordance with operating area limiting control. The control driving commands Yα, Yβ, Yγ are transmitted to the third control unit  33 , and the actual excavation depth h is transmitted to the display unit  41 . Further, the third control unit  33  performs control computation using the received control driving commands Yα, Yβ, Yγ as with that shown in FIG. 1, whereby the operating area limiting control of the excavating device  7  can be performed. The display unit  41  displays the actual excavation depth h. 
     Thus, with this embodiment, system change from the system shown in FIG. 1 can be easily realized just by replacing the fifth control unit  40  and the display unit  41  without changing the common communication line  39 . 
     In addition, since a communication device and a minimum processing means, which are similar to those in the first embodiment, are provided in each of the control units, this second embodiment can also provide similar advantages as with the first embodiment, such as easy addition and exclusion of one or more control units, appropriate setting of the transmission cycle, and efficient utilization of the common communication line  39  due to identification of received data using ID numbers. 
     FIG. 34 shows, as a third embodiment of the present invention, an electronic control system wherein a hydraulic system installed in a hydraulic excavator, to which the electronic control system is applied, is changed from the one in the embodiment shown in FIG.  1 . In FIG. 34, the same numerals as those in FIG. 1 denote the same components. 
     Referring to FIG. 34, the electronic control system of this embodiment differs from the system of the embodiment shown in FIG. 1 in that the present invention is applied to a hydraulic excavator  1  including a hydraulic system  55 A which comprises control valves  24 A,  25 A,  26 A controlled respectively by hydraulic pilot-operated valves  30 A,  31 B,  32 A. Between the hydraulic pilot-operated valves  30 A,  31 B,  32 A and the control valves  24 A,  25 A,  26 A, a proportional solenoid valve unit  44  is provided for adjusting pilot pressures produced by the hydraulic pilot-operated valves  30 A,  31 B,  32 A with the pilot pressure from an auxiliary pump  43  used as a source pressure. 
     In the electronic control system of this embodiment which is applied to the hydraulic excavator including the hydraulic system  55 A, the fourth control unit  37  for the excavating device  7 , shown in FIG. 1, is replaced by a sixth control unit  42  for a hydraulic pilot-operating system, and the third control unit  33  for the operating system is excluded. The proportional solenoid valve unit  44  is controlled by a signal from the sixth control unit  42  to modify the pilot pressures from the hydraulic pilot-operated valves  30 A,  31 B,  32 A, thereby performing the operation control of the excavating device  7 . The first control unit  17 , the second control unit  23  and the display unit  38  are the same as those in the embodiment shown in FIG.  1 . 
     In this embodiment, the sixth control unit  42  computes the actual excavation depth h, and this data is transmitted to the display unit  38 . The display unit  38  displays the actual excavation depth h. 
     Further, in this embodiment, since the third control unit  33  in the embodiment shown in FIG. 1 is excluded, the second control unit  23  cannot receive the operation signals X 1 , X 2 , X 3  used in the computation performed therein. However, since a minimum processing means similar to that in the first embodiment is provided in each of the control units, the second control unit  23  can perform the computation using the initial values of the operation signals X 1 , X 2 , X 3  which are data to be received. Accordingly, there is no need of replacing the second control unit  23 . 
     Thus, even where the control unit  33  transmitting the operation signals X 1 , X 2 , X 3 , which are signals to be received by the second control unit  23  and used for the computation performed therein, is excluded as a result of replacing the fourth control unit  37  by the sixth control unit  42 , system change can be easily realized from the electronic control system shown in FIG. 1 to that for the hydraulic system  55 A including the hydraulic pilot-operated valves  30 A,  31 B,  32 A, just by removing the third control unit  33  and replacing the fourth control unit  37  by the sixth control unit  42  without changing the common communication line  39 . 
     In addition, since a communication device and a minimum processing means, which are similar to those in the first embodiment, are provided in each of the control units, this third embodiment can also provide similar advantages as with the first embodiment, such as easy addition and exclusion of one or more control units, appropriate setting of the transmission cycle, and efficient utilization of the common communication line  39  due to identification of received data using ID numbers. 
     FIG. 35 shows, as a fourth embodiment of the present invention, another example of a electronic control system wherein a hydraulic system, to which the electronic control system is applied, is changed from the one in the embodiment shown in FIG.  1 . In FIG. 35, the same numerals as those in FIG. 1 denote the same components. 
     Referring to FIG. 35, the electronic control system of this embodiment differs from the system of the embodiment shown in FIG. 1 in that the present invention is applied to a hydraulic excavator including a hydraulic system  55 B which comprises two hydraulic pumps  18 A,  18 B and provides higher working efficiency. A hydraulic fluid delivered from the hydraulic pump  18 A is supplied to the boom cylinder  11  and the arm cylinder  12  through control valves  24 A,  25 , respectively, and a hydraulic fluid delivered from the hydraulic pump  18 B is supplied to the boom cylinder  11  and the bucket cylinder  13  through control valves  24 B,  26 , respectively. In other words, the hydraulic fluids delivered from the two hydraulic pumps  18 A,  18 B are supplied to the boom cylinder  11  in a joined way. 
     In the electronic control system of this embodiment which is applied to the hydraulic excavator including the hydraulic system  55 B, the control unit  23  for the hydraulic pump  18 , shown in FIG. 1, is replaced by a seventh control unit  23 A which controls the two hydraulic pumps  18 A,  18 B, and the third control unit  33  for the control levers  27 ,  28 ,  29 , shown in FIG. 1, is replaced by an eighth control unit  33 A which controls the four control valves  24 A,  24 B,  25 ,  26 . The seventh control unit  23 A reads respective delivery pressures of the hydraulic pumps  18 A,  18 B as the pressure signal Pd to be read in step  312  of FIG. 25, and also reads respective swash-plate tilting angles of the hydraulic pumps  18 A,  18 B as the swash-plate position signal θ. The eighth control unit  33 A computes shift amounts by which the control valves  24 A,  24 B are to be operated, respectively, based on the operation signal X 1 . The control units  17 ,  37  and the display unit  38  are the same as those shown in FIG.  1 . 
     In this embodiment, the seventh control unit  23 A controls the swash-plate positions of the two hydraulic pumps  18 A,  18 B by using the operation signals X 1 , X 2 , X 3  which are data to be received from the eighth control unit  33 A and the revolution speed signals Nr, Ne which are data to be received from the first control unit  17 . 
     In a hydraulic excavator, it is often practiced to include a plurality of hydraulic pumps  18  to constitute a hydraulic system for improving operability of the excavating device  7  and enabling power to be utilized more effectively, to divide actuators into separate groups drive by the respective hydraulic pumps, and to employ hydraulic fluids from the hydraulic pumps  18  in a joined way. In such a case, it is often desired to control the plurality of hydraulic pumps by a single control unit from the functional point of view because a hydraulic power control system is required to be able to comprehensively control individual delivery rates of the plurality of hydraulic pumps. On that occasion, the desired function can be achieved with the construction of this embodiment by substituting only the control units  23 A,  33 A for the corresponding ones. 
     Thus, with this embodiment, system change can be easily realized from the electronic control system shown in FIG. 1 to that for the hydraulic system  55 B including the two hydraulic pump  18 A,  18 B, just by removing the second control unit  23  and the third control unit  33  and providing the seventh control unit  23 A and the eighth control unit  33 A instead without changing the common communication line  39 . 
     In addition, since a communication device and a minimum processing means, which are similar to those in the first embodiment, are provided in each of the control units, this fourth embodiment can also provide similar advantages as with the first embodiment, such as easy addition and exclusion of one or more control units, appropriate setting of the transmission cycle, and efficient utilization of the common communication line  39  due to identification of received data using ID numbers. 
     FIG. 36 shows, as a fifth embodiment of the present invention, another example of a electronic control system wherein a hydraulic system installed in a hydraulic excavator, to which the electronic control system is applied, is changed from the one in the embodiment shown in FIG.  1 . In FIG. 36, the same numerals as those in FIGS. 1 and 35 denote the same components. 
     Referring to FIG. 36, the electronic control system of this embodiment differs from the system of the embodiment shown in FIG. 1 in that the present invention is applied to a hydraulic system which includes a hydraulic circuit for driving the swing body  3  to be able to swing and a hydraulic circuit for driving the track body  2  to be able to travel. 
     More specifically, a hydraulic system  55 C in this embodiment comprises a swing motor  45  for driving the swing body  3 ; a right-hand track motor (not shown) for driving a right-hand track of the track body  2 ; a left-hand track motor  46  for driving a left-hand track of the track body  2 ; a control valve  47  for the right-hand track motor; a control valve  48  for the left-hand track motor  46 ; a control valve  49  for the swing motor  45 ; a control lever unit  50  for outputting an operation signal X 1  for the boom  8  upon lever manipulation in one of crossed directions and outputting an operation signal X 2  for the bucket  10  upon lever manipulation in the other of the crossed directions; a control lever unit  51  for outputting an operation signal X 3  for the arm  9  upon lever manipulation in one of crossed directions and outputting an operation signal X 4  for the swing body  3  upon lever manipulation in the other of the crossed directions; a control lever unit  52  for outputting an operation signal X 5  for the left track; and a control lever unit  53  for outputting an operation signal X 6  for the right track. 
     In the electronic control system of this embodiment which is applied to the hydraulic excavator including the hydraulic system  55 C, the eighth control unit  33 A for the operating system, shown in FIG. 35, is replaced by a ninth control unit  33 B so that the control valves  24 A,  24 B,  25 ,  26 ,  47 ,  48 ,  49  are shifted in accordance with the operation signals from control lever units  50 ,  51 ,  52 ,  53 . Also, corresponding to the replacement of the eighth control unit  33 A, the seventh control unit  23 A, which a control unit for the hydraulic pumps, is replaced by a tenth control unit  23 B. The ninth control unit  33 B transmits the operation signals X 1 -X 6  to the common communication line  39 , and the tenth control unit  23 B computes a demanded flow rate using the transmitted operation signals X 1 -X 6 . The control units  17 ,  37  and the display unit  38  are the same as those shown in FIG.  35 . 
     Thus, with this embodiment, system change can be easily realized from the electronic control system shown in FIG. 1 or  35  to that for the hydraulic system  55 C including the hydraulic circuit for driving the swing body  3  to be able to swing and the hydraulic circuit for driving the track body  2  to be able to travel, just by substituting the ninth control unit  33 B and the tenth control unit  23 B for the corresponding control units without changing the common communication line  39 . 
     Also, with this embodiment, since the actuator operating system is totally constructed in an electronic control manner, distribution of a flow rate of the hydraulic fluid to the respective actuators can be controlled comprehensively. Further, since data for distribution of a flow rate of the hydraulic fluid to the respective actuators can be processed by a single control unit, a processing and computing time for each actuator can be cut and the actuator control performance can be improved. 
     In addition, since a communication device and a minimum processing means, which are similar to those in the first embodiment, are provided in each of the control units, this fifth embodiment can also provide similar advantages as with the first embodiment, such as easy addition and exclusion of one or more control units, appropriate setting of the transmission cycle, and efficient utilization of the common communication line  39  due to identification of received data using ID numbers. 
     Additionally, the above embodiments have been described as implementing the minimum processing means by a method of appropriately setting an initial value. It is however as a matter of course that the minimum processing means may be implemented by any other method so long as each control unit can execute the least necessary processing by itself when no data is transmitted via the communication line. 
     FIG. 37 shows an electronic control system for a hydraulic excavator according to a sixth embodiment of the present invention, along with the hydraulic excavator and a hydraulic system installed therein. In FIG. 37, the same numerals as those in FIG. 1 denote the same components. 
     Referring to FIG. 37, a first control unit  117 , a second control unit  123 , a third control unit  133  and a fourth control unit  137  are provided for the prime mover  14 , the hydraulic pump  18 , the control valves  24 ,  25 ,  26  and the excavating device  7 , respectively, and a display/setting unit  138  is also provided. 
     The third control unit  133  and the fourth control unit  137  are interconnected by a common communication line  139  to transmit and receive data between them via the communication line  139 . Also, the display/setting unit  138  is connected to the fourth control unit  137  by a serial communication line  60  to transmit and receive data between them via the serial communication line  60 . 
     The hardware configurations of the third and fourth control units  133 ,  137  are essentially the same as those of the third and fourth control units  33 ,  37  in the first embodiment shown in FIGS. 4 and 5. For the sake of later description, communication devices in the third and fourth control units  133 ,  137  are denoted respectively by  133   a ,  137   a . The configuration and functions of the communication devices  133   a ,  137   a  are also essentially the same as those of the communication devices in the first embodiment shown in FIGS. 8,  21  and  22 . In this embodiment, however, a specific number is affixed to a message comprising a plurality of data grouped into a unit, and the communication devices  133   a ,  137   a  manage transmission and reception based on the message number (as described later). 
     In view of the case where work carried out by the hydraulic excavator  1  imposes a limitation in height or depth of the operating region of the excavating device  7 , the hydraulic excavator has a function of keeping the excavating device  7  from entering an area set by the operator. Such a function will be referred to as area limiting control hereinafter. Taking as an example processing to realize the area limiting control, processing functions of the third and fourth control units  133 ,  137  will be described with reference to flowcharts shown in FIGS. 38 to  40 . 
     FIGS. 38 and 39 show, in the form of a flowchart, processing sequences in the third control unit  133  and the fourth control unit  134  when the third control unit  133  and the fourth control unit  137  are both connected to the common communication line  139  as shown in FIG.  37 . FIG. 40 shows, in the form of a flowchart, an essential processing sequence in the third control unit  133  when the fourth control unit  137  is not connected. Further, in FIG. 38, portions indicated by broken lines represent processing portions executed in the third control unit  133  when the fourth control unit  137  is not connected, and processing in those portions is not executed when the fourth control unit  137  is connected as shown in FIG.  37 . 
     Referring to FIG. 38, when the fourth control unit  137  is connected to the common communication line  139 , the processing executed in the third control unit  133  includes main processing, timer interrupt processing, and reception interrupt processing. 
     In the main processing, target pilot pressures to be supplied to the control valves  24 ,  25 ,  26  are computed in accordance with the received operation signals X 1 , X 2 , X 3  (STEP  405 ). 
     In the timer interrupt processing, processing to take in the operation signals X 1 , X 2 , X 3  from the lever operating units  30 ,  31 ,  32  associated with the control levers  27 ,  28 ,  29  (STEP  401 ) and processing to issue output command values received from the fourth control unit  137  to the shifting sectors  24 L,  24 R,  25 L,  25 R,  26 L,  26 R of the control valves  24 ,  25 ,  26  (STEP  402 ) are performed each time the timer interrupt processing is activated, i.e., at intervals of a certain time. Also, as shown in FIG. 39, a count value in a transmission time management table (described later) is incremented at intervals of a certain time (STEP  430 ), and the count value is compared with a cycle provided as a predetermined transmission set time (STEP  431 ). Then, if the count value has become equal to the cycle, the communication device  133   a  is activated to execute processing to transmit the target pilot pressures, which are computed in the main processing, as data (STEP  403 ). 
     In the reception interrupt processing, each time the communication device  133   a  receives the data (output command values to be issued to the shifting sectors  24 L,  24 R,  25 L,  25 R,  26 L,  26 R of the control valves  24 ,  25 ,  26 ) received from the fourth control unit  137  and a reception interrupt signal is transmitted to the microcomputer side, the reception interrupt processing is activated to store the data received by the communication device  133   a  (STEP  404 ). 
     The processing executed in the fourth control unit  137  also includes main processing, timer interrupt processing, and reception interrupt processing. 
     In the main processing, the end position of the excavating device  7  is computed from the received angle α, β, γ, and attitude control computation is performed based on the computed position data, the target pilot pressures received from the third control unit  133  and the setting data for area limiting control received from the display/setting unit  138  (STEP  409 ). Finally, output command values are computed based on the results of the control computation (STEP  410 ). 
     In the timer interrupt processing, the angle signals α, β, γ from the angle sensors  34 ,  35 ,  36 , which are angle data of the excavating device  7 , are received each time the timer interrupt processing is activated, i.e., at intervals of a certain time (STEP  406 ). Also, as described above in connection with FIG. 39, a count value in a transmission time management table (described later) is incremented at intervals of a certain time (STEP  430 ), and the count value is compared with a cycle provided as a predetermined transmission set time (STEP  431 ). Then, if the count value has become equal to the cycle, the communication device  137   a  is activated to execute processing to transmit the output command values for the solenoid valves, which are computed in the main processing, as data to the third control unit  133  (STEP  407 ). 
     In the reception interrupt processing, each time the communication device  137   a  receives the data (target pilot pressures) received from the third control unit  133  and a reception interrupt signal is transmitted to the microcomputer side, the reception interrupt processing is activated to store the data received by the communication device  137   a  (STEP  408 ). 
     When the fourth control unit  137  is not connected to the common communication line  139 , processing  420 ,  421  indicated by broken lines in FIG. 38 are executed in the third control unit  133 . Also, when the fourth control unit  137  is not connected, parts of the timer interrupt processing, i.e., the sequences of processing  430 ,  431  shown in FIG.  39  and the processing  403  shown in FIG. 38, are activated. However, because the fourth control unit  137  is not connected, the third control unit  133  is substantially in the same condition as when those sequences of processing are not activated. Further, when the fourth control unit  137  is not connected, the reception interrupt signal is not transmitted and therefore the reception interrupt processing  404  is in a standby state. Note that, of the timer interrupt processing, the data transmission processing  403  shown in FIG. 38 may be switched over to be not activated (executed) when the fourth control unit  137  is not connected. 
     FIG. 40 shows, in the form of a flowchart, essential processing in the third control unit  133  when the fourth control unit  137  is not connected. In this case, the processing includes main processing and timer interrupt processing. 
     In the main processing, target pilot pressures to be supplied to the control valves  24 ,  25 ,  26  are computed in accordance with the received operation signals X 1 , X 2 , X 3  (STEP  405 ). Filtering and other processing are performed as required (STEP  420 ). Then, command values issued to the shifting sectors, i.e., the solenoid valves  24 L,  24 R,  25 L,  25 R,  26 L,  26 R, for providing the pilot pressures to be outputted are determined (STEP  421 ). 
     In the timer interrupt processing, processing to take in the operation signals X 1 , X 2 , X 3  from the lever operating units  30 ,  31 ,  32  associated with the control levers  27 ,  28 ,  29  (STEP  401 ) and processing to issue the output command values determined in the main processing to the shifting sectors  24 L,  24 R,  25 L,  25 R,  26 L,  26 R of the control valves  24 ,  25 ,  26  (STEP  402 ) are performed each time the timer interrupt processing is activated, i.e., at intervals of a certain time. 
     The processing in the third control unit  133  is changed over depending on whether the fourth control unit  137  is connected to the common communication line  139 . To that end, the third control unit  133  has the function of detecting whether the fourth control unit  137  is connected to the common communication line  139 , and the processing changeover function of changing over the processing depending on a detected result. Those detecting function and processing changeover function will be described below with reference to FIGS. 41-46. 
     FIG. 41 shows, in the form of a functional block diagram, overall processing functions of software installed in microcomputers of the third control unit  133  and the fourth control unit  137 . 
     Referring to FIG. 41, the third control unit  133  comprises a communication management section  71 , a flag setting section  72 , a processing selecting/executing section  73 , a processing section  74  for executing processing A, a processing section  75  for executing processing B, a processing section  76  for executing processing C, and a processing section  77  for executing processing D. Here, the processing A corresponds to the processing of STEP  405  shown in FIGS. 38 and 40. The processing B corresponds to the processing of STEP  420  shown in FIG.  38 . The processing C corresponds to the processing of STEP  421  shown in FIG.  38 . The processing D corresponds to the processing of STEP  401 ,  402  shown in FIGS. 38 and 40. 
     Also, the fourth control unit  137  comprises a communication management section  81 , a flag setting section  82 , a processing selecting/executing section  83 , a processing section  84  for executing processing E, a processing section  85  for executing processing F, and a processing section  86  for executing processing G. Here, the processing E corresponds to the processing of STEP  409  shown in FIG.  40 . The processing F corresponds to the processing of STEP  410  shown in FIG.  38 . The processing G corresponds to the processing of STEP  406  shown in FIG.  38 . 
     The communication management section  71  of the third control unit  133  has the function of transmitting data at intervals of a certain time in STEP  403  and STEP  430 ,  431  of the timer interrupt processing shown in FIGS. 38 and 39, and the function of STEP  404  of the reception interrupt processing shown in FIGS.  38 . The communication management section  81  of the fourth control unit  137  has the function of transmitting data at intervals of a certain time in STEP  407  and STEP  430 ,  431  of the timer interrupt processing shown in FIGS. 38 and 39, and the function of STEP  408  of the reception interrupt processing shown in FIGS.  38 . Further, the communication management sections  71 ,  81  each have, as a part of the reception interrupt processing, the function of detecting based on reception of data whether another control unit is connected. 
     The processing selecting/executing section  73  of the third control unit  133  and the processing selecting/executing section  83  of the fourth control unit  137  are each constituted as a program for selecting and executing the sequences of processing A-D or E-G at intervals of a required time by using the timer function (not shown) incorporated in the microcomputer. 
     The function of transmitting data at intervals of a certain time in the timer interrupt processing in each of the communication management sections  71 ,  81  will be first described more concretely with reference to FIGS. 42-46. FIGS. 42-46 show various transmission/reception management tables created and used in the communication management sections  71 ,  81 . 
     FIG. 42 shows one example of a data definition table usable in common to the respective control units. Each data is assigned with a specific ID. For example, a data ID “1” represents a boom-raising operation signal from the boom control lever  27 , and indicates the fact that the data having such an ID is data expressed by 2 bytes (16 bits). 
     FIG. 43 shows one example of a message definition table stored in the ROM of the fourth control unit  137 . Here, a “message” comprises a plurality of data grouped into a unit, and a specific number is affixed to each message. For example, a message number “1” represents a data group of boom-and-arm operation signals having data IDs  1 ,  2 ,  3  and  4  in the data definition table shown in FIG.  42 . Further, data is transmitted and received in units of message. The item “transmission/reception” represents whether the message is to be transmitted and received, and the item “transmission cycle” represents a cyclic interval for transmission of messages. Since the table of FIG. 43 is incorporated in the fourth control unit  137 , the data is computed in the fourth control unit  137 . A message  3  given as a data group of the output command values, which are issued to the shifting sectors (solenoid valves)  24 L,  24 R,  25 L,  25 R,  26 L,  26 R of the control valves  24 ,  25 ,  26 , has a transmission attribute, while messages  1  and  2  given as data groups of the operation signals received from the third control unit  133  have a reception attribute. 
     FIG. 44 shows one example of a transmission time management table created in the RAM of the fourth control unit  137 . For example, the message number  3  has a transmission cycle of 5 ms, and a count value counted by a timer incorporated in the fourth control unit  137  is entered in the item “counter”. 
     FIG. 45 shows one example of a message definition table stored in the ROM of the third control unit  133 . In this table, the messages  1  and  2  given as data groups of the operation signals from the control levers, which are computed in the third control unit  133 , have a transmission attribute, while the message  3  given as a data group of the output command values transmitted from the fourth control unit  137  have a reception attribute. 
     FIG. 46 shows one example of a transmission time management table created in the RAM of the third control unit  133 . For example, the message numbers  1  and  3  each have a transmission cycle of 5 ms, and a count value counted by a timer incorporated in the third control unit  133  is entered in the term “counter”. 
     For data transmission, the communication management section  71  of the third control unit  133  performs the above-described processing of STEP  430 ,  431  (FIG. 39) and STEP  403  (FIG. 38) in the timer interrupt processing of FIGS. 38 and 39. More specifically, in STEP  430 ,  431  of FIG. 39, the following processing is performed using the transmission time management table shown in FIG. 46. A count value in the transmission time management table is incremented at intervals of a certain time (STEP  430 ), and the count value is compared with a cycle provided as a predetermined transmission set time (STEP  431 ). Then, if the count value has become equal to the cycle, the communication device  133   a  is activated to execute the processing to transmit the target pilot pressures, which are computed in the main processing, as data in STEP  393  of FIG.  38 . 
     For data transmission, the communication management section  81  of the fourth control unit  137  performs the above-described processing of STEP  430 ,  431  (FIG. 30) and STEP  397  (FIG. 38) in the timer interrupt processing of FIGS. 38 and 39. More specifically, in STEP  430 ,  431  of FIG. 39, the following processing is performed using the transmission time management table shown in FIG. 44. A count value in the transmission time management table is incremented at intervals of a certain time (STEP  430 ), and the count value is compared with a cycle provided as a predetermined transmission set time (STEP  431 ). Then, if the count value has become equal to the cycle, the communication device  137   a  is activated to execute the processing to transmit the output command values for the solenoid valves, which are computed in the main processing, as data to the third control unit  133  in STEP  397 . 
     Next, a description is made of the function of detecting whether another control unit is connected, the function being performed by each of the communication management sections  71 ,  81  based on reception of data. 
     FIG. 47 is a flowchart showing details of the above-described reception interrupt processing in STEP  404 ,  408  of FIG.  38 . 
     First, the communication devices  133   a ,  137   a  each have such a function that when the communication device receives a message comprising a plurality of data grouped into a unit, as described above, via the common communication line  138 , it determines whether the received message is necessary for the associated control unit, by using as an identifier the message number affixed to the message, and that if the received message is necessary, it sends a reception interrupt signal to the microcomputer side. Upon receiving the reception interrupt signal, each of the communication management sections  71 ,  81  on the microcomputer side activates the reception interrupt processing and performs the following processing. 
     Referring to FIG. 47, each communication management section stores, in the RAM, the message comprising a plurality of data grouped into a unit, which has been received by the communication device  133   a ,  137   a  via the common communication line  139  (STEP  450 ). Then, the communication management section writes “1” in a flag in the flag setting section  72  or  82  (STEP  451 ), thereby ending the processing. 
     Here, the flag in the flag setting section  72  or  82  is initialized immediately after power-on of the control unit  133 ,  137 , and an initial value of “0” is written in the flag. Accordingly, the fact that the reception interrupt processing is activated as shown in FIG.  38  and “1” is written in the flag in STEP  451  of FIG. 47 is equivalent to detection of such a condition that another control unit from which data is to be received (i.e., the fourth control unit  137  when the processing is performed in the third control unit  133 ) is connected to the common communication line  139 . Depending on a detection result, the processing selecting/executing section  73 ,  83  changes over the processing to be performed in the control unit. 
     The processing performed by the processing selecting/executing section  73  of the third control unit  133  will be described with reference to a flowchart shown in FIG.  48 . 
     Referring to FIG. 48, the processing selecting/executing section  73  checks the status of the flag in the flag setting section  72  (STEP  460 ), and if the flag is “0”, it executes the processing A, B, C, D (STEP  461 ), thereby ending the processing. If the flag is “1”, it executes the processing A, D (STEP  462 ), thereby ending the processing. 
     The processing performed by the processing selecting/executing section  83  of the fourth control unit  137  will be described with reference to a flowchart shown in FIG.  49 . 
     Referring to FIG. 49, the processing selecting/executing section  83  checks the status of the flag in the flag setting section  82  (STEP  470 ), and if the flag is “1”, it executes the processing E-G (STEP  471 ), thereby ending the processing. If the flag is “0”, it executes nothing and ends the processing (STEP  472 ). 
     With this embodiment thus constructed, when the fourth control unit  137  is not connected to the common communication line  139  as shown in FIG. 50, the third control unit  133  operates, as described above, such that the reception interrupt processing in the communication management section  71  is not activated, the flag remains at the initial value “0”, and the processing selecting/executing section  73  selects and activates the processing A-D. In FIG. 50, processing blocks under activation are indicated by hatching. Therefore, the processing shown in FIG. 38 is executed and ordinary work using the operation signals from the control levers can be performed. 
     On the other hand, when the fourth control unit  137  is additionally connected to the common communication line  139  as shown in FIGS. 37 and 41, the third control unit  133  operates, as described above, such that the communication management section  71  writes “1” in the flag and the processing selecting/executing section  73  selects and activates the processing A, D. Also, the fourth control unit  137  operates, as described above, such that the communication management section  81  writes “1” in the flag and the processing selecting/executing section  83  selects and activates the processing E-G. In FIG. 41, processing blocks under activation are indicated by hatching. Therefore, the processing shown in FIG. 38 is executed and the area limiting control using the fourth control unit  137  can be performed. 
     Accordingly, even in the case of making system change from the electronic control system not including the fourth control unit  137 , shown in FIG. 50, to the electronic control system including the fourth control unit  137 , shown in FIG. 37, for adding an function of the hydraulic excavator, i.e., for the purpose of upgrading the machine, the system change can be realized just by additionally connecting the fourth control unit  137  to the common communication line  139 . There is no need of changing the program in the third control unit  133  or replacing the third control unit  133  itself. 
     Also, in the case of making system change from the electronic control system shown in FIG. 37 to the electronic control system shown in FIG. 50 for simplifying the functions of the hydraulic excavator, the system change can be realized just by disconnecting the fourth control unit  137  from the common communication line  139 . There is no need of changing the program in the third control unit  133  or replacing the third control unit  133  itself. 
     Thus, the control unit  137  can be additionally connected to the common communication line  139  or can be disconnected from the common communication line  139  without changing the program in the existing third control unit  133  or replacing the third control unit  133  itself. It is hence possible to easily realize system change including addition, exclusion and replacement of the control unit, and to hold down an increase of the development cost. 
     Another advantage is that the functions of a hydraulic excavator can be upgraded just by adding the control unit  137  having a new function incorporated therein to the existing control unit  133 , and the number of steps to be carried out by a worker for maintenance can be reduced. 
     A seventh embodiment of the present invention will be described with reference to FIGS. 51-63. In FIGS. 51-63, equivalent components to those in FIGS. 37-50 are denoted by the same numerals. This embodiment is based on the concept of the sixth embodiment and is adapted for a system including three or more control units connected to a common communication line. 
     FIG. 51 shows the overall configuration of an electronic control system of this embodiment. In FIG. 51, numerals  123 A,  137 A,  138  denote respectively a second control unit, a fourth control unit, and a display/setting unit, which constitutes a fifth control unit, according to this embodiment. The second control unit  123 A, the third control unit  133 , the fourth control unit  137 A, and the display/setting unit  138 A are connected to the common communication line  139  to transmit and receive data among them via the communication line  139 . 
     The hardware configurations of the second and fourth control units  123 A,  137 A and the display/setting unit  138 A are substantially the same as those of the second and fourth control units  23 ,  37  and the display unit  38  in the first embodiment shown in FIGS. 3,  5  and  6 . Communication devices of the second and fourth control units  123 A,  137 A and the display/setting unit  138 A are denoted by  123   a,    137   b,    138   a  in FIG.  51 . The configurations and functions of the communication devices  123   a,    137   b,    138   a  are also substantially the same as those in the first embodiment shown in FIGS. 8,  21  and  22  except that transmission and reception are managed using message numbers. 
     FIG. 52 shows, in the form of a functional block diagram, overall processing functions of software installed in microcomputers of the second control unit  123 A, the third control unit  133 , the fourth control unit  137 A, and the display/setting unit  138 A 
     In FIG. 52, the processing function of the third control unit  133  is substantially the same as that in the sixth embodiment shown in FIG.  41 . 
     The processing function of the fourth control unit  137 A is substantially the same as that in the sixth embodiment shown in FIG. 41 except for the following point. 
     In the fourth control unit  137 A, when performing the function of transmitting data at intervals of a certain time in STEP  397  of the timer interrupt processing shown in FIG. 38, a communication management section  81 A instructs the communication device  137   b  to perform processing to transmit, as data, not only the output command values issued to the shifting sectors (solenoid valves)  24 L,  24 R,  25 L,  25 R,  26 L,  26 R of the control valves  24 ,  25 ,  26 , but also the angle signal α from the boom rotational angle sensor  34  and the angle signal β from the arm rotational angle sensors  35 . 
     FIG. 53 shows one example of a message definition table serving as a transmission/reception management table which is created and used in the communication management section  81 A of the fourth control unit  137 A. FIG. 54 shows one example of a transmission time management table similarly created and used in the fourth control unit. In the communication management section  81 A, as shown in FIG. 53, a message  4  given as a data group of the boom angle signal α and the arm angle signal β also has a transmission attribute in addition to the above-mentioned message  3  given as a data group of the output command values supplied to the solenoid valves. Further, as shown in FIG. 54, the message numbers  3  and  4  each have a transmission cycle of 5 ms, and a count value counted by a timer incorporated in the fourth control unit  137 A is entered in the item “counter”. 
     In FIG. 52, the second control unit  123 A comprises a communication management section  91 , a flag setting section  92 , a processing selecting/executing section  93 , a processing section  94  for executing processing L, a processing section  95  for executing processing M, and a processing section  96  for executing processing N. Here, the processing L is processing to perform ordinary pump tilting control, the processing M is processing to perform pump tilting control in accordance with the operation signals from the control levers, and the processing N is processing to receive the pressure signal Pd and the swash-plate position signal θ. 
     The display/setting unit  138 A comprises a communication management section  101 , a flag setting section  102 , a processing selecting/executing section  103 , a processing section  104  for executing processing P, and a processing section  105  for executing processing Q. Here, the processing P is processing to display the pump delivery pressure, and the processing Q is processing to display a setting value entered by the operator. 
     The communication management section  91  of the second control unit  123 A and the communication management section  101  of the display/setting unit  138 A has, as with those of the third and fourth control units  133 ,  134 , the function of transmitting data at intervals of a certain time in the timer interrupt processing and the function of storing received data in the reception interrupt processing. Also, the communication management sections  91 ,  101  each have, as a part of the reception interrupt processing, the function of detecting based on reception of data whether another control unit is connected. 
     The processing selecting/executing section  93  of the second control unit  123 A and the processing selecting/executing section  103  of the display/setting unit  138 A are each constituted as a program for selecting and executing the sequences of processing A-D or P, Q at intervals of a required time by using the timer function (not shown) incorporated in the microcomputer. 
     The function of transmitting data at intervals of a certain time in each of the communication management sections  91 ,  101  will be described more concretely with reference to FIGS. 55-58. FIGS. 55-58 show various transmission/reception management tables created and used in the communication management sections  91 ,  101 . 
     FIG. 55 shows one example of a message definition table stored in the ROM of the second control unit  123 A, and FIG. 56 shows one example of a transmission time management table created in the RAM of the second control unit  123 A. Referring to FIG. 55, in the communication management section  91 , a message  5  given as data of the pump delivery pressure used in the display/setting unit  138 A has a transmission attribute, while messages  1  and  2  given as data groups of the operation signals received from the third control unit  133 , and a message  3  given as a data group of the output command values and a message  4  given as a data group of the angle signals, which are received from the fourth control unit  137 A, have a reception attribute. 
     FIG. 57 shows one example of a message definition table stored in the ROM of the display/setting unit  138 A, and FIG. 58 shows one example of a transmission time management table created in the RAM of the display/setting unit  138 A. Referring to FIG. 57, in the communication management section  101 , a message  6  given as data of the setting value used in the fourth control unit  137 A has a transmission attribute, while the messages  1  and  2  given as data groups of the operation signals received from the third control unit  133 , the message  3  given as a data group of the output command values and the message  4  given as a data group of the angle signals, which are received from the fourth control unit  137 A, and the message  5  given as data of the pump delivery pressure received from the second control unit  123 A have a reception attribute. In FIG. 58, for example, the message number  6  has a transmission cycle of 5 ms, and a count value counted by a timer incorporated in the display/setting unit  138 A is entered in the item “counter”. 
     For data transmission, the communication management section  91  of the second control unit  123 A and the communication management section  101  of the display/setting unit  138 A each perform the processing to transmit data similarly to STEP  430 ,  431  (FIG. 39) and STEP  403  or  407  (FIG. 38) in the timer interrupt processing of FIGS. 38 and 39, which have been described above in the first embodiment, by using the transmission time management tables shown in FIGS. 56 and 58. 
     Next, a description is made of the reception interrupt functions of the communication management section  91  of the second control unit  123 A and the communication management section  101  of the display/setting unit  138 A, as well as of the function of detecting whether another control unit is connected, this detecting function being performed as a part of the reception interrupt function based on reception of data. 
     Prior to describing those functions, FIGS. 59A and 59B show respectively the flag configuration in the flag setting section  92  of the second control unit  123 A and the flag setting section  102  of the display/setting unit  138 A. The flag configuration in each flag setting section consists of four bits as shown. A bit  1  is assigned to reception of a message from the second control unit  123 A. A bit  2  is assigned to reception of a message from the third control unit  133 . A bit  3  is assigned to reception of a message from the fourth control unit  137 A. A bit  4  is assigned to reception of a message from the display/setting unit  138 A. Each bit is turned on (written to “1”) by a corresponding reception interrupt signal. 
     FIG. 60 is a flowchart showing the reception interrupt processing performed by the communication management section  91  of the second control unit  123 A. 
     First, the communication device  123   a  of the second control unit  123 A has such a function that when the communication device receives a message comprising a plurality of data grouped into a unit, as described above, via the common communication line  139 , it determines whether the received message is necessary for the second control unit  123 A, by using as an identifier the message number affixed to the message, and that if the received message is necessary, it sends a reception interrupt signal to the microcomputer side. Upon receiving the reception interrupt signal, the communication management section  91  on the microcomputer side activates the reception interrupt processing and performs the following processing. 
     Referring to FIG. 60, the communication management section  91  stores, in the RAM, the message comprising a plurality of data grouped into a unit, which has been received by the communication device  123   a  via the common communication line  139  (STEP  455 ). Then, the communication management section  91  determines from which control unit the message has been transmitted, by using as an identifier the message number affixed to the message. It turns on (writes “1” in) the bit  2  of the flag if the message is from the third control unit  133 , turns on (writes “1” in) the bit  3  of the flag if the message is from the fourth control unit  137 A, and turns on (writes “1” in) the bit  4  of the flag if the message is from the display/setting unit  138 A (STEP  451 ), thereby ending the processing. 
     Here, each bit of the flag in the flag setting section  92  is initialized immediately after power-on of the second control unit  123 A, and an initial value of “0” is written in the bit. Accordingly, the fact that the reception interrupt processing shown in FIG. 60 is activated and “1” is written in each bit of the flag in STEP  456  is equivalent to detection of such a condition that another control unit from which data is to be received (i.e., the third control unit  133 , the fourth control unit  137 A or the display/setting unit  138 A when the processing is performed in the second control unit  123 A) is connected to the common communication line  139 . Depending on a detection result, the processing selecting/executing section  93  changes over the processing to be performed in the second control unit  123 A. 
     The processing performed by the processing selecting/executing section  93  will be described with reference to a flowchart shown in FIG.  61 . 
     Referring to FIG. 61, the processing selecting/executing section  93  checks the status of bit  2  of the flag in the flag setting section  92  (STEP  480 ), and if the bit  2  is turned off (“0”), it executes the processing L, N (STEP  481 ), thereby ending the processing. If the bit  2  is “1”, it executes the processing M, N (STEP  482 ), thereby ending the processing. 
     Consequently, in the second control unit  123 A, when connection of the third control unit  133  is detected upon receiving the message having the message number  1  or  2 , the processing M, N are selected to perform computation of an output value to the swash-plate position regulator  20  of the hydraulic pump  18  by employing the operation signals from the control levers. When no data is received from the third control unit  133  and connection of the third control unit  133  is not detected, the processing L, N are selected to perform computation of an output value to the swash-plate position regulator  20  without employing data of the operation signals. Though not shown, the hydraulic pump  18  may be controlled by checking the on/off status of the bit  3  or  4  and selecting the processing to perform the pump tilting control using the output command values for control of the solenoid valves which are given as a message received from the fourth control unit  137 A, or using the setting value which is given as a message received from the display/setting unit  138 A. 
     FIG. 62 is a flowchart showing the reception interrupt processing performed by the communication management section  101  of the display/setting unit  138 A. 
     First, the communication device  138   a  of the display/setting unit  138 A has such a function that when the communication device receives a message comprising a plurality of data grouped into a unit, as described above, via the common communication line  139 , it determines whether the received message is necessary for the display/setting unit  138 A, by using as an identifier the message number affixed to the message, and that if the received message is necessary, it sends a reception interrupt signal to the microcomputer side. Upon receiving the reception interrupt signal, the communication management section  101  on the microcomputer side activates the reception interrupt processing and performs the following processing. 
     Referring to FIG. 62, the communication management section  101  stores, in the RAM, the message comprising a plurality of data grouped into a unit, which has been received by the communication device  138   a  via the common communication line  139  (STEP  457 ). Then, the communication management section  101  determines from which control unit the message has been transmitted, by using as an identifier the message number affixed to the message. It turns on (writes “1” in) the bit  1  of the flag if the message is from the second control unit  123 A, turns on (writes “1” in) the bit  2  of the flag if the message is from the third control unit  133 , and turns on (writes “1” in) the bit  3  of the flag if the message is from the fourth control unit  137 A (STEP  458 ), thereby ending the processing. 
     Here, each bit of the flag in the flag setting section  102  is initialized immediately after power-on of the display/setting unit  138 A, and an initial value of “0” is written in the bit. Accordingly, the fact that the reception interrupt processing shown in FIG. 62 is activated and “1” is written in each bit of the flag in STEP  458  is equivalent to detection of such a condition that another control unit from which data is to be received (i.e., the second control unit  123 A, the third control unit  133  or the fourth control unit  137 A when the processing is performed in the display/setting unit  138 A) is connected to the common communication line  139 . Depending on a detection result, the processing selecting/executing section  103  changes over the processing to be performed in the display/setting unit  138 A. 
     The processing performed by the processing selecting/executing section  103  will be described with reference to a flowchart shown in FIG.  63 . 
     Referring to FIG. 63, the processing selecting/executing section  103  checks the status of bit  1  of the flag in the flag setting section  102  (STEP  490 ), and if the bit  1  is turned off (“0”), it executes the processing Q (STEP  491 ), thereby ending the processing. If the bit  1  is “1”, it executes the processing P (STEP  492 ), the reby ending the processing. 
     Consequently, the display/setting unit  138 A displays the pump delivery pressure Pd based on the message  5  transmitted from the second control unit  123 A in accordance with an operator&#39;s demand. Though not shown, necessary data may be displayed by checking the on/off status of the bit  2  or  3  and selecting the processing to display the operation signals of the control levers based on the messages having the message numbers  1 ,  2  transmitted from the third control unit  133 , or selecting the processing to display the angles α, β of the excavating device based on the message having the message number  3  transmitted from the fourth control unit  137 A. 
     In this embodiment thus constructed, the third control unit  133  and the fourth control unit  137 A function in a like manner to those in the first embodiment. 
     When the third control unit  133  is not connected to the common communication line  139  on the contrary to the electronic control system shown in FIG. 51, the second control unit  123 A operates, as described above, such that the bit  2  of the flag remains turned off (at the initial value “0”) during the reception interrupt processing in the communication management section  91 , and the processing selecting/executing section  93  selects and activates the processing L, N. As a result, the second control unit  123 A performs computation of an output value to the swash-plate position regulator  20  without employing data of the operation signals, thereby controlling the delivery rate of the hydraulic pump. 
     On the other hand, when the third control unit  133  is additionally connected to the common communication. line  139  as shown in FIG. 51, the second control unit  123 A operates, as described above, such that the communication management section  91  turns on (writes “1” in) the bit  2  of the flag in the reception interrupt processing and the processing selecting/executing section  93  selects and activates the processing M, N. As a result, the second control unit  123 A performs computation of an output value to the swash-plate position regulator  20  for the hydraulic pump  18  by employing the operation signals from the control levers, thereby controlling the delivery rate of the hydraulic pump  18  in accordance with the input amounts by which the control levers are operated. 
     Also, when the second control unit  123 A is not connected to the common communication line  139  on the contrary to the electronic control system shown in FIG. 51, the display/setting unit  138 A operates, as described above, such that the bit  1  of the flag remains turned off (at the initial value “0”) during the reception interrupt processing in the communication management section  101 , and the processing selecting/executing section  103  selects and activates the processing Q. As a result, the display/setting unit  138 A displays the setting value entered by the operator. 
     On the other hand, when the second control unit  123 A is additionally connected to the common communication line  139  as shown in FIG. 51, the display/setting unit  138 A operates, as described above, such that the communication management section  101  turns on (writes “1”) in the bit  1  of the flag in the reception interrupt processing and the processing selecting/executing section  103  selects and activates the processing P. As a result, the display/setting unit  138 A displays the pump delivery pressure Pd based on the message  5  transmitted from the second control unit  123 A in accordance with an operator&#39;s demand. 
     Accordingly, with this embodiment, even in the case where a system includes a plurality of control units receiving data, such as the second control unit  123 A and the display/setting unit  138 A, and the processing sections  94 - 96  or the processing sections  104 ,  105  perform computational processing using data transmitted from the plurality of control units, the communication management sections  91 ,  101  detect whether the plurality of control units are connected, by checking the on/off status of the flag bit for each control unit, and renders the processing selecting/executing sections  93 ,  103  to appropriately change over the computational processing so as to execute the selected computational processing. Therefore, the control unit  133  or  123 A can be additionally connected to the common communication line  139  or can be disconnected from the common communication line  139  without changing the software in the existing control unit  123 A or  138 A or replacing the control unit  123 A or  138 A itself. It Is hence possible to easily realize system change Including addition, exclusion and replacement of the control unit, and to hold down an increase of the development cost. 
     Another advantage is that the functions of a hydraulic excavator can be upgraded just by adding the control unit  133  or  123 A having a new function incorporated therein to the existing control unit  123 A or  128 A, and the number of steps to be carried out by a worker for maintenance can be reduced. 
     It is to be noted that the above embodiments have been described in connection with a hydraulic excavator as a typical example of construction machines, the present invention is also similarly applicable to any type of construction machines so long as the machine includes a plurality of control systems or operating systems provided with respective control units, such as a working device and a control system thereof, hydraulic equipment and a control system or an operating system thereof, etc. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, in an electronic control system for a construction machine which includes a plurality of control units interconnected via a common communication line, one or more control units can be additionally connected to the common communication line or can be disconnected from the common communication line without changing software in the existing control units or replacing the control units themselves. Also, system change including addition, exclusion and replacement of the control unit can be easily realized. As a result, a variety of electronic control systems adapted for different customer-demanded functions can be provided inexpensively. 
     Also, according to the present invention, since the control unit on the transmitting side sets a transmission time interval required for each transmitted data, the common communication line can be utilized efficiently and the control performance is avoided from being affected by a lowering of the communication efficiency. In addition, even with an increase in the number of control units, the system is less susceptible to such a trouble as disabling communication due to excessive traffic on the common communication line. 
     Further, according to the present invention, since each control unit identifies and receives only necessary one of data flowing over the communication line based on a specific ID assigned to each data, the control unit can receiver only the data necessary for itself in spite of various data flowing over the communication line.