Patent Publication Number: US-6661528-B2

Title: Image forming system including a first printer connected to a host computer and a second printer connected to the host computer via a switching device provided in the first printer

Description:
This is a divisional of application Ser. No. 09/907,274 filed Jul. 17, 2001, now U.S. Pat. No. 6,559,959 which is a divisional of application Ser. No. 09/479,777 filed Jan. 7, 2000, now U.S. Pat. No. 6,301,014 which is a divisional of application Ser. No. 08/922,579 filed Sep. 3, 1997, now U.S. Pat. No. 6,025,924. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an image forming system and, more particularly, to an image forming system including a plurality of image forming apparatuses selectively connected to a host computer. 
     2. Description of the Related Art 
     Generally, an image forming apparatus such as a printer is connected to a personal computer to output image data or information on a recording paper. Typically, the personal computers are connected to a single printer so that the personal computer is connected to the printer on a one to one basis. A centronics interface is the most popular way to connect a printer to a personal computer. Normally, the printer is connected to the personal computer via a connector conforming to the centronics standards. 
     There are many types of printers such as laser printers, inkjet printers, etc. Additionally, there are also monochrome and color types of printers. Demand for use of different types of printers with a single personal computer has been increasing. A description will now be given of a typical case in which a monochrome laser printer and a color inkjet printer are available. If only the monochrome laser printer is connected to a personal computer, the need for color printing cannot be satisfied. Thus, it is also desirable to have a color inkjet printer connected to the personal computer. On the other hand, if only the color inkjet printer is connected to the personal computer, although monochrome printing can be performed as well as color printing by the color inkjet printer, this does have its disadvantages. For example, monochrome printing by a color inkjet printer has a disadvantage in that the printing speed is much slower than that of the monochrome laser printer. Additionally, the image quality of monochrome printing performed by the color inkjet printer is much lower than that of the monochrome laser printer. Accordingly, considering the fact that most printing operations are require monochrome printing, it is preferable that the monochrome printer also be connected to the personal computer. 
     In most cases, when two printers are commonly used with one personal computer, the user must change from one printer to the other printer by disconnecting the centronics connector of one printer and connecting the centronics connector of the other printer. This changing operation is very inconvenient for the user. 
     In order to eliminate such inconvenience, a switching device  101  such as shown in FIG. 1 is commercially available. The switching device  101  is provided with three connectors CN 1 , CN 2  and CN 3  so that a personal computer  102  and two printers  103  and  104  are interconnected via the switching device  101 . The switching device  101  is provided to selectively connect one of the printers  103  and  104  to the personal computer  102  by control of the personal computer  102 . Accordingly, the switching device  101  is intended to eliminate the disconnecting and connecting operations of the connectors of the printers  103  and  104 . However, the switching operation performed by the switching device  101  lacks reliability at the present time. This may be caused by poor compatibility between the personal computer  102  and printers  103  and  104 . More specifically, although the personal computer  102  and the printers  103  and  104  have an interface conforming to the centronics standard, each of the interfaces may actually conform to various local standards which are based on the centronics standard. This situation creates a problem that a personal computer may not have compatibility with a particular printer. When the above-mentioned switching device  101  is used, as viewed from the personal computer  102 , it is not recognizable as to what types of printers are connected to the switching device  101 . 
     In order to eliminate the above-mentioned problem with respect to compatibility, the printers to be connected to the switching device  101  must be limited to ones which are compatible with the personal computer  102 . Accordingly, the types of printers which can be connected to the switching device  101  is limited. This results in poor reliability of the switching function of the switching device  101 . That is, since generally the personal computer is not intended to be used with two printer drivers, there may be a case in which the printer is not operatively connected to the personal computer. Additionally, even if the printers  102  and  104  can be operatively connected to the personal computer  102  via the switching device  101 , the total length of the cables between the personal computer  102  and each of the printers  103  and  104  becomes excessively long, resulting in an increased possibility of noise intrusion. Further, the commercially available switching device  101  may perform a switching operation which requires electrical power which cannot be provided from the personal computer  102  via the connecter C 1 . That is, there may be a case in which the switching device  101  must be directly connected to an external power source. 
     In the above-mentioned case, the personal computer  102  described as being connected to the printers  103  and  104 . However, one or both of the printers  103  and  104  may be replaced by a multi-function image processing apparatus such as a multi-function peripheral (MFP) device which may include a facsimile function, a scanning function and a printing function. This results in similar problems to those discussed above. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide an improved and useful image forming system in which the above-mentioned problems are eliminated. 
     A more specific object of the present invention is to provide an image forming system including a first image forming apparatus which can be connected to a host computer as an image data supplying source and also to a second image forming apparatus so that the host computer can be selectively and reliably connected to one of the first and second image forming apparatuses. 
     According to an aspect of the present invention, an image forming system includes as least one user device for providing image data to be printed, the at least one user device selected from one of a group of a host computer and a network. A first peripheral device is connected to the at least one user device for receiving the image data from the at least one user device and for processing the received image data. A second peripheral device is operatively connected to the at least one user device via the first peripheral device for processing image data sent from the at least one user device via the first peripheral device. A switching device is provided in and controlled by the first peripheral device. The switching device automatically switches the connection of the at least one user device so that one of the first peripheral device and the second peripheral device is operatively connected to the at least one user device. 
     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention are described herein below with reference to the accompanying drawings, wherein; 
     FIG. 1 is an illustration of a conventional image forming system using a switching device; 
     FIG. 2 is an illustration of an image forming system according to a first embodiment of the present invention; 
     FIG. 3 is an illustration of an entire system according to the first embodiment of the present invention; 
     FIG. 4 is a flowchart of an operation performed by a personal computer shown in FIG. 3; 
     FIG. 5 is a flowchart of an operation performed in a first printer shown in FIG. 3; 
     FIG. 6 is an illustration of a dialog window displayed on a personal computer provided in an image forming system according to a second embodiment of the present invention; 
     FIG. 7 is an illustration of an operational panel of a printer provided in an image forming system according to a third embodiment of the present invention; 
     FIGS. 8A-8C are flowcharts of operations that can be performed by a printer provided in the image forming system according to embodiments of the present invention; 
     FIG. 9 is a flowchart of an operation performed by a printer provided in an image forming apparatus according to a fourth embodiment of the present invention; 
     FIG. 10 is a flowchart of an operation performed by a printer provided in an image forming apparatus according to a fifth embodiment of the present invention; 
     FIG. 11 is an illustration of an image forming system according to a sixth embodiment of the present invention; 
     FIG. 12 is a block diagram of a multi-function peripheral device shown in FIG. 11; 
     FIG. 13 is a circuit diagram of a switching device shown in FIG. 11; 
     FIG. 14 is a flowchart of an operation of a personal computer shown in FIG. 11; 
     FIG. 15 is a flowchart of an operation of an MFP device shown in FIG. 11; and 
     FIG. 16 is a circuit diagram of a switch shown in FIG.  13 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description will now be given, with reference to FIGS. 2 to  5 , of a first embodiment of the present invention. FIG. 2 is a circuit diagram of a printer interface switching device of a monochrome laser printer included in an image forming system according to the first embodiment of the present invention. 
     The monochrome laser printer  1  shown in FIG. 2 is connected to a color inkjet printer  2  and also connected to a personal computer  3 . That is, the personal computer  3  and two printers  1  and  2  together constitute the image forming system according to the present invention. Hereinafter, the monochrome laser printer  1  is referred to as a first printer, and the color inkjet printer  2  is referred to as a second printer. 
     In the image forming system according to the present invention, the personal computer  3  is connected to the second printer  2  via the first printer  1  as shown in FIG.  3 . The first printer  1  is provided with the printer interface switching device  4  having a switching function. The printer interface switching device  4  is basically an interface conforming to the centronics standard. 
     The printer interface switching device  4  is provided with a first connector  5  and a second connector  6 . The first connector  5  is connected to a centronics standard connector of the personal computer  3 . The second connector  6  is connected to a centronics standard connector of the second printer  2 . Since the printer interface switching device  4  is incorporated into the first printer  1 , the printer interface switching device  4  is connected to an electrical circuit of the first printer  1  by an internal wiring system in the first printer  1 . Accordingly, there is no printer connector needed for the connection of the first printer  1 . 
     The first printer  1  comprises therein a controller  8  including a central processing unit (CPU)  7 . The connector  5  is connected to the controller  8  by a first printer signal transmission path  9 . The first printer signal transmission path  9  comprises an eight-wire data bus  9   a , a four-wire reception control line  9   b  and a five-wire transmission control line  9   c . Additionally, the connector  6  is connected to the controller  8  by a second printer signal transmission path  10 . The second printer signal transmission path  10  comprises a data bus  10   a , a reception control line  10   b  and a transmission control line  10   c  which are branched from the data bus  9   a , the reception control line  9   b  and the transmission control line  9   c , respectively. 
     A switching unit  11  is provided to select one of the printer signal transmission paths  9  and  10  in response to a switching signal provided by the controller  8 . The switching unit  11  comprises buffers  12   a ,  12   b ,  12   c ,  13   b  and  13   c  and a switch  14 . The buffers  12   a ,  12   b  and  12   c  can be set to either an active state or an inactive state, and are provided on the data bus  9   a , the reception control line  9   b  and the transmission control line  9   c , respectively. The buffers  13   b  and  13   c  can be set to either an active state or an inactive state, and are provided on the reception control line  10   b  and the transmission control line  10   c , respectively. The switch  14  is controlled to be turned on or off, and is provided on the data bus  10   a . The buffer  12   a  is a bidirectional buffer. 
     The buffer  12   a  is connected to the controller  8  via a control line  15   a  so as to provide a switching signal which controls the operational state of the buffer  12   a . The buffers  12   b  and  13   b  are connected to the controller  8  via the control line  15   b  so as to provide a switching signal which selectively sets one of the buffers  12   b  and  13   b  to the activated state. The buffers  12   c  and  13   c  are connected to the controller  8  via the control line  15   c  so as to provide a switching signal which selectively sets one of the buffers  12   c  and  13   c  to the activated state. The switch  14  is connected to the controller  8  via a control line  15   d  so as to provide a switching signal which controls the operational state of switch  14 . Additionally, inverters  16   b  and  16   c  are provided to the control lines  15   b  and  15   c  connected to the buffers  13   b  and  13   c , respectively, so as to invert the switching signals on the control lines  15   b  and  15   c.    
     A power source  17  is provided to operate the switching unit  11 . Power is provided to the power source  17  from the controller  8  via a power source line  18 , and also provided from the second printer  2  via a peripheral logic high (P.L.H.) line  19  of the second connector  6 . The P.L.H. line  19  is provided for checking whether power of the second printer  2  is turned on. Specifically, the centronics interface is provided with a small current from the second printer  2 . Since diodes  20  and  21  are provided on the power source line  18  and the P.L.H. line  19 , respectively, the two lines  18  and  19  are connected in a logic summing manner. 
     When the first printer  1  is turned on, the switching signals provided to the control lines  15   a ,  15   b  and  15   c  are initially set to a high level, and the switching signal provided to the control line  15   d  is initially set to a low level. In this state, since the buffers  12   a  to  12   c  are in the active state, the switch  14  is turned off and the buffers  13   b  and  13   c  are in the inactive state, and the first printer signal transmission path  9  is set to be effective. 
     Additionally, the personal computer  3  has a function for selectively designating one of the first printer  1  and the second printer  2 . The personal computer  3  also has a switching command sending function for selecting a printer driver based on the designated selection. Specifically, for example, when a printing operation for an application of a word processor is performed, the first printer  1  can be automatically designated by selecting the printer driver for the first printer  1 . Alternatively, when a printing operation for an application of a word processor is performed, the second printer  2  can be automatically designated by selecting the printer driver for the second printer  2 . This function is performed by a CPU (not shown in the figure) of the personal computer  3 . 
     The first printer  1  includes, in addition to the CPU  7 , a ROM fixedly storing data such as programs and a RAM rewritably storing various information. The CPU  7  performs various processes and controls in accordance with the program data stored in the ROM  7 . For example, in the present embodiment, a determining function and a path state switching function are provided. The determining function determines, when a switching command is sent by the switching command sending function performs by personal computer  3  which is used for selecting a printer driver in accordance with the operation for designating the printer performed on the personal computer  3  side, whether the switching command designates the first printer  1  or the second printer  2 . Additionally, the path state switching function switches the state of paths by operating the switching unit  11  so as to effect one of the first printer signal reception path  9  and the second printer signal reception path  10  in accordance with the result of the determination made by the determining function. 
     In the above-mentioned structure in which two printers  1  and  2  are connected to the personal computer  3 , when all power is turned on, the controller  8  changes the switching signals on the control lines  15   a ,  15   b  and  15   c  to the high level. The switching signal on the control line  15   d  is at the low level. Accordingly, the first printer signal transmission path  9  is selected, resulting in an equivalent state where the first printer  1  is connected to the personal computer  3 . Thus, a printing operation can be performed by the first printer  1  by sending character print data, other data or signals. In this case, monochrome printing can be provided with a high quality image and at a high-speed. 
     A description will now be given, with reference to FIGS. 3 and 4, of switching to the second printer  2  and switching back to the first printer  1 . The flowchart shown in FIG. 4 is a procedure performed by the personal computer  3 . In order to switch to a printer to be used, the printer driver corresponding to the printer to be used is selected on the personal computer  3  when a printing operation is performed in the word processor application. Accordingly, it is determined first, in step S 1 , whether or not an operation for selecting the printer is performed. If it is determined that the operation for selecting is performed, it is then determined, in step S 2 , whether or not to switch from the first printer  1  to the second printer  2 . If the determination of step S 2  is negative, it is determined, in step S 4 , whether or not to switch from the second printer  2  to the first printer  1 . On the other hand, if the determination of step S 2  is affirmative, the routine proceeds to step S 3  where the personal computer  3  sends to the first printer  1  a switching command which indicates to switch from the first printer  1  to the second printer  2 . 
     If it is determined, in step S 4 , that the switching is to be from the second printer  2  to the first printer  1 , the routine proceeds to step S 5 . In step S 5 , the personal computer  3  sends to the first printer  1  a switching command which indicates to switch from the second printer  2  to the first printer  1 . The process of steps S 3  to S 5  is performed as a function of the switching command sending function performed in the personal computer  3 . 
     According to the present embodiment, personal computer  3  executes a program which monitors the currently selected printer driver. If a printer driver for driving printer  1  is selected, a command is sent to switch to printer  1 . On the other hand, if a printer driver for driving printer  2  is selected, a command is sent to switch to printer  2 . 
     When a command is sent from the personal computer  3  to the first printer  1  via the connector  5 , the command is received in the CPU  7  of the controller  8 . It is then determined, in step S 6  of FIG. 5, whether or not the command is a switching command for the printer. If it is determined that the command is the switching command, the switching unit  11  is operated, in step S 7 , so as to switch from the present printer to the other printer. That is, when the switching command is input and if the command represents a command for switching from the first printer  1  to the second printer  2 , the switching signal provided to the switching unit  11  is changed from the high level to the low level. Thereby, the second printer signal transmission path  10  is effected. This state is equivalent to the state where only the second printer  2  is connected to the personal computer  3 . Thus, a printing operation is performed by the second printer by sending character image data or other necessary data to the second printer  2  via the second printer signal transmission path  10 . In this case, a multi-color printing can be provided. When viewing from the first printer  1 , the character image data passes through the first printer to the second printer  2 . 
     On the other hand, when the switching command is input and if the command represents a command for switching from the second printer  2  to the first printer  1 , the switching signal provided to the switching unit  11  is changed from the low level to the high level. Thereby, the first printer signal transmission path  9  is effected. This state is equivalent to the state where only the first printer  1  is connected to the personal computer  3 . Thus, a printing operation is performed by the first printer by sending character image data or other necessary data to the first printer  1  via the first printer signal transmission path  9 . Accordingly, the process of step S 6  is performed as the function of the determining function, and the process of step S 7  is performed as the function of the path state switching function. 
     As mentioned above, according to the present embodiment, the first printer  1  itself has the connector function and the switching function so that the printers  1  and  2  are connected to a single centronics interface line extending from the personal computer  3 . That is, a system can be provided in which a remote switching control is performed by the personal computer  3 . Especially, according to the present embodiment, since the system is constructed so that the first printer  1  is the center of the system by providing the switching function to the interface unit of the first printer  2  which is one of the objects to switch, at least all of the command system of the first printer  1  is known. Thus, even if the command system of the second printer  2  is unknown, switching between the first printer  1  and the second printer  2  can be reliably performed. Additionally, since the first printer itself has the switching function in the interface unit, the connection of the first printer  1  can be achieved by internal wiring without a connector. Thus, a length of wire or cable can be shorter than that of a case in which a separate switching device  101  is used. This improves antinoise characteristics. Further, since the first printer  1  has the switching device therein, a power source for the switching device can be the power source for the first printer  1 . That is, a separate power source is not needed as in the case of the conventional switching device  101 . 
     A description will now be given, with reference to FIG. 6, of a second embodiment of the present invention. An image forming system according to the second embodiment is similar to that of the above-mentioned first embodiment except for a dialog program being provided in the personal computer  3  as a means for designating one of the first printer  1  (internal printer) and the second printer  2  (external printer). FIG. 6 shows a dialog window  32  for designating one of the internal printer and the external printer. Before starting a printing operation, the dialog window  32  is displayed, and a user can select one of the internal printer and the external printer by moving a pointer or cursor  33  by a mouse or similar cursor control means. When one of the internal printer and the external printer is selected in the dialog window  32 , the switching command is sent from the personal computer  3  to the first printer  1  similar to the process of steps S 3  and S 5  shown in FIG.  4 . Then, the process of steps S 6  and S 7  shown in FIG&gt; 5  is performed in the first printer  1 . 
     Thus, according to the present embodiment, the first printer itself has the connector function and the switching function so that the two printers  1  and  2  are connected to the personal computer  3  via a single centronics interface line extending from the personal computer  3 . That is, a system can be provided in which a remote switching control is performed by the personal computer  3 . 
     A description will now be given, with reference to FIGS.  7  and  8 A- 8 C, of other embodiments of the present invention. An image forming system according to these embodiments is similar to that of the above-mentioned first embodiment except for an operational panel  34  shown in FIG. 7 being provided on the first printer  1 . The operational panel  34  has a printer designating part  35  for designating the printer to be used. Since the operational panel  34  is provided on the first printer  1 , the printer designating part  35  is operated only when the second printer  2  is to be used. For example, the printer designating part  35  can be in the form of a key for inputting a command to switch to the second printer. Thus, the first printer  1  is always monitoring whether or not the printer designating part  35  is operated as indicated by step S 11  of the flowchart of FIG.  8 A. When it is determined that the printer designating part  35  is operated, the state of the switching signal is changed, in step S 12 , so that the second printer signal transmission path  10  is effected. The process of step S 12  is performed as the function of the path state switching function which switches the state of the path so as to effect one of the first printer signal transmission path  9  and the second printer signal transmission path  10  in response to the operation of the printer designating part  35 . 
     Of course, variations of the embodiment depicted in FIG. 8A are possible. As shown in FIG. 8B, when it is detected in step S 11  that the printer designating part  35  has been operated, a determination is made in step S 11 A whether the system is idle (e.g., whether the printer  1  is currently printing). The system waits until the system is idle (Yes in step S 11 A) and then switched to the printer  2  in step S 12 . In addition, as described with respect to FIG. 8A, if the printer  2  is currently selected, if it is determined in step S 11  that a printer designating operation has again been performed (e.g., a user desires to switch back to the printer  1 ), the switching operation can proceed immediately to step S 12  for switching back to the printer  1 . In the alternative, as shown in FIG. 8C, if it is determined in step S 11  that the printer designating operation has again been performed (e.g., the user desires to switch back to the printer  1 ), a determination is made in step S 11 A whether the system is idle. If the system is not idle, (e.g., printer  215  is currently printing), a warning can be issued in step S 11 B. The warning can be issued to the user by displaying a message indication that the printer  2  is currently printing and the printer  1  can not be selected. If the system is idle, the process proceeds to step S 12  and a switching command is issued. 
     The determination of whether the system is idle can be made by controller  8  by controlling bidirectional buffer  12   a  so as to monitor the data on the data bus  9   a . Thus, a determination can be mode by the printer  1  that the system is busy, even if the printer  2  is the currently selected printer. 
     Thus, according to the present embodiment, the first printer itself has the connector function and the switching function so that the two printers  1  and  2  are connected to the personal computer  3  via a single centronics interface line extending from the personal computer  3 . That is, a system can be provided in which a switching control can be performed by an operation of the printer designating part  35 . 
     A description will now be given, with reference to FIG. 9, of a fourth embodiment of the present invention. In the present embodiment, the CPU  7  performs the processes corresponding to the determining function and the path state switching functions as follows. The determining function determines a time for switching the printer by monitoring whether the state where no command data is provided continues for a predetermined time period while the second printer signal transmission path  10  is effective. Additionally, the path state switching function switches the state of the path so as to effect the first printer signal transmission path  9  by operating the switching unit  11  when it is determined by the determining function that it is time for switching the printer. That is, the present embodiment automatically returns the system to a state where the first printer  1  is usable when the system is set to a state where the second printer  2  is usable. It should be noted that a switching operation to the second printer  2  may be performed in an appropriate manner such as disclosed in one of the aforementioned embodiments. Additionally, the present embodiment is provided with a timer, which is started at a predetermined time and counts a clock signal, so as to specify the predetermined time period for use of the determining function. 
     Referring to FIG. 9, in the present embodiment, the CPU  7  always monitors the state of the printer interface switching device  4  so as to determine, in step S 21 , whether or not the second printer signal transmission path  10  is effective. This determination can be performed based on the present state of the switching signal. If the system is in the state where the first printer  1  is usable, there is no need to switch the printer. If the system is in the state where the second printer  2  is usable, it is determined, in step S 22 , whether or not the command data signal is sent from the personal computer  3  to the printer interface switching device  4 . This determination can be performed by monitoring data on the data bus  10   a , which data is sent from the personal computer  3  while the bidirectional buffer  12   a  on the data bus  9   a  is continuously controlled in an active state. If command data is sent while the system is in the state where the second printer is usable, a switching operation is not performed since the second printer  2  is being used. On the other hand, if the command data is not sent in the state where the second printer  2  is usable, the operation of a timer (not shown) is started in step S 23 . Then, it is determined, in step S 24 , whether or not the command data is sent. If it is determined that the command data is sent from the personal computer  3  to the printer interface switching circuit  4  during the counting operation of the timer, the routine is ended so that no switching operation is performed and the system is maintained in the state where the second printer is usable. On the other hand, in step S 25 , it is determined whether or not the counting operation of the timer has terminated. When no command data is sent from the personal computer  3  until the counting operation has terminated, the state of the switching signal provided to the switching unit  11  is changed, in step S 26 , from the low level to the high level. Thereby, the first printer signal transmission path  9  is effected, and the system is set to the state which is equivalent to a state where only the first printer  1  is connected to the personal printer  3 . Accordingly, the process of steps S 21  to S 25  is performed as a function of the determining function, and the process of step S 26  is performed as a function of the path state switching function. 
     According to the present embodiment, the system is automatically returned to the state where the first printer  1  is usable if the second printer  2  is not used for the predetermined continuous time period even if the system is set to the state where the second printer  2  is usable. That is, since the system according to the present embodiment is constructed as the first printer  1  is the center of the system, the state where the first printer  1  is usable is given priority. This condition facilitates the control of the system, over the state where the second printer  2  is usable by automatically setting the first printer signal transmission path  9  to be effective. 
     A description will now be given, with reference to FIG. 10, of a fifth embodiment of the present invention. In the present embodiment, the CPU  7  performs the processes corresponding to the determining function and the path state switching function as follows. The determining function determines, when a command is sent from the personal computer  3 , whether or not the command conform to the system command. The path state switching function changes the state of the path so as to effect the second printer signal transmission path  10  when it is determined that an unreadable command is sent from the personal computer  3  while the first printer signal transmission path  9  is effective. On the other hand, the path state switching function changes the state of the path so as to effect the first printer signal transmission path  9  when it is determined that a readable command is sent from the personal computer  3  while the second printer signal transmission path  10  is effective. It should be noted that a FIFO buffer is provided in the RAM of the controller  8  so as to temporarily store information provided by the personal computer  3 . 
     In the present embodiment, when the command is sent from the personal computer  3  to the printer interface switching device  4 , the command is taken by the CPU  7  of the controller  8  via the bidirectional buffer  12   a  on the data bus  9   a . It is then determined, in step S 31 , whether or not the command is readable by the CPU  7 . If the command is designated for the first printer  1 , the command must be readable by the CPU  7 . Thus, in this case, it is determined, in step S 32 , whether or not the first printer signal transmission path  9  is effective. If it is determined, in step S 31 , that the command is not readable by the CPU  70 , the routine proceeds to step S 34  where it is determined whether the second printer signal transmission path  10  is effective. If it is determined that the second printer signal transmission path  10  is effective, the routine proceeds to step S 35 . In step S 35 , the switching signal provided to the switching unit  11  is changed from the low level to the high level so that the first printer signal transmission path  9  becomes effective. On the other hand, if it is determined, in step S 31 , that the command from the personal computer  3  is not readable by the CPU  7 , this means that the command is not designated for the first printer  1 , and the routine proceeds to step S 32 . In step S 32 , it is determined whether or not the first printer signal transmission path  9  is effective. If it is determined that the first printer signal transmission path  9  is not effective, that is, the second printer signal transmission path  10  is effective, the routine is ended. On the other hand, if it is determined, in step S 32 , that the first printer signal transmission path  9  is effective, the switching signal provided to the switching unit  11  is changed from the high level to the low level so that the second printer signal transmission path  10  becomes effective. 
     As mentioned above, in the present embodiment, a different operation is performed based on the determination of whether the command sent from the personal computer  3  is readable or unreadable. When the command is readable, it is interpreted that the command is designated for the first printer  1 , and the first printer signal transmission path  9  is maintained effective or changed to be effective. On the other hand, if the command is unreadable, it is interpreted that the command is designated for the second printer, and the second printer signal transmission path  10  is maintained effective or changed to be effective. It should be noted that the process of step S 31  is performed as a function of the determining function, and the process of steps S 32  to S 35  is performed as a function of the path state switching function. 
     It should be noted that, in the present embodiment, the command sent from the personal computer  3  is subjected to a reading operation by the CPU  7 , and simultaneously stored in the FIFO buffer. Then, if the command is not readable by the CPU  7 , the command temporarily stored in the FIFO buffer is sent to the second printer  2  through the second printer signal transmission path  10 . If the FIFO buffer is not provided, the command which was received and interpreted as not readable is lost, and the command cannot be sent to the printer  2 . 
     According to the present embodiment, since the switching function for the printers is provided in the interface unit of the first printer  1 , the first printer  1  is located in the center of the system. Thus, taking into consideration that the command system of the first printer is known, if the command is not readable, this does not assure that the command is designated for the printer  2  but it can be interpreted that the command is at least not designated for the printer  1  and the switching operation is performed. Thus, the switching of the printers can be appropriately performed by utilizing the command itself. 
     A description will now be given of a sixth embodiment of the present invention. FIG. 11 is an illustration of an entire image forming system according to the sixth embodiment of the present invention. In this system, a multi-function peripheral (MFP) device  51  and a second peripheral device  72  are used instead of the first printer  1  and the second printer  2  in the above-mentioned embodiments, respectively. The MFP device  51  has an interface switching device  73  which includes a first connector  74  and a second connector  75 . A personal computer  71  is connected to the MFP device  51  via the first connector  74 . The second peripheral device  72  is connected to the MFP device  51  via the second connector  75 . A facsimile line  54  is connected to the MFP device  51 . 
     FIG. 12 is a block diagram of the MFP device  51  shown in FIG.  11 . The MFP device  51  comprises a scanner unit  52 , a printer unit  53 , a network control unit (NCU)  55 , a modem  56 , an image memory  57 , a central processing unit (CPU)  58 , a memory  59 , an operational unit  60 , the first connector interface  74  and the second connector interface  75  which are interconnected by a bus line  61 . 
     The scanner unit  52  scans an original image and outputs image data corresponding to the original image. The printer unit  53  outputs an image on a recording sheet according to image data supplied thereto. The NCU  55  corresponds to a facsimile communication unit which transmits and receives data through the facsimile line  54 . The image memory  57  stores the image data generated by the scanner unit  52  or image data received by the NCU  55 . The operational unit  10  is provided for inputting various instructions to control the MFP device  51 . Thus, the MFP device  51  has a scanner function, a printer function and a facsimile function. 
     The scanner unit  52  is a sheet scanner which scans images of original documents fed by an automatic document feeder (ADF) (not shown in the figure) by a contact-type optoelectric conversion element fixedly provided at a scanning position. Thus the original image is read and electrical signals corresponding to the original images are generated. The printer unit  53  prints on a recording sheet based on digital image data. The printer unit  53  comprises, for example, a laser printer which includes a monochrome laser writing system and an electrophotographic device. The memory  59  comprises a ROM for fixedly storing data such as programs and a RAM for rewritably storing various types of information. The CPU  8  performs various processes and controls in accordance with the programs stored in the ROM. 
     As mentioned above, the MFP device  51  together with the personal computer  71  and the second peripheral device  72  are components of an image forming system in which the personal computer  71  can be connected to one of the MFP device  51  and the second peripheral device  72 . In the present embodiment, the second peripheral device  72  is a color inkjet printer which has a printing capability different from that of the printer unit  53  of the MFP device  51 . Hereinafter, the second peripheral device  72  may be referred to as a second printer  72 . 
     The personal computer  71  is connected to the second printer  72  via the MFP  1 . Thus, the MFP  1  is provided with the interface switching device  73  which has a switching function. Basically, the interface switching device  73  conforms to the centronics standard. 
     FIG. 13 is a circuit diagram of the switching device  73 . In FIG. 13, parts that are the same as the parts shown in FIG. 2 are given the same reference numerals. 
     The switching device  73  is provided with a first connector interface  74  and a second connector interface  75 . The first connector interface  74  is connected to a centronics standard connector of the personal computer  71 . The second connector interface  75  is connected to a centronics standard connector (not shown in the figure) of the second printer  72 . Since the switching device  73  is incorporated into the MFP device  51 , the switching device  73  is connected to the electrical circuit of the MFP device  73  by an internal wiring system in the MFP device  51 . Accordingly, there is no printer connector needed for the connection of the MFP device  51 . 
     The MFP device  1  comprises a controller  76  including the central processing unit (CPU)  58 . The first connector interface  74  is connected to the controller  76  by a first printer signal transmission path  9 . The first printer signal transmission path  9  comprises an eight-wire data bus  9   a , a four-wire reception control line  9   b  and a five-wire transmission control line  9   c . Additionally, the second connector interface  75  is connected to the controller  76  by a second printer signal transmission path  10 . The second printer signal transmission path  10  comprises a data bus  10   a , a reception control line  10   b  and a transmission control line  10   c  which are branched from the data bus  9   a , the reception control line  9   b  and the transmission control line  9   c , respectively. 
     A switching unit  11  is provided so as to effect one of the printer signal transmission paths  9  and  10  in response to a switching signal provided by the controller  76 . The switching unit  11  comprises buffers  12   a ,  12   b ,  12   c ,  13   b  and  13   c  and a switch  14 . The buffers  12   a ,  12   b  and  12   c  can be set to either an active state or inactive state, and are provided on the data bus  9   a , the reception control line  9   b  and the transmission control line  9   c , respectively. The buffers  13   b  and  13   c  can be set to either an active state or inactive state, and are provided on the reception control line  10   b  and the transmission control line  10   c , respectively. The switch  14  is controlled to be turned on or off, and is provided on the data bus  10   a . The buffer  12   a  is a bidirectional buffer. 
     The buffer  12   a  is connected to the controller  76  via a control line  15   a  so as to provide a switching signal which controls the operational state of the buffer  12   a . The buffers  12   b  and  13   b  are connected to the controller  76  via the control line  15   b  so as to provide a switching signal which selectively sets one of the buffers  12   b  and  13   b  to the activated state. The buffers  12   c  and  13   c  are connected to the controller  76  via the control line  15   c  so as to provide a switching signal which selectively sets one of the buffers  12   c  and  13   c  to the activated state. The switch  14  is connected to the controller  76  via a control line  15   d  so as to provide a switching signal which controls the operational state of switch  14 . Additionally, inverters  16   b  and  16   c  are provided to the control lines  15   b  and  15   c  connected to the buffers  13   b  and  13   c , respectively, so as to invert the switching signals on the control lines  15   b  and  15   c.    
     A power source  17  is provided to operate the switching unit  11 . Power is provided to the power source  17  from the controller  76  via a power source line  18 , and also provided from the second printer  2  via a peripheral logic high (P.L.H.) line  19  of the second connector interface  75 . The P.L.H. line  19  is provided for checking whether power of the second printer  2  is turned on. Specifically, the centronics interface is provided with a small current from the second printer  2 . Since diodes  20  and  21  are provided on the power source line  18  and the P.L.H. line  19 , respectively, the two lines  18  and  19  are connected in a logic summing manner. 
     When the MFP device  51  is turned on, the switching signals provided to the control lines  15   a ,  15   b  and  15   c  are initially set to a high level, and the switching signal provided to the control line  15   d  is initially set to a low level. In this state, since the buffers  12   a  to  12   c  are in the active state, the switch  14  is turned off and the buffers  13   b  and  13   c  are in the inactive state, the first printer signal transmission path  9  is set to be effective. 
     Additionally, the personal computer  71  has a function for selectively designating one of the MFP device  51  and the second printer  72 . The personal computer  71  also has a switching command sending function for selecting a printer driver based on the selective designation. Specifically, for example, when a printing operation for an application of a word processor is performed, the MFP device  51  can be automatically designated by selecting the printer driver for the MFP device  51 . Alternatively, when a printing operation for an application of a word processor is performed, the second printer  72  can be automatically designated by selecting the printer driver for the second printer  72 . This function is performed by a CPU (not shown in the figure) of the personal computer  71 . 
     In the MFP device  51 , the CPU  58  performs various processes and controls in accordance with the program data stored in the ROM of the memory  59 . In the present embodiment, a determining function, a path state switching function and a concurrent printing function are provided. The determining function determines, whether the switching command designates the MFP device  51  or the second printer  72 , when the switching command is sent by the switching command sending function so as to select the printer driver in accordance with the operation for designating the printer by the personal computer  71 . Additionally, the path state switching function switches the state of paths by operating the switching unit  11  so as to effect one of the first printer signal reception path  9  and the second printer signal reception path  10  in accordance with the determination result of the determining function. Further, the concurrent printing function functions to provide image data generated by the scanner unit  52  of the MFP device  51  to both the printer unit  53  and the second printer  72  so as to concurrently print the image data by both the MFP device  51  and the second printer  72 . 
     In the above-mentioned structure in which the MFP device  51  and the second printer  72  are connected to the personal computer  71 , when all power is turned on, the controller  76  changes the switching signals on the control lines  15   a ,  15   b  and  15   c  to the high level. The switching signal on the control line  15   d  is at the low level. Accordingly, the first printer signal transmission path  9  becomes effective, resulting in a state where only the MFP device  51  is connected to the personal computer  71 . Thus, a printing operation can be performed by the MFP device  51  by sending character print data, other data or signals. In this case, monochrome printing can be provided with a high quality image at a high-speed. Additionally, the image data generated by the scanner unit  52  of the MFP device  51  can be sent to the personal computer  71  so as to store the image data in the personal computer  71 . 
     A description will now be given, with reference to flowcharts shown in FIGS. 14 and 15, of a switching operation from the MFP device  51  to the second printer  2  and switching from the second printer  72  to the MFP device  51 . In order to switch to a printer for a printing operation, the printer driver corresponding to the printer to be used is selected by the personal computer  71  when a printing operation is performed in the word processor application. Accordingly, it is determined first, in step S 51 , whether or not an operation for selecting the printer is performed. If it is determined that the operation for selecting is performed, it is then determined, in step S 52 , whether or not the switching is from the MFP device  51  to the second printer  72 . If the determination of step S 52  is negative, it is determined, in step S 54 , whether or not the switching is from the second printer  72  to the MFP device  51 . On the other hand, if the determination of step S 52  is affirmative, the routine proceeds to step S 53  where the personal computer  71  sends to the MFP device  51  a switching command for switching from the MFP device  51  to the second printer  72 . 
     If it is determined, in step S 54 , that the switching is from the second printer  72  to the MFP device  51 , the routine proceeds to step S 55 . In step S 55 , the personal computer  71  sends to the MFP device  51  a switching command for switching from the second printer  72  to the MFP device  51 . The process of steps S 53  to S 55  is performed as a function of the switching command sending function in the personal computer  71 . 
     When a command is sent from the personal computer  71  to the MFP device  51  via the connector interface  74 , the command is received in the CPU  58  of the controller  76 . It is then determined, in step S 56  of FIG. 15, whether or not the command is the switching command for the printer. If it is determined that the command is the switching command, the switching unit  11  is operated, in step S 57 , so as to switch from the MFP device  51  to the second printer  72 . That is, when the switching command is input and if the command represents the switching from the MFP device  51  to the second printer  72 , the switching signal provided to the switching unit  11  is changed from the high level to the low level. Thereby, the second printer signal transmission path  10  is effected. This state is equivalent to the state where only the second printer  2  is connected to the personal computer  71 . Thus, a printing operation is performed by the second printer by sending character image data or other necessary data to the second printer  72  via the second printer signal transmission path  10 . In this case, a multi-color printing can be provided. When viewing from the MFP device  51 , the character image data goes through to the second printer  72 . 
     On the other hand, when the switching command is input and if the command represents the switching from the second printer  72  to the MFP device  51 , the switching signal provided to the switching unit  11  is changed from the low level to the high level. Thereby, the first printer signal transmission path  9  is effected. This state is equivalent to the state where only the MFP device  51  is connected to the personal computer  71 . Thus, a printing operation is performed by the MFP device  51  by sending character image data or other necessary data to the MFP device  51  via the first printer signal transmission path  9 . Accordingly, the process of step S 56  is performed as the function of the determining function, and the process of step S 57  is performed as a function of the path state switching function. 
     As mentioned above, according to the present embodiment, the MFP device  51  itself has the connector function and the switching function so that the MFP device  51  and the second printer  72  are connected to a single centronics interface line extending from the personal computer  71 . That is, a system can be provided in which a remote switching control is performed by the personal computer  71 . Especially, according to the present embodiment, since the system is constructed so that the MFP device  51  is the center of the system by providing the switching function to the interface unit of the MFP device  51  which is one of objects to be switched, at least all of the command system of the MFP device  51  is known. Thus, even if the command system of the second printer  72  is unknown, switching between the MFP device  51  and the second printer  72  can be reliably performed. Additionally, as the MFP device  51  itself has the switching function in the interface unit, the connection of the MFP device  51  can be achieved by internal wiring without a connector. Thus, a length of wire or cable can be shorter than that of a case in which a separate switching device is used. This improves antinoise characteristics. Further, since the MFP device  51  has the switching device therein, a power source for the switching device can be the power source for the MFP device  51 . That is, a separate power source is not needed as in the case of the separate switching device. 
     A description will now be given of an operation for printing the image data generated by the scanner unit  52  of the MFP device  51 . Since the MFP device  51  has the scanner unit  52  and the printer unit  53 , the image data generated by scanning an original document by the scanner unit  52  can be printed by the printer unit  53 . This means that the MFP device  51  includes a copying function. The MFP device according to the present embodiment has an ability to print the image data generated by the scanner unit  52  by both the printer unit  53  of the MFP device  51  and the second printer  72  at the same time. This function is performed by the concurrent printing function under the control of the CPU  58 . When a copy mode is designated by the operational unit  60  in a printing mode where a plurality of copies are printed, concurrent printing is performed. That is, the controller  76  changes the switching signals on the control lines  15   a  and  15   d  to the high level so that the data buses  9   a  and  10   a  are effective to connect the controller  76  to the second connector interface  75 . Thus, the MFP device  51  is connected to the second printer  72 . In this state, when the scanner unit  52  scans an original image and generates image data, the image data is sent to the printer unit within the MFP device  51 , and is simultaneously sent to the second printer  72  via the data busses  9   a  and  10   a  and the second connector interface  75 . Accordingly, both the printer unit  53  and the second printer  72  print the image data at the same time. This achieves a circumstances in which the personal computer  71  is connected to two printers in parallel. Thus, since the printing operation of a plurality of copies can be distributed to the two printers, that is, the printer unit  53  and the second printer  72 , the printing operation for a plurality of copies can be performed at a high speed. The present embodiment takes advantage of the fact that the personal computer  71  is substantially connected to the two printers. 
     It should be noted that, as for the determining function and the path state switching function, the present invention is not limited to that specifically disclosed, and other appropriate determining and path state switching functions or units may be used. 
     A description will now be given of a seventh embodiment of the present invention. The structure of the seventh embodiment is basically the same with that of the sixth embodiment, and a description will be given with reference to FIG.  13 . In the present embodiment, the CPU  58  of the MFP device  51  performs a memory control function in addition to the determining function and the path state switching function. When the image data sent from the personal computer  71  is printed by the second printer  72 , the memory control function operates the image memory  57  as a printer buffer by connecting between the connector interface  74  and the MFP device  51 . 
     In the present embodiment, when a print mode is designated in which the second printer  72  is used to print image data generated by the personal computer  71 , the second printer signal transmission path  10  is not effective but the first printer signal transmission path  9  is turned on so as to be effective and provide the image data from the personal computer  71  to the image memory  57  of the MFP device  51  via the connector interface  74  and the bidirectional buffer  12   a . The image data is then temporarily stored in the image buffer  57 . The image data is then sent to the second printer  72  via the data buses  9   a  and  10   a  by effecting the data buses  9   a  and  10   a  between the controller  76  and the connector interface  75 . The data busses  9   a  and  10   a  are effected by changing the switching signals on the control lines  15   a  and  15   d  to the high level. Accordingly, the image memory  57 , which is originally provided for facsimile transmission, can be used as a buffer for printing operation. This eliminates the need for using a buffer for printing operations in the personal computer  71 . Thus, there is no time loss caused by the buffer in the personal computer  71 . That is, this takes advantage of the fact that the personal computer  71  is connected to the MFP device  51  which has the image memory therein. 
     A description will now be given of an eighth embodiment of the present invention. A structure of the present embodiment is basically the same as that of the seventh embodiment, and a description will be given with reference to FIG.  13 . 
     In this embodiment, a scanner is used as the second peripheral device. That is, the scanner is used instead of the second printer  72 . Hereinafter, the scanner is referred to as a second scanner  72 . As for the second scanner  72 , for example, a color scanner or a high-resolution scanner may be used. In the present embodiment, the CPU  58  of the MFP device  51  performs functions as an operation control function in addition to the determining function and the path state switching function. The operation control function is performed when the image data generated by the second scanner  72  is sent to the personal computer  71 . Specifically, the second connector interface  75  and MFP device  51  are connected, and the image memory  57  is used as a date buffer so that the image data generated by the second scanner  72  is temporarily stored in the image memory  57 . 
     In the present embodiment, when a print mode is designated in which image data generated by the second scanner  72  is sent to the personal computer  71 , the data buses  10   a  and  9   a  are turned on so as to be effective and the second scanner  72  is operatively connected to the MFP device  51  by changing the switching signals on the control lines  15   a  and  15   d  to the high level. Thus, the image data generated by the second scanner can be temporarily stored in the image memory  57  in the MFP device  51  as a data buffer. That is, the image memory  57  serves as a data buffer similar to the manner when image data received by the NCU  55  for facsimile communication is stored or image data generated by the scanner unit  52  is stored. Thereafter, the first print signal transmission path  9  is turned on so as to be effective and to operatively connect the MFP device  51  to the personal computer  71 . Thus, the image data in the image memory  57  can be sent to the personal computer  71  via the data bus  9   a . Thereby, the image data generated by the second scanner  72  can be sent to the personal computer  71  via the image memory  57 . 
     Accordingly, in the present embodiment, a high-speed reading operation can be performed by the second scanner  72  since a delay in the operation speed of the application can be eliminated as compared to a case in which the second scanner is directly connected to the personal computer  71 . Additionally, when the second scanner  72  is directly connected to the personal computer  71 , image data generated by the second scanner  72  may be reduced so as to avoid a lack of capacity of a memory provided in the personal computer  71 . The reduction of image data is generally performed, for example, by intermittently eliminating image data generated by the scanner. Thus, in such a case, the image quality is deteriorated even when a high-resolution scanner is used as the second scanner  72 . In this respect, according to the present invention, since the image memory  57  is used as a data buffer which is originally for facsimile communication and has inherently a large capacity, there is no need to reduce the amount of image data. Thus, the image quality can be improved as compared to the case where the scanner is directly connected to the personal computer  71 . That is, the image reading capability of the scanner  72  is not deteriorated due to lack of memory capacity. This embodiment takes advantage of the fact that the second scanner  72  is connected to the personal computer  71  via the MFP device  51  having the image memory  57  therein. 
     A description will now be given of a ninth embodiment of the present invention. The basic structure and operation of the ninth embodiment is the same as that of the first embodiment, and a description will be given, with reference to FIG. 2, of structures and operations different from that of the first embodiment. 
     In this embodiment, when the second printer is used, an ESC command is sent by the personal computer  3 . The ESC command comprises a printer selection code in accordance with an escape sequence. The controller  8  receives the ESC command via the first printer signal transmission path, and the CPU  7  recognizes the reception of the command. Thus, the CPU  7  changes the switching signals on the control lines  15   a  to  15   c  to the low level, and changes the switching command on the control line  15   d  to the high level. Thereby, the buffers  13   b  and  13   c  are set to the active state, and the switch  14  is closed so as to be turned on, resulting in the second printer signal transmission path being effective. Accordingly, the system is set in a state equivalent to a state where only the second printer  2  is connected to the personal computer  3  via the connectors  5  and  6 . Thus, a printing operation can be performed by the second printer  2  by sending from the personal computer  3  image data, character data or other data and signals. In this embodiment, multi-color printing can be performed. 
     On the other hand, in order to return the system to the state where the first printer  1  is usable, the personal computer  3  sends a printer initialization signal (/INIT) to the printer interface switching device  4  via the first connector  5 . The printer initialization signal (/INIT) is a signal conforming to the centronics standard, and thus this signal is readable by the CPU  7 . When the printer initialization signal (/INIT) is received, the CPU  7  operates the switching unit  11  to be returned to the initial state after the power is turned on. Thus, the first printer signal transmission path  9  is turned on so as to be effective instead of the second printer signal transmission path  10 . In such a switching operation, it is better to use a command readable by the second printer  2 . However, the command system of the second printer is unknown as far as the first printer  1  is concerned. Thus, the command system of the second printer  2  does not always conform to the command system of the first printer  1 . Rather, in many cases, the command systems of the first and second printers are different from each other. In such a case, if a command conforming to the command system of the second printer is sent to the first printer  1 , the command cannot be read by the CPU  7  of the first printer  1 , and thus the switching operation from the second printer  2  to the first printer  1  cannot be performed. Accordingly, in the present embodiment, the switching operation is performed by using a command which is positively readable by the CPU  7  of the first printer  1 . Therefore, control of the first printer  1  and the second printer  2  can be arbitrarily performed by the personal computer  3 . 
     It is possible that a situation could occur in which the first printer  1  is turned off and the second printer and the personal computer  3  are turned on. Even in such a situation, it is desirable that the second printer  2  can be usable by the personal computer  3 . Since the printer interface switching device  4  is incorporated into the first printer  1  and the power of the first printer is turned off, the switching unit  11  cannot be operated to switch the connection to the second printer  2  when the power is provided to the switching unit  11  only from the first printer  1 . However, in the present embodiment, the power is provided to the switching unit  11  from the second printer via the connector  6  and the switching unit  11  can be operated when the second printer  2  is turned on. Additionally, in this embodiment, the second printer signal transmission path  10  is effective even when the controller  8  of the first printer  1  does not operate due to the first printer  1  being turned off since the low level of the switching signal provided to the switching unit  11  is set to a ground level which is provided when the first printer  1  is turned off. Accordingly, the second printer  2  can be operatively connected to the personal computer even when the power of the first printer  1  is turned off. 
     A description will now be given, with reference to FIG. 16, of a structure and an operation of the switch  14  of FIG.  2 . The switch  14  may be a mechanical switch. However, it is preferable that the switch  14  is an analog IC switch  22  constructed by an integrated circuit. In this case, in order to protect the analog switch  22  from an external noise, protective diodes  23   a  to  23   d  are interposed between the input and output signal terminals and the power source as shown in FIG.  16 . However, if only the protective diodes  23   a  to  23   d  are provided, a state of the power source of the switch  14  is equivalent to a state where the power source is grounded. Thus, it is possible that a reverse current could flow into the analog IC switch  22  via the signal line. This is not preferable since the reverse current may destroy the analog IC switch  22  which comprises an integrated circuit. Accordingly, in the present embodiment, in order to prevent the reverse current from flowing into the analog IC switch  22 , a diode  24  (correspond to the diode  20  in FIG. 2) is provided on the power source line  18  in a direction opposite to the direction of the diodes  23   a  to  23   d . Thus, even if the first printer  1  is turned off and the personal computer  3  is turned on, the reverse current does not flow into the analog IC switch  22  from the signal line. Thus, the analog IC switch  22  is positively protected. 
     The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention. For example, although the above-described embodiments refer to a host computer as providing the data, any other type of user device as a network, for example, can provide the data. In addition, although the embodiments refer to the use of printers or multi-functional peripherals, other types of peripheral devices such as facsimile and scanner type device may be used.