Abstract:
Disclosed herein is a printing device connected to a client, the printing device including: a reception unit which receives plural types of logic packets from the client; a hardware packet processing unit which processes the logic packets by hardware; a software packet processing unit which includes a microprocessor and processes the logic packets by software; and a sorting unit which sorts the plural types of logic packets into a logic packet to be processed by the hardware packet processing unit and a logic packet to be processed by the software packet processing unit, wherein the sorting unit sorts a reply requiring logic packet including contents replying to the client as the logic packet to be processed by the hardware packet processing unit when sorting the plural types of logic packets.

Description:
The entire disclosure of Japanese Patent Application No. 2006-253094, filed Sep. 19, 2006 is expressly incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a technology for suppressing a microprocessor used in a printing process from becoming a high load status in a printing device. 
     2. Related Art 
     In a printing system in which a client (a personal computer or the like) is connected to a printing device, data communication between the client and the printing device has been performed by transmitting/receiving a logic packet. For example, a printing system in which a client is connected to a printer through a universal serial bus (USB), print data or a control command is transmitted/received by means of a logic packet defined by a D4 protocol (hereinafter, referred to as a “D4 packet”). The D4 protocol is a communication protocol defined by the IEEE 1284.4. Examples of a system for transmitting/receiving print data or a control command by means of the D4 packet include a network system disclosed in JP-A-2006-139585. 
     In general, a printing device includes a microprocessor which executes predetermined software (firmware) so as to drive a printing mechanism (printer engine), thereby performing a printing process. In a printing device for performing data communication with a client by transmitting/receiving a logic packet, the microprocessor analyzes the logic packet to perform a predetermined process in addition to a printing process. In such a configuration, if a processing load of the logic packet is increased due to the frequent reception of the logic packet from the client, the microprocessor is in a high load status and thus the printing process becomes stagnant. Thus, it is take much time to perform the printing process. 
     SUMMARY 
     An advantage of some aspects of the invention is that it provides a technology for suppressing a microprocessor used in a printing process from becoming a high load status in a printing device for transmitting/receiving a logic packet to/from a client. 
     According to an aspect of the invention, there is provided a printing device connected to a client, the printing device including: a reception unit which receives plural types of logic packets from the client; a hardware packet processing unit which processes the logic packets by hardware; a software packet processing unit which includes a microprocessor and processes the logic packets by software; and a sorting unit which sorts the plural types of logic packets into a logic packet to be processed by the hardware packet processing unit and a logic packet to be processed by the software packet processing unit. 
     In the printing device, since the plural types of logic packets are sorted into the logic packet to be processed by the hardware packet processing unit and the logic packet to be processed by the software packet processing unit, it is possible to reduce load of the software packet processing unit compared with a configuration in which all the logic packets are processed by the software packet processing unit. Accordingly, it is possible to suppress the microprocessor included in the software packet processing unit from becoming a high load status. 
     In the print device, the sorting unit may sort a reply-requiring logic packet, including contents for replying to the client, as the logic packet to be processed by the hardware packet processing unit when sorting the plural types of logic packets. 
     By this configuration, since the logic packet which requires a reply to the client to cause high processing load is processed by the hardware packet processing unit, it is possible to reduce the processing load of the microprocessor. 
     In the printing device, the reply-requiring logic packet may be a credit request packet for requesting a credit for controlling a flow between the client and the printing device. 
     By this configuration, since the credit request packet which is frequently transmitted/received in order to control the flow is processed by the hardware packet processing unit, it is possible to remarkably reduce the processing load of the microprocessor. 
     In the printing device, a print mode may include at least a high quality print mode and a low quality print mode, and the sorting unit may sort a credit request packet for transmitting print data as the packet to be processed by the hardware packet processing unit when the high quality print mode is set as the print mode and sorts the credit request packet for transmitting the print data as the software packet processing unit when the low quality print mode is set as the print mode. 
     By this configuration, when the printing process is performed in the high quality print mode, the processing load of the microprocessor is increased in order to a large amount of print data. However, since the credit request packet for transmitting the print data used in the printing process can be processed by the hardware, it is possible to suppress the microprocessor from becoming a very high load status. 
     In the printing device, each of the logical packets may include a header portion and a body portion, and the types of the logic packets may be identified by an identifier included in the header portion or a combination of the identifier included in the header portion and an identifier included in the body portion. 
     By this configuration, the type of the logic packet received from the client can be identified by the identifier included in the header portion and the identifier included in the body portion. Accordingly, the sorting unit can sort the logic packet by identifying the type of the logic packet using the identifiers. 
     In the printing device, the processing mode of the logic packets may include a first processing mode for processing a specific type of logic packet by the hardware packet processing unit or a second processing mode for processing the specific type of logic packet by the software packet processing unit. The printing device may include a processing mode switching unit which switches the processing mode to the first processing mode or the second processing mode according to an operation status of the printing device. 
     By this configuration, when the operation status of the printing device is an operation status which increases the processing load of the microprocessor, the processing mode is switched to the first processing mode so as to reduce the processing load of the microprocessor and, when the operation status of the printing device is an operation status which hardly increases the processing load of the microprocessor, the processing mode is switched to the second processing mode so as to process the logic packet. 
     In the printing device, the processing mode switching unit may switch the processing mode to the second processing mode when the operation status of the printing device is an error status and may switch the processing mode to the first processing mode when the operation status of the printing device is a normal status. 
     By this configuration, since the specific type of logic packet received from the client is processed by the software when the printing device is in the error status, it is possible to freely change the processing contents according to an error correction status. Accordingly, it is possible to suppress the logic packet from being uniformly processed by the hardware even in the error status and to suppress the same credit number as the normal status (error-free status) from being notified to the client even when the printing process cannot be performed. 
     The invention can be implemented by a variety of forms, such as a logic packet processing method, a computer program for realizing the logic packet processing method or the function of the printing device, a recording medium including the computer program recorded therein, and a data signal implemented in a carrier including the computer program. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a schematic block diagram showing the configuration of a printer  100  according to a first embodiment of the invention. 
         FIG. 2  is a view showing a hierarchical structure of a function for transmitting/receiving data between a personal computer  10  and the printer  100 . 
         FIG. 3  is a view showing the configuration of a D4 packet (credit request/reply packet). 
         FIG. 4  is a sequence view showing an example of an operation for transmitting print data from the personal computer  10  to the printer  100 . 
         FIG. 5  is a flowchart showing a packet reception process according to the first embodiment of the invention. 
         FIG. 6  is a schematic block diagram showing the configuration of a printer according to a second embodiment of the invention. 
         FIG. 7  is a flowchart showing a packet reception process according to the second embodiment of the invention. 
         FIG. 8  is a view showing a data flow in the printer  101  in a software processing mode. 
         FIG. 9  is a schematic block diagram showing the configuration of a printer according to a third embodiment of the invention. 
         FIG. 10  is a flowchart showing a packet reception process according to the third embodiment of the invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, embodiments of the invention will be described in the following order. 
     A. First Embodiment 
     B. Second Embodiment 
     C. Third Embodiment 
     D. Modified Example 
     A. First Embodiment 
     A1. Schematic Configuration of Printing Device 
       FIG. 1  is a schematic block diagram showing the configuration of a printer  100  according to a first embodiment of the invention. The printer  100  includes a USB control module  110 , a D4 management module  120 , a RAM management module  130 , a RAM  140 , a microprocessor  150  and a printer engine  160 . The USB control module  110  includes a physical interface for connecting the printer  100  to a personal computer  10  through a USB and analyzes a USB protocol such that the printer  100  functions as a USB device. The D4 management module  120  sorts a D4 packet received from the personal computer  10  and performs a predetermined process according to a D4 protocol with respect to a specific type of D4 packet (credit request packet). The RAM management module  130  writes data to the RAM  140 , reads data from the RAM  140 , or manages a write address/read address in the RAM  140 . 
     The D4 management module  120  includes a header/command analysis unit  121 , a data buffer  122  and a reply generation unit  123 . The header/command analysis unit  121  analyzes a header of the D4 packet and a portion of a command of a packet having a specific header. The data buffer  122  stores a specific D4 (D4 packet excluding a credit request packet and a print data packet). The reply generation unit  123  generates a D4 packet for a credit request reply. 
     The RAM management module  130  includes a store module  131 , a store address management unit  132 , a load address management unit  133 , a RAM free capacity calculation unit  134  and a load module  135 . The store module  131  writes data received from the header/command analysis unit  121  in the RAM  140 . The store address management unit  132  manages a data write address in the RAM  140 . The load address management unit  133  manages a data read address in the RAM  140 . The free capacity calculation unit  134  periodically acquires addresses which are respectively managed by the store address management unit  132  and the load address management unit  133 , calculates a free capacity of the RAM  140  on the basis of these addresses, and periodically notifies the reply generation unit  123 . 
     The USB control module  110 , the D4 management module  120 , the RAM management module  130  and the RAM  140  are configured by hardware. 
     The microprocessor  150  executes predetermined software (firmware) stored in the RAM  140  and functions as a D4 protocol processing unit  151 , a print processing unit  152  and a printer management unit  153 . The D4 protocol processing unit  151  analyzes the D4 protocol and executes a predetermined process. The print processing unit  152  drives a printer engine  160  on the basis of print data (data written in a page description language) received from the personal computer  10  and performs a printing process. The printer management unit  153  manages a status such as the residual quantity of ink or existence of an error. In  FIG. 1 , an arrow of a thick solid line indicates the flow of a credit request/reply packet and an arrow of thick dotted line indicates the flow of print data. 
     The USB control module  110  corresponds to a reception unit, the reply generation unit  123  corresponds to a hardware packet processing unit, the D4 protocol processing unit  151  corresponds to a software packet processing unit, and a header/command analysis unit  121  corresponds to a sorting unit. 
       FIG. 2  is a view showing a hierarchical structure of a function for transmitting/receiving the D4 packet between a personal computer  10  and the printer  100 . In the printer  100 , the USB control module  110  becomes a lowermost layer and the D4 management module  120  and the D4 protocol processing unit  151  are located thereon. The USB control module  110  includes a USB device interface  111  for providing a physical interface, a USB logic device  112  and a printer interface  113 . The USB logic device  112  includes a control end point  112   e , and the printer interface  113  includes a bulk OUT end point  113   e  and a bulk IN end point  113   f.    
     The term “end point” described herein is a logic resource for performing communication with a host. The control end point  112   e  is an end point for transmitting/receiving a standard device request. The term “standard device request” described herein is a basic request which needs to be supported by all USBs. The bulk OUT end point  113   e  and the bulk IN end point  113   f  are end points for transmitting/receiving a message for performing the printing process. 
     The personal computer  10  includes a USB host interface (hardware)  11 , USB system software  12 , a USB printer class driver  13  and a D4 protocol processing unit  14 , all of which are located in this order from the lowermost layer. A control pipe is provided between the USB system software  12  and the control end point  112   e . The control pipe is a dedicated logic channel for controlling a variety of settings in USB communication. A bulk OUT pipe is provided between the USB printer class driver  13  and the bulk OUT end point  113   e  and a bulk IN pipe is provided between the USB printer class driver and the bulk IN end point  113   f . The bulk OUT pipe and the bulk IN pipe are dedicated logic channels for transmitting/receiving a message for performing the printing process. 
     In addition to the above-described logic channels, three logic channels (a print data channel, a D4 control channel and a print control channel) for transmitting/receiving the D4 packet are established between the personal computer  10  and the printer  100 . The three logic channels are terminated by the D4 management module  120  or the D4 protocol processing unit  151  in the printer  100 . The print data channel is a logic channel for transmitting/receiving the D4 packet including the print data. The D4 control channel is a logic channel for transmitting/receiving the D4 packet for controlling the transmission/reception of the D4 packet. The print control channel is a logic channel for transmitting/receiving the D4 packet for controlling the printing process. 
       FIG. 3A  shows the configuration of the D4 packet transmitted/received between the personal computer  10  and the printer  100 . The D4 packet includes 6-byte header portion (D4 header) and a body portion. The header portion includes a primary socket ID (PSID) field, a secondary socket ID (SSID) field, a packet length field, and a control information field. The PSID field and the SSID field are used as logic channel IDs for identifying the three logic channels (print data channel, the D4 control channel and the print control channel). Since a plurality of logic channels can be set as the print data channel, the logic channel IDs are used as the IDS for identifying the plurality of print data channels. In the control information field, information controlling the transmission of data in the D4 protocol is stored. In the body portion, a D4 command and a parameter thereof, a reply for the D4 command or the print data is stored. 
     In the D4 protocol, a flow control process is performed in the transmission/reception of the D4 packet. In more detail, a transmitter of data requests to a receiver for the notification of the number of receivable data packets (also called a credit number), and the receiver notifies the transmitter of the credit number. The transmitter transmits a predetermined number of data packets including the notified credit number to the receiver. 
       FIG. 3B  shows the configuration of the body portion of the D4 packet used in a credit request. The body portion of a credit request packet includes a command field, a PSID field, a SSID field, a request credit number field and a maximum necessity credit number field. In the command field, a credit request command is stored. In the PSID field and the SSID field, the ID of a logic channel for transmitting the D4 packet using a credit is stored. In the request credit number field and the maximum necessity credit number field, a value which is set according to the size of data to be transmitted or the size of a transmission buffer is stored. 
       FIG. 3C  shows the configuration of the body portion of the D4 packet used in a reply for the credit request. The body portion of a credit request reply packet includes a command field, a result field, a PSID field, a SSID field and a provided credit number field. In the command field, a credit request reply command is stored. In the result field, information indicating the result (success/failure) of the credit request is stored. In the PSID field and the SSID field, the IDs of logic channels for requesting the credit are stored. In the provided credit number field, the credit number notified by the receiver is stored. In the credit request packet and the credit request reply packet, the ID of a logic channel (D4 control channel) for transmitting/receiving the credit request/reply packet is stored as the logic channel ID of the header portion. 
     A2. Print Data Transmission Sequence 
       FIG. 4  is a sequence view showing an example of an operation for transmitting print data from the personal computer  10  to the printer  100 . In a step [ 1 ], the personal computer  10  transmits an open channel request packet to the printer  100  and, in a step [ 2 ], the printer  100  transmits an open channel request reply packet to the personal computer  10 . The open channel request packet is transmitted in order to open a print data channel for transmitting print data which will be transmitted. In the example of  FIG. 4 , the printer  100  returns a message (Accept) that the requested print data channel is opened as the open channel request reply packet. The open channel request packet and the open channel request reply packet are transmitted through the D4 control channel. 
     In a step [ 3 ], the personal computer  10  transmits a credit request packet to the printer  100 . In a step [ 4 ], the printer  100  transmits a credit request reply packet to the personal computer  10 . The credit request packet and the credit request reply packet are transmitted through the D4 control channel. In the example of  FIG. 4 , the credit number is “4”. 
     In a step [ 5 ], the personal computer  10  transmits print data packets. Here, since the personal computer  10  receives “4” as the credit number in the step [ 4 ], four print data packets are continuously transmitted. The print data packets are transmitted through the opened print data channel. 
     When the transmission of the print data of the credit number is completed, the personal computer  10  transmits the credit request packet to the printer  100  again in a step [ 6 ], and the printer  100  transmits the credit request reply packet to the personal computer  10  in a step [ 7 ]. In the example of  FIG. 4 , the personal computer  10  receives the credit number “2” in the step [ 7 ]. Accordingly, in a step [ 8 ], the personal computer  10  continuously transmits two print data packets to the printer  100 . Whenever the credit number is received from the printer  100 , the continuous transmission of the print data packets of the credit number is repeated. When the transmission of all print data is completed, the personal computer  10  transmits a close channel request packet to the printer  100  in a step [ 9 ]. In a step [ 10 ], the printer  100  transmits a close channel request reply packet to the personal computer  10 . The close channel request packet is transmitted in order to close the print data channel used in the transmission of the print data. The close channel request packet and the close channel request reply packet are transmitted through the D4 control channel. Although not shown, the status (the residual quantity of ink or the existence of the error) of the printer  100  is periodically notified to the personal computer  10  using the printer control channel between the personal computer  10  and the printer  100 . 
     Various types (commands) of D4 packets are transmitted/received between the personal computer  10  and the printer  100  through the three logic channels (the print data channel, the D4 control channel and the print control channel). As shown in  FIG. 4 , in the transmission/reception of the print data, the credit request packet, the credit request reply packet and the print data packet are frequently transmitted/received between the personal computer  10  and the printer  100 . Accordingly, in the printer  100  according to the present embodiment, the credit request packet, the credit request reply packet and the print data packet which are frequently transmitted/received are processed by hardware in a packet reception process such that the microprocessor  150  is suppressed from becoming a high load status. 
     The credit request packet corresponds to a reply-requiring logic packet. Three D4 packets (a D4 packet for the print data, a D4 packet for the D4 control and a D4 packet for print control) which are respectively transmitted through the three logic channels (the print data channel, the D4 control channel and the print control channel) correspond to plural types of logic packets, and the various type (commands) of D4 packets (the credit request packet, the print data packet or the like) transmitted through the logic channels correspond to the plural types of logic packets. That is, the plural types of packets classified by a predetermined identifier included in the D4 packet correspond to the plural types of logic packets. 
     A3. Packet Reception Process of Printer  100   
       FIG. 5  is a flowchart showing a packet reception process according to the first embodiment of the invention. In the printer  100 , if a power source is turned on, the packet reception process is started in order to receive various D4 packets. 
     When the packet reception process shown in  FIG. 5  is started, the header/command analysis unit  121  ( FIG. 1 ) stands by until the D4 packet is received through the USB control module  110  (step S 305 ). When the D4 packet is received, the header/command analysis unit  121  analyzes the D4 header and the command of the body portion of the received D4 packet (step S 310 ). Next, the header/command analysis unit  121  determines whether the received D4 packet is the credit request packet (step S 315 ). In more detail, the header/command analysis unit  121  determines whether the received D4 packet is the “D4 control channel” by the logic channel ID of the header of the received D4 packet and determines whether a value indicating the “credit request” is stored in the command field of the body portion if it is determined to the D4 control channel. 
     When the received D4 packet is the credit request packet, the header/command analysis unit  121  sends the body portion of the received D4 packet to the reply generation unit  123 . The reply generation unit  123  calculates the credit number on the basis of the free capacity of the RAM  140  notified from the RAM free capacity calculation unit  134  (step S 320 ). As described above, since the free capacity of the RAM  140  is periodically notified from the RAM free capacity calculation unit  134 , the reply generation unit  123  divides a new free capacity by the size of the D4 packet and calculates the credit number. Next, the reply generation unit  123  generates the credit request reply packet for notifying the calculated credit number and transmits the credit request reply packet to the personal computer  10  through the USB control module  110  (step S 325 ). The steps S 305  to S 325  are performed by the hardware without through the microprocessor  150 , as denoted by the arrow of the thick line of  FIG. 1 . 
     In contrast, when the received D4 packet is not the credit request packet, the header/command analysis unit  121  determines whether the received D4 packet is the print data packet (step S 345 ). This determination can be executed by determining whether the logic channel ID of the received D4 packet is the ID of the print data channel. 
     When the received D4 packet is the print data, the header/command analysis unit  121  sends the body portion (print data) of the received D4 packet to the store module  131 . The store module  131  writes the print data in the address of the RAM  140  instructed by the store address management unit  132  (step S 350 ). Next, the load module  135  reads the print data from the address of the RAM  140  specified by the load address management unit  133  and sends the print data to the microprocessor  150  (step S 355 ). In the microprocessor  150 , the print processing unit  152  drives the printer engine  160  and performs the printing process on the basis of the print data received from the load module  135  (step S 360 ). The steps S 345  to S 355  are performed by the hardware without through the microprocessor  150 , as denoted by the arrow of the thick dotted line of  FIG. 1 . 
     In the step S 345 , if it is determined that the received D4 packet is not the print data, the header/command analysis unit  121  writes the received D4 packet in the data buffer  122  (step S 375 ) and transmits an interrupt signal to the microprocessor  150  (step S 380 ). The interrupt signal is used to notify the data buffer  122  that the D4 packet is written. In the microprocessor  150 , when the interrupt signal is received, the D4 protocol processing unit  151  reads the D4 packet from the data buffer  122  (step S 385 ), and analyzes the D4 packet to perform a predetermined D4 protocol (step S 390 ). The steps S 385  and S 390  are performed by software. 
     In more detail, for example, when the received D4 packet is the open channel request packet (the step [ 1 ] of  FIG. 4 ), the packet is sent to the D4 protocol processing unit  151  through the data buffer  122  and is processed by the D4 protocol processing unit  151 , because the received D4 packet is not the credit request packet and the print data packet. The D4 protocol processing unit  151  transmits the open channel request reply packet to the personal computer  10  through the USB control module  110  when the requested print data channel can be opened (the step [ 2 ] of  FIG. 4 ). 
     As described above, in the printer  100 , the D4 packet received from the personal computer  10  is sorted by the logic channel ID of the header and the value of the command field of the body portion. The credit request packet and the print data packet are processed by the hardware and the other packet is processed by the software. Since the credit request packet and the print data packet are frequently received from the personal computer  10  and the credit request packet requires the reply, high load is applied to the module for performing the process when these packets are processed. In the printer  100 , since these packets are processed by the hardware, it is possible to suppress the microprocessor  150  from becoming a high load status. 
     B. Second Embodiment 
       FIG. 6  is a schematic block diagram showing the configuration of a printer according to a second embodiment of the invention. The printer  101  is equal to the printer  100  shown in  FIG. 1  except that a mode specifying value register  124  is included in the D4 management module. In the printer  100  according to the first embodiment, the D4 packet received from the personal computer  10  is sorted into the packet to be processed by the hardware (the credit request packet and the print data packet) and the packet to be processed by the software (the other packet) and is processed. In contrast, in the printer  101  according to the second embodiment, a mode for sorting and processing the D4 packet (sort processing mode) and a mode for processing all the D4 packets by the software (software processing mode) are set and are switched to process the D4 packet. The sort processing mode corresponds to a first processing mode and the software processing mode corresponds to a second processing mode. The printer management unit  153  corresponds to a processing mode switching unit. 
     The switching of the processing mode is controlled. In more detail, the printer management unit  153  acquires alert information such as the residual quantity of the ink or a paper jam from the printer engine  160  and manages the status of the printer  100 . The printer management unit  153  controls the processing mode to be switched from the sort processing mode to the software processing mode when the residual quantity of the ink is very low or the paper jam occurs such that the operation status of the printer  100  becomes an error status. The reason why such control is performed is as follows. In the sort processing mode, the credit number is notified to the personal computer  10  by the hardware process, as described in the first embodiment. Accordingly, even when the printing process cannot be performed because the operation status of the printer  10  is the error status, the print data is sequentially received from the personal computer  10 . Accordingly, when the operation status is the error status, the processing mode is switched to the software processing mode and the credit number is notified in consideration of the free capacity of the RAM  140  and the operation status of the printer  100 . 
     The mode specifying value register  124  ( FIG. 6 ) is used to store a specifying value for specifying the processing mode. In more detail, the printer management unit  153  writes “1” indicating the software processing mode in the mode specifying value register  124  when the operation status of the printer  100  is the error status and writes “0” indicating the sort processing mode when the operation status of the printer  100  is a normal status (an error-free status) 
       FIG. 7  is a flowchart showing a packet reception process according to the second embodiment of the invention. The process according to the second embodiment is equal to the process according to the first embodiment except that a step S 312  is added to the flowchart of  FIG. 5 . When the header and the command of the D4 packet is analyzed in the step S 310 , the header/command analysis unit  121  reads the value of the mode specifying value register  124  and determines whether the mode specifying value is “0” (step S 312 ). When the mode specifying value is “0”, the sort processing mode is specified and the printer  100  performs the same process as the first embodiment. Accordingly, the flow of the credit request packet and the credit request reply packet and the flow of the print data packet of  FIG. 7  are equal to those of  FIG. 1 . In contrast, when the mode specifying value is “1”, the software processing mode is specified. In this case, the processing mode is switched to the software processing mode and the steps S 375  to S 390  are performed. 
       FIG. 8  is a view showing a data flow in the printer  101  in the software processing mode. In the software processing mode, since the steps S 375  to S 390  ( FIG. 7 ) are performed, all the D4 packets are sent to the D4 protocol processing unit  151  through the data buffer  122 . Accordingly, the credit request packet, the credit request reply packet and the print data packet are processed by the D4 protocol processing unit  151  (the arrow of the thick solid line and the arrow of the dotted line). In the example of  FIG. 8 , since the paper jam occurs in the printer engine  160 , the mode specifying value of “1” is written in the mode specifying value register  124 . Accordingly, since the credit request packet is sent to the D4 protocol processing unit, the D4 protocol processing unit  151  can notify the personal computer  10  of the credit number according to the operation status of the printer  100 . For example, in the error status such as the paper jam, the D4 protocol processing unit  151  may transmit the credit number of “0” such that the print data cannot be received from the personal computer  10 . In a predetermined period immediately after solving the paper jam and returning to the normal status, the credit number of “1” is transmitted and, after a lapse of a predetermined period, “0” is written in the mode specifying value register  124  such that the processing mode is switched to the sort processing mode. 
     C. Third Embodiment 
       FIG. 9  is a schematic block diagram showing the configuration of a printer according to a third embodiment of the invention. The printer  102  is equal to the printer  100  shown in  FIG. 1  except that a credit number acquiring method specifying value register  125  is included in the D4 management module. In the second embodiment, the processing mode of the printer  101  is switched to the sort processing mode or the software processing mode such that the credit number according to the operation status of the printer  101  is notified to the personal computer  10 . In contrast, in the third embodiment, only the sort processing mode is used as the processing mode and the credit number according to the operation status of the printer  102  is notified to the personal computer  10 . 
     In the printer  102 , as a method of obtaining the credit number by the reply generation unit  123  includes a method of calculating the credit number on the basis of the free capacity of the RAM  140  notified from the RAM free capacity calculation unit  134  and a method of obtaining the credit umber by the notification (specifying) of the printer management unit  153 . When the operation status of the printer  102  is the error status, the reply generation unit  123  acquires the credit number by the notification of the printer management unit  153  and calculates and acquires the credit number in the normal status, similar to the first embodiment. 
     The switching of the method of acquiring the credit number is controlled by the printer management unit  153 . In more detail, the printer management unit  153  writes “0” as a credit number acquiring method specifying value in the credit number acquiring method specifying value register  125  when the operation status of the printer  102  is the error status. The specifying value of “0” is a value for specifying the method of acquiring the credit number by the notification of the printer management unit  153  and a value for specifying the credit number of “0”. In contrast, the printer management unit  153  writes “1” in the credit number acquiring method specifying value register  125  when the operation status of the printer  102  is the normal status. The specifying value of “1” is a value for specifying the method of calculating the credit number on the basis of the free capacity notified from the RAM free capacity calculation unit  134 . 
       FIG. 10  is a flowchart showing a packet reception process according to the third embodiment of the invention. The process according to the third embodiment is equal to the process according to the first embodiment except that steps S 318  and S 405  are added to the flowchart shown in  FIG. 5 . In a step s 315 , if it is determined that the received D4 packet is the credit request packet, the reply generation unit  123  reads the credit number acquiring method specifying value from the credit number acquiring method specifying value register  125  and determines whether the credit number acquiring method specifying value is 0 (step S 318 ). If the specifying value is not 0, that is, if the specifying value is 1, the reply generation unit  123  performs the steps S 320  and S 325 , calculates the credit number on the basis of the free capacity of the RAM  140 , and notifies the personal computer  10  of the credit number. In contrast, if the credit number acquiring method specifying value is 0, the reply generation unit  123  notifies the personal computer  10  of the credit number of 0 (step S 405 ). 
     By the above-described configuration, the credit request packet is processed by the hardware and, when the operation status of the printer  102  is the error status, the credit number of 0 is transmitted such that the print data is not received from the personal computer  10 . 
     D. Modified Example 
     Among the elements of the embodiments, elements excluding elements described in independent claims may be omitted. The invention is not limited to the embodiments or the examples and may be variously changed without departing from the scope of the invention. The modified examples are as follows. 
     D1. Modified Example 1 
     Although the credit request packet, the credit request reply packet and the print data packet are used as the D4 packets processed by the hardware in the above-described embodiments, the invention is not limited to this and other D4 packets may be processed by the hardware. In more detail, a status request packet and a status request reply packet may be processed by the hardware. As described above, the status request packet is used when the personal computer  10  requests for the statuses (the residual quantity of the ink) of the printers  100  to  102  and the status request reply packet is used when the personal computer  10  notifies the statuses of the printers  100  to  102 . In the status request reply packet, the operation statuses of the printers  100  to  102  such as the residual quantity of the ink are described as parameters. The status such as the residual quantity of the ink is acquired by the software. In more detail, the printer management unit  153  acquires the status from the printer engine  160 . In order to transmit the status request reply packet by the hardware, the printer management unit  153  acquires the status when the processing load of the microprocessor  150  is relatively low, and records the residual quantity of the ink in a status recording register (not shown) for recording the status. When the status request packet is received, the reply generation unit  123  reads the value of the status recording register and generates and transmits the status request reply packet including the value. Since the status request packet is periodically transmitted/received through the printer control channel, the status request packet is a reply-requiring logic packet which has a relatively high frequency of performing the process and requires a reply. Accordingly, it is possible to remarkably reduce the processing load of the microprocessor  150  by processing the status request packet by the hardware. 
     D2. Modified Example 2 
     Although the processing mode is switched according to whether the operation status of the printer  101  is the error status or not in the second embodiment, the invention is not limited to this and the processing mode may be switched by any operation status. For example, the processing mode may be switched to the sort processing mode during warming-up immediately after start-up and may be switched to the software processing mode when the warming-up is completed. The processing mode may be switched according to the degree of the processing load of the microprocessor  150 . In more detail, the processing mode may be switched to the sort processing mode when the usage rate of the microprocessor  150  is equal to or greater than 80% and may be switched to the software processing mode when the usage rate of the microprocessor  150  is less than 80%. 
     D3. Modified Example 3 
     Although the processing mode is switched according to the operation status of the printer  101  in the second embodiment, the invention is not limited to this and the processing mode may be switched according to a predetermined parameter specified by the personal computer  10 . For example, when the printer  101  has a high quality print mode for performing a high-quality printing process and a high speed print mode for performing a high-speed printing process with low quality as a print mode, the processing mode may be switched by the print mode specified by the personal computer  10 . In more detail, when the high quality print mode is specified by the personal computer  10  and the print data is received from the personal computer  10 , the processing mode is switched to the sort processing mode and, when the high speed print mode is specified by the personal computer  10  and the print data is received from the personal computer  10 , the processing mode is switched to the software processing mode. The reason why the processing mode is switched according to the print mode is as follows. That is, in the high quality print mode, since a large amount of print data is transmitted from the personal computer  10  to the printer  101 , the microprocessor  150  is susceptible to a high load status in order to process the large amount of print data. Accordingly, in this case, the processing mode is switched to the sort processing mode to perform the process of a portion of the D4 packets such that the microprocessor  150  is suppressed from becoming the high load status. The specifying of the print mode is described in a top of the print data received from the personal computer  10  and the printer  101  analyzes the print data to identify the specified print mode. 
     In general, in the printer, the cancel of a print job may be specified by receiving a print data packet in which a specific value is stored in a control information field. When such a print data packet is received, the processing mode may be switched from the sort processing mode to the software processing mode. 
     D4. Modified Example 4 
     Although the software process (the process using the D4 protocol processing unit  151 ) is performed with respect to all the D4 packets in the software processing mode in the second embodiment, the invention is not limited to this and the hardware process may be switched to the software process with respect to a portion of the D4 packets. For example, in the software processing mode, the hardware process may be switched to the software process with respect to the credit request packet and the hardware process may be performed with respect to the print data packet regardless of the processing mode. Even by this configuration, it is possible to notify the personal computer  10  of the credit number according to the operation status of the printer. 
     D5. Modified Example 5 
     Although the credit number transmitted to the personal computer  10  is changed according to the operation status of the printer  101  or  102  in the second embodiment or the third embodiment, the invention is not limited to this and an interval of transmitting the credit number may be changed instead of the credit number. In more detail, when the operation status of the printer is the error status, an interval from a time point when the credit request packet is received to a time point when the credit request reply packet is transmitted may be larger than that of the normal status. In the error state, since an interval of transmitting the print data is large in the personal computer  10 , it is possible to prevent a large amount of print data from being continuously received in a state in which the printing process cannot be performed. 
     D6. Modified Example 6 
     Although the logic packets processed in the packet reception process are the D4 packets in the embodiments, the invention is not limited to this. The above-described packet reception process may be performed with respect to any logic packet if the logic packets are plural types of logic packets which are transmitted/received between the personal computer and the printer.