Patent Publication Number: US-6665088-B1

Title: Page printer and page print system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a page printer used with a computer system, etc., and in particular to control of the operation of storing image data in buffer memory, reading the image data from the buffer memory, and transferring the image data to a print engine. 
     The present application is based on Japanese Patent Applications No. Hei. 10-275074 and No. Hei. 11-97997, which are incorporated herein by reference. 
     2. Description of the Related Art 
     In a page printer, it is a general rule that the speed at which image data is expanded into a bit map and written into buffer memory differs from the speed at which image data is read from the buffer memory and is transferred to a print engine. Often, buffer memory has a small capacity and cannot store one page of image data. Thus, particularly for a complicated image of a large amount of data, image data expansion and write into the buffer memory is too late for the transfer timing to the print engine and an error in which data to be read from the buffer memory runs out during printing the page and the printing cannot be continued may occur (the error is referred to as an underrun error in the specification). In contrast, image data expansion and write into the buffer memory is too fast and the buffer memory becomes full, then an error in which the subsequent data cannot be written may occur (the error is referred to as an overrun error in the specification). 
     To solve this problem, in a page printer in Japanese Patent No. 2663861, a match between the buffer memory write address and read address is detected, whereby error occurrence is detected and the host is informed of the error occurrence, then informs the user of the error occurrence so that the user can take proper steps. 
     However, the advantage of the related art described above is only to inform the user of error occurrence when an error occurs for taking proper steps. For the user, it is ideal that no error occurs and print always succeeds. It is more desirable that if an error occurs, the system automatically takes proper steps for handling the error. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to prevent an overrun error from occurring. 
     It is another object of the invention to prevent an underrun error from occurring. 
     It is still another object of the invention to automatically take proper steps and retry printing, thereby causing the retried printing to succeed if an underrun error occurs. 
     According to a first aspect of the invention, there is provided a page printer comprising memory containing a buffer, an image output section for forming an image based on data and outputting the image, a reception section for writing data received from a host into the buffer, a transfer section for transferring the data in the buffer to the image output section, and a control section for controlling the reception section and the transfer section. The control section permits the transfer section to transfer the data in the buffer after at least a part as much as a predetermined amount P, of one page of data from the host is stored in the buffer. The predetermined amount P is equal to or less than the size of the buffer and is set so that the predicted point in time at which the transfer section will complete transferring of all of one page of data from the buffer becomes later than the predicted point in time at which the reception section will complete writing of all of the page of data into the buffer. 
     According to the page printer, the transfer start timing is controlled so that write of received data into the buffer always precedes transfer of the data from the buffer in the process of reception and transfer of one page of data, thus an underrun error is prevented. 
     Preferably, just before reception of each page is started, the predetermined amount P is determined so as to substantially satisfy the relation 
     
       
           M≧P&gt;T (α−( x/y )) 
       
     
     where x is predicted data reception speed from the host, y is predicted data transfer speed to the image output section, α is a predicted compression rate of data from the host, T is the total number of data pieces of one page, and M is the buffer size. 
     Preferably, the predetermined amount P is determined by the printer itself. Preferably, the host determines the predetermined amount P and specifies the amount for the printer. When the host determines the predetermined amount P, the predetermined amount P is easy to set appropriately because generally the CPU of the host is high performance. 
     Preferably, if the buffer has an available capacity reduced to a predetermined minimum amount even before data as much as the predetermined amount P is stored in the buffer, the printer spontaneously starts transferring the data in the buffer, whereby if the predetermined amount P is not appropriate, an event in which the buffer becomes full is avoided. 
     According to a second aspect of the invention, there is provided a page printer comprising memory containing a buffer, an image output section for forming an image based on data and outputting the image, a reception section for writing data received from a host into the buffer, a transfer section for transferring the data in the buffer to the image output section, and a control section for controlling the reception section and the transfer section. The control section permits the transfer section to transfer the data in the buffer at a predetermined start timing. The start timing is set variably in response to a parameter affecting an increase or decrease in the data in the buffer. For example, the parameters affecting an increase or decrease in the data in the buffer are predicted data reception speed from the host, predicted data transfer speed to the image output section, a predicted compression rate of data from the host, the total number of data pieces of one page, the buffer size, etc. The start timing can be set in response to at least one of the parameters. Alternatively, the start timing can be set in response to different information substantially corresponding to the predicted data reception speed from the host, for example, the communication port, communication mode, etc., used with communication between the host and the printer. 
     According to the page printer, appropriate start timing such that no underrun error occurs and that the buffer does not become full, for example, for each page can be set, for example, for each page. 
     Preferably, the start timing is applied when at least a part as much as a predetermined amount P, of one page of data from the host is stored in the buffer, and the predetermined amount P is set so as to substantially satisfy 
     
       
           M≧P&gt;T (α−( x/y )) 
       
     
     where x is predicted data reception speed from the host, y is predicted data transfer speed to the image output section, α is a predicted compression rate of data from the host, T is the total number of data pieces of one page, and M is the buffer size. 
     According to a third aspect of the invention, there is provided a page printer comprising memory containing a buffer; an image output section for forming an image based on data and outputting the image, a reception section for writing data received from a host into the buffer, a transfer section for transferring the data in the buffer to the image output section, an error detection section for detecting an underrun error caused by the fact that a read address of the transfer section from the buffer catches up with a write address of the reception section into the buffer, and an error notification section for notifying the host of occurrence of the underrun error. When the host is notified of underrun error occurrence from the page printer, it resends the data to the page printer under a condition where the underrun error is harder to occur. 
     When the underrun error occurs, the data is automatically resent under a condition where the underrun error is harder to occur (the process is repeated stepwise depending on the situation), whereby retry printing can be made to succeed as the data is resent without causing an underrun error to occur. Although the print image quality is somewhat degraded by dropping the resolution, print can be reliably made to succeed without troubling the user. 
     The condition where the underrun error is harder to occur is, for example, to delay the current transfer start timing of the data in the buffer by the transfer section or to make the current data resolution lower. Preferably, when an underrun error occurs, first the transfer start timing of the data in the buffer by the transfer section is set to a later timing and the data is resent. If an underrun error occurs still after the start timing is delayed, then the resolution is lowered and the data with the resolution lowered is resent. 
     Preferably, when detecting an underrun error, the printer temporarily stores occurrence information of the underrun error in the memory without immediately notifying the host of the underrun error. Upon reception of a request from the host, the printer transmits the underrun error occurrence information to the host. The host requests the printer to send error information at printer&#39;s convenience, for example, each time the host attempts to transmit one-band data. This method enables host processing to be simplified as compared with the method wherein the host is informed of an underrun error immediately when the underrun error is detected. 
     According to a fourth aspect of the invention, there is provided a page printer comprising memory containing a buffer, an image output section for forming an image based on data and outputting the image, a reception section for writing data received from a host into the buffer, a transfer section for transferring the data in the buffer to the image output section, and an available buffer capacity notification section for notifying the host of information concerning the available capacity of the buffer. The host transmits data of a size that can be received at the printer to the printer based on the notification from the page printer, whereby an overrun error is prevented. 
     Preferably, the host sends data compressed in predetermined units, for example, band units or data not compressed to the printer and gets the available buffer capacity from the printer just before transmission of each band data is started. If the available buffer capacity is equal to or larger than the size of the band data, the host starts transmitting the band data; if the available buffer capacity is less than the size of the band data, the host waits until the available buffer capacity becomes equal to or larger than the size of the band data before the host starts transmitting the band data. 
     Preferably, the page printer further has a function of detecting a printer error such as an underrun error and various types of printer status such as print success of each page. In this case, the printer once stores information of the detection result in the memory and transmits the information in the memory upon reception of a request from the host. Just before the host attempts to start transmitting each band data, it sends a request to the printer for getting the printer status. 
     According to a fifth aspect of the invention, there is provided a page printer comprising memory containing a buffer, an image output section for forming an image based on data and outputting the image, a reception DMA section for receiving data in a reception data size unit from a host and writing the data into the buffer by DMA, a transfer DMA section for transferring the data in the buffer to the image output section in a transfer data unit by DMA, and a control section for controlling the reception DMA section and the transfer DMA section. Here, the reception data size unit is the same as the transfer data size unit, for example, one band. The control section checks whether or not an underrun error occurs based on the transfer DMA start address of the transfer DMA section and the current reception DMA address of the reception DMA section whenever the transfer DMA section attempts to start data transfer in the transfer data size unit. If the underrun error is detected, the control section notifies the host of occurrence of the underrun error. 
     Thus, the presence or absence of an underrun error is detected at an appropriate timing not too frequent and the host can be notified of the underrun error if the error occurs. 
     Preferably, when detecting an underrun error, the printer once stores information concerning the underrun error in the memory and sends the information to the host upon reception of a request from the host. Just before the host attempts to start transmitting data in each reception data size unit, for example, each band, it sends a request to the printer for getting the error information. 
     The host of the invention typically is a computer and a computer program for use with the host (computer), for example, a printer driver program can be installed in or loaded into the computer through various media such as disk-type storage, semiconductor memory, and a communication network. 
    
    
     Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a block diagram to show the configuration of a first embodiment of the invention; 
     FIG. 2 is a flowchart of control performed by a host  1  for each page when the host  1  sends image data to a printer  3 ; 
     FIG. 3 is a flowchart of control of a printer CPU  23  when the printer  3  receives data from the host  1 ; 
     FIG. 4 is a flowchart of control of one-band data reception; 
     FIG. 5 is a flowchart of control of video transfer; FIG. 6 is a memory map of DRAM  21 ; 
     FIGS. 7A and 7B are drawings to show two states of a reception buffer  43 ; 
     FIG. 8 is diagram to show the configuration of a circuit (or software) used for calculating an available buffer capacity; 
     FIG. 9 is a flowchart of control performed by a host  1  for each page when the host  1  sends image data to a printer  3  in a second embodiment of the invention; 
     FIG. 10 is a flowchart of error handling of the host in the second embodiment of the invention; 
     FIG. 11 is a flowchart of control of a printer CPU  23  when the printer  3  receives data from the host  1  in the second embodiment of the invention; and 
     FIG. 12 is a flowchart of control of video transfer in the second embodiment of the invention, 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows the configuration of a first embodiment of the invention. 
     A page printer  3  of the embodiment is a host-based printer for receiving image data expanded into a bit map in a host  1  from the host  1  and printing the data. The printer  3  is connected to the host  1  via a dedicated interface such as a parallel interface or a network such as a LAN (local area network) so that the printer  3  can communicate with the host  1  bidirectionally. That is, the printer  3  can receive image data expanded into a bit map from the host  1  and can transmit various types of status of the printer, such as available buffer capacity, error information, and print success information, to the host  1 . A reception buffer for storing the image data received from the host  1  is provided with a fixed or variable capacity in DRAM (dynamic random access memory)  21  in the printer  3 . The reception buffer has a capacity smaller than the size of one page of image data. A work area of a CPU (central processing unit)  23  described later is also provided in the DRAM  21  and status information to be sent to the host  1  is stored in the work area. 
     The printer  3  has a print engine  27  of a mechanism for executing an electrophotographic process and a series of processing circuits for receiving image data from the host  1 , applies necessary processing to the received image, and passes the resultant data to the print engine  27 , namely, an interface circuit  11 , a DMA (dynamic memory access) controller  13 , a data decompression circuit  15 , a video controller  17 , and a postprocessing circuit  19 . The processing circuits  11 ,  13 ,  15 ,  17 , and  19  are formed of dedicated hardware logic circuitry. Further, the printer  3  has a microcomputer consisting of the above-mentioned CPU  23 , ROM (read-only memory)  25  storing a program and fixed data, the above-described DRAM, etc., for interpreting a request received from the host  1 , detecting an error, and managing the printer status. The functions of the components are as follows: 
     The interface circuit  11  performs control of bidirectional communication with the host  1 , such as receiving a request and image data from the host  1  and transmitting status information to the host  1 . The request received at the interface circuit  11  is read directly by the CPU  23 . The CPU  23  interprets the request and executes control based on the request. The DMA controller  13  executes DMA for writing and reading image data into and from the reception buffer in the DRAM  21 . The input and output paths of image data to and from the DRAM  21  by DMA are two channels of reception DMA  31  and transfer DMA  33 . The image data from the host  1  is written into the DRAM  21  through the reception DMA  31 . The image data is transferred from the DRAM  21  to the data decompression circuit  15  through the transfer DMA  33 . 
     If the image data from the host  1  is compressed, the data decompression circuit  15  decompresses the compressed image data to the original data and then passes the data to the video controller  17  at the following stage. If the image data from the host  1  is not compressed, the data decompression circuit  15  passes the data to the video controller  17  as it is. The video controller  17  controls the timing of image data transfer to the print engine  27 , called video transfer. 
     The postprocessing circuit  19  applies postprocessing of resolution conversion for matching the resolution of image data with that of the print engine  27 , edge smoothing for smoothing edges of characters, etc., gradation control for adjusting a gradation value considering a gamma characteristic, etc., and the like to the image data and sends the resultant data to the print engine  27 . For example, when the print engine  27  has a resolution of 600 dpi, if the data received from the host is 300 dpi, the resolution conversion function is to convert the resolution 300 dpi into the same resolution of the print engine  27 , 600 dpi. As described later, for example, if the host  1  first sends image data with 600 dpi and an underrun error occurs and the print results in a failure, then the resolution conversion function makes it possible to drop the resolution of the image data from the host  1  to 300 dpi and retry printing. 
     The CPU  23  sends printer information (fixed state information to some extent in one print job, such as communication mode and RAM size) to the host  1  upon reception of a request from the host  1  at the print starting time, gets the current status of the printer (state information varying from time to time, such as engine state, available buffer capacity, error occurring, and print success or failure for each page) in real time and records the status in the work area in the DRAM  21  during the printing, and sends the status to the host in response to a request from the host  1 . As a function particularly related directly to the invention among the functions, the CPU  23  calculates an available capacity of the reception buffer in the DRAM  21  based on the write address of the DMA controller  13  into the DRAM  21  (reception DMA address) and the read address (transfer DMA address) and records the available capacity in the work area in the DRAM  21 , then sends the available reception buffer capacity to the host  1  in response to a request from the host  1 . 
     Next, the operation in the described configuration is as follows: 
     FIG. 2 shows a flow of control performed by the host  1  for each page when the host  1  sends image data to the printer  3 . 
     First, the host  1  determines whether the page to be printed is the first page or the second or later page at step  1 . If the page is the first page, the host  1  gets printer information, such as communication mode (communication mode established between the printer  3  and the host  1 , for example, Compatibility or ECP if parallel communication is executed) and DRAM size (if the printer information is not ready, information indicating “preparation”) at step S 2 , and grasps the limitations on the printer  3  based on the gotten information. For the second or later page, printer information is not gotten. In addition to the first page, the host  1  also gets printer information after a print error occurs at step S 6 , and checks whether or not the printer  3  is recovered from the error. 
     Upon reception of printer information from the printer  3 , the host  1  determines whether the printer information is valid or invalid, namely, is valid printer information indicating the communication mode, the DRAM size, etc., or invalid printer information indicating preparation at step S 3 . If the printer information is preparation (invalid), the host  1  reads printer information repeatedly until reception of valid printer information. The possible reason why the printer information becomes preparation (invalid) is that the printer  3  is being initialized or is being recovered from an error. 
     When the host  1  gets valid printer information, it determines the image data transmission mode of resolution, etc., based on the communication mode and DRAM size of the printer  3 . Next, the host  1  transmits host information concerning the image data to be printed, such as image data resolution and the total number of bands, to the printer  3  at step S 4 . Upon reception of the host information, the printer  3  performs register setting and page setting of buffer clear, etc., based on the host information. 
     Next, the host  1  receives printer status, such as engine state, available buffer capacity, error occurring, and print success or failure for each page, from the printer  3  at step S 5  and checks whether or not the printer  3  can receive one band of image data based on the received printer status at steps S 6  and S 8 . That is, the host  1  determines that the printer  3  can receive one band of image data if the following conditions (1) to (4) are satisfied: 
     (1) The print engine  27  can perform the print operation. That is, the print engine  27  can receive data, a print-impossible state, such as no paper, paper jam, cover open, or engine error, is not entered, and the fuser temperature is a specified temperature. 
     (2) Page setting of printer  3  is complete. 
     (3) The reception buffer has an available capacity of one band or more. 
     (4) No error occurs. 
     The printer  3  sets a print success/failure flag each time print of one page is complete. The host  1  sees the print success/failure flag contained in the printer status gotten from the printer  3  at step S 6 . If the flag indicates “success,” the host  1  deletes the page of the image data from the host  1 ; if the flag indicates “failure,” the host  1  returns to step S 1  and retransmits the image data starting at the failing page. To retransmit the image data, if the print failure is caused by an underrun error, the host  1  converts the image data resolution into a lower value (for example, from 600 dpi to 300 dpi) at step S 7  and retransmits the image data. Since the data size is lessened by dropping the resolution, an underrun error becomes hard to occur when the image data is retransmitted as described later in detail. 
     The host  1  transmits one band of image data at step S 9  only if it determines that the printer  3  can receive one band of image data at step S 8 , whereby an overrun error is avoided. Before transmitting band image data, the host  1  determines whether or not the band data is reduced in size if the band data is compressed. If the band data is reduced in size, the host compresses the data; if the band data is not reduced in size, the host does not compress the data. Before transmitting band image data, the host  1  informs the printer  3  whether or not the band data is compressed data (compression ON/OFF) and of the compressed band data size (compression ON) or the original band data size (compression OFF). The printer  3  stores the band image data in the reception buffer in the DRAM  21  and starts video transfer upon completion of storing a predetermined number of bands of data. 
     Next, the host  1  checks whether or not all one page of band data has been transmitted at step S 10 . If not all one page has yet transmitted, the host  1  returns to step S 5 . Thus, the host  1  transmits image data to the printer  3  in band units and checks the printer status each time before transmission to see if the printer  3  can receive one band of data. Therefore, when the printer  3  has an available buffer capacity of at least one band, it receives data of one-band size at the most (one band in compression ON; less than one band in compression OFF), whereby an overrun error is avoided as described above. 
     FIG. 3 shows an operation flow of the printer  3  when the printer  3  receives data from the host  1 . 
     First, if the host  1  makes a request to send printer information (YES at step S 1 ), the CPU  23  of the printer  3  transmits printer information of the communication mode, DRAM size, etc., to the host  1  at step S 13 . If the printer information is not ready (NO at step S 12 ), the CPU  23  of the printer  3  informs the host  1  that the printer is in preparation (specifically, the printer is being initialized or is being recovered from an error) at step S 14 . 
     After transmitting the printer information to the host  1 , the CPU  23  of the printer  3  receives host information concerning image data (resolution 300 or 600 dpi and the total number of bands) from the host  1  and performs register setting concerning data reception and page setting of buffer clear, etc., based on the received host information at step S 15 . 
     Subsequently, a process to receive one band of image data from the host  1  is entered at step S 16 . FIG. 4 shows a control flow of the one-band data reception. 
     As shown in FIG. 4, when a printer status request comes from the host  1  (YES at step S 21 ), the CPU  23  sends the current printer status to the host  1  at steps S 22 , S 23 , and S 24 . Specifically, if a print error occurs at present, the CPU  23  sends the error status of the print error type, etc., to the host  1  at step S 23  and returns to step S 11  in FIG.  3 . If no error occurs, the CPU  23  sends status information, such as available buffer capacity, engine state, end of page setting, and page print success or failure, to the host  1  at step S 24 . The available buffer capacity is calculated from the transfer DAM address and the reception DMA address of the DMA controller  13  as described above. The host  1  determines whether or not the printer  3  can receive one band of data based on the status information and if the host  1  determines that the printer  1  can receive one band of data, it sends band information of compression ON/OFF, band compression size, etc., followed by the band of image data to the printer, as described above. 
     The CPU  23  of the printer  3  sets parameters concerning reception DMA of the DMA controller  13 , such as the start point of the reception DMA address and the number of bytes of band data, based on the band information of compression ON/OFF, band compression size, etc., at step S 26  and permits the DMA controller  13  to receive data at step S 27 . Then, the DMA controller  13  receives one band of image data on the path of the reception DMA  31  from the host  1  and writes the data into the reception buffer in the DRAM  21 . If band information does not come from the host  1  (NO at step S 25 ), control returns to step S 21 . 
     The DMA controller  13 , which starts data reception, receives the image data as much as the number of bytes set at step S 26 , namely, one band of image data. Upon completion of receiving the data, the DMA controller  13  informs the CPU  23  of the fact, and control of the CPU  23  goes to step S 17  shown in FIG.  3 . 
     The CPU  23  checks whether or not image data as much as a predetermined number of bands is stored in the reception buffer in the DRAM  21  at step S 17 . If data as much as the predetermined number of bands is not stored, the CPU  23  again returns to step S 16  and repeats reception of subsequent band data. If data as much as the predetermined number of bands is stored, the CPU  23  permits the DMA controller  13  to execute video transfer at step S 18 . Then, video transfer is started. That is, the DMA controller  13  starts the operation of reading image data from the DRAM  21  and transferring the read data to the data decompression circuit  15  and the data decompression circuit  15 , the video controller  17 , and the postprocessing circuit  19  also start their respective processing, then the print engine  27  starts printing. Still after the video transfer is started, data reception in band units from the host  1  is continued likewise at step S 19  (details are as shown in FIG.  4 ). Upon completion of one page of data (YES at step S 20 ), control returns to step S 11  and data reception of the next page is started in a similar sequence. 
     The “predetermined number of bands” determined at step S 17  is determined by the CPU  23  based on the following principle for the purpose of preventing an underrun error: 
     Let the data reception speed from the host  1  be x [bytes/sec], the video transfer speed be y [bytes/sec], data compression ratio be a, the total number of data pieces of one page be T [bytes], the reception buffer size be M [bytes], and the size of data stored in the reception buffer at the time at which video transfer is started be P [bytes]. Normally, the video transfer speed y is higher than the data reception speed x and if the transfer DMA address of the high-speed video transfer catches up with the reception DMA address of the low-speed data reception, an underrun error occurs. However, if P is set to as satisfy 
     
       
           P≦M   (1) 
       
     
     
       
         (α XT−P )/ x&lt;T/y   (2) 
       
     
     reception of one page of data is complete before the transfer DMA address of the video transfer catches up with the reception DMA address of the data reception, thus an underrun error does not occur. From expressions (1) and (2), 
     
       
           M≧P&gt;T (α− x/y )  (3) 
       
     
     is derived. If P is set so as to satisfy this expression (3), an underrun error does not occur. In the embodiment, the predetermined number of bands rather than bytes is adopted, thus the minimum P [bytes] satisfying expression (3) is divided by the compression band size [bytes] and one is added to the quotient of the division, then the result may be set as the “predetermined number of bands.” 
     At the instant at which data as much as the “predetermined number of bands” has been stored in the reception buffer, the video transfer is started, whereby an underrun error can be avoided. In addition, starting of the video transfer is delayed only necessary minimum, thus the print speed is also high. 
     If the “predetermined number of bands” is determined at the beginning of a page, there appears a possibility that an underrun error will occur if the compression rate α lowers or the data reception speed from the host  1  lowers. In this case, it is necessary to set the buffer size M larger or drop the image resolution to lessen the total data size of one page T. Particularly, the resolution conversion effect is large; for example, if the resolution is dropped from 600 dpi to 300 dpi, the data size becomes a quarter. Therefore, when data is retransmitted after an underrun error once occurs, execution of resolution conversion (at step S 7  in FIG. 2) produces a large effect of preventing an underrun error from again occurring. Although the print image quality somewhat worsens by executing resolution conversion, it is more preferred for the user than the case where print is impossible. 
     FIG. 5 shows a flow of control performed by the CPU  23  for video transfer (S 18 ). 
     The CPU  23  checks whether or not video transfer is executable (namely, image data is already received as much as the predetermined number of bands or more) at step S 31 . If image data is already received, the CPU  23  performs register setting concerning video transfer based on the host information concerning the image data, such as resolution and the total number of bands, at step S 32 . Next, the CPU  23  sets parameters concerning transfer DMA of the DMA controller  13 , such as the start point of the transfer DMA address and the number of bytes of band data, and parameters of the data decompression circuit  15 , such as expansion ON, OFF, etc., based on the band information of the compression:band size, etc. When setting the start point of the transfer DMA address the CPU  23  compares the start point of the reception DMA address of the current DMA being executed (or to be started) with the start point of the transfer DMA address to be set at step S 33 . If both match, the CPU  23  determines that an underrun error occurs, and sets an underrun error flag in the work area in the DRAM  21  at step S 34 . The host  1  is informed of the underrun error by reading the printer status before transmission of each band as described above (step S 5  in FIG.  2 ). At the instant at which the host  1  reads the printer status, the underrun error flag is reset. As described above, if the host  1  detects an underrun error, it drops the resolution of the error occurring image data and retransmits the data starting at the first band, then also transmits the subsequent bands of image data with the resolution dropped. 
     If an underrun error does not occur, the CPU  23  of the printer  3  permits the DMA controller  13  to execute one-band transfer DMA at step S 35 . Whenever one-band transfer DMA terminates, the DMA controller  13  informs the CPU  23  of the fact. Then, the CPU  23  returns to step S 33 . One-band transfer DMA setting and one-band transfer DMA execution are thus repeated until one-page transfer is complete. Upon completion of transferring one page (YES at step S 36 ), the CPU  23  sets a print success flag in the work area in the DRAM  21  at step S 37 . When the host  1  reads the printer status, it is informed of the print success flag. At the instant at which the host  1  reads the printer status, the print success flag is reset. As described above, if the host  1  receives the print success flag, it deletes the page of the image data. 
     Upon completion of the one-page video transfer, control returns to step S 31  and video transfer of the next page is started in a similar sequence. If the image data compression rate is high or the reception buffer size is sufficiently large, more than one page of image data is stored in the reception buffer  43  and is printed successively at the maximum throughput of the print engine  27 . 
     FIG. 6 shows the structure of the DRAM  21 . 
     As previously described, the work area  41  and the reception buffer  43  are provided in the DRAM  21 . The work area  41  is used as heap memory and stack memory. In the example shown in the figure, of all area of the DRAM  21 , the area of buffer top address BUFTOP to buffer end address BUFBTM provides the reception buffer  43 . In reception DMA, address point (reception DMA address) RAD is advanced in the direction from the buffer top address BUFTOP to the buffer end address BUFBTM and when the address point RAD reaches the buffer end address BUFBTM, it returns to the buffer top address BUFTOP. Likewise, also in transfer DMA, address point (transfer DMA address) TAD is advanced in the direction from the buffer top address BUFTOP to the buffer end address BUFBTM and when the address point TAD reaches the buffer end address BUFBTM, it returns to the buffer top address BUFTOP. Thus, the reception buffer  43  is used as a ring buffer. 
     The example shown in the figure shows a state in which B page of data  53  is being stored in the reception buffer  43  and video transfer of A page of data  51  preceding the B page is about to start. Each page of data  51 ,  53  contains the host information concerning each page image, such as resolution and the total number of bands, information on each band, such as compression ON, OFF and compression band size, each band of image data, and the like. 
     In the embodiment, as described above, to prevent an overrun error from occurring, the host  1  transmits data in band units and the printer  3  calculates the available capacity of the reception buffer  43  each time before transmission of the data and informs the host  1  of the available capacity. The host  1  executes transmission only if the available capacity is one band or more. A method of calculating the available capacity is as follows: 
     FIGS. 7A and 7B show two states of the reception buffer  43 . That is, FIG. 7A shows the state of “reception DMA address RAD&gt;transfer DMA address TAD and FIG. 7B shows the state of “reception DMA address RAD&lt;transfer DMA address TAD.” Reception data is stored in the hatched areas and the areas described as “available” is available areas. 
     When “reception DMA address RAD&gt;transfer DMA address TAD” shown in FIG.  7 A, 
     
       
         available capacity=( TAD−BUFTOP )+( BUFBTM−RAD ) 
       
     
     and when “reception DMA address RAD&lt;transfer DMA address TAD” shown in FIG.  7 B, 
     
       
         available capacity= TAD−RAD   
       
     
     FIG. 8 shows a circuit (or a software processing flow) used for calculating the available capacity. 
     In FIG. 8, TADST denotes the start point of the transfer DMA address and RADST denotes the start point of the reception DMA address; as previously described, TADST and RADST are set by the CPU  23  when transfer DMA of each band and reception DMA of each band are started. TADCNT denotes a counter indicating the address of the reception buffer to execute transfer DMA and RADCNT denotes a counter indicating the address of the reception buffer to execute reception DMA. 
     While transfer DMA of each band is executed, a transfer DMA address counter  61  uses the transfer DMA start address TADST as the initial value of the output value and advances the output value one by one so as to circulate from the reception buffer top address BUFTOP to the reception buffer end address BUFBTM like a ring. The output value of the transfer DMA address counter  61  is the transfer DMA address TAD. 
     Likewise, while reception DMA of each band is executed, a reception DMA address counter  63  uses the reception DMA start address RADST as the initial value of the output value and advances the output value one by one so as to circulate from the reception buffer top address BUFTOP to the reception buffer end address BUFBTM like a ring. The output value of the reception DMA address counter  63  is the reception DMA address RAD. 
     A comparator  65  compares the transfer DMA address TAD with the reception DMA address RAD and sets an address comparison flag ADCMPR indicating TAD or RAD, whichever is the greater. Subtractors  67 ,  69 , and  71  calculate “TAD−BUFTOP=TSUBTP,” “TAD−RAD=TSUBR,” and “BUFBTM−RAD=BTSUBR” respectively. 
     The CPU  23  sees the address comparison flag ADCMPR. When “reception DMA address RAD&gt;transfer DMA address TAD,” the CPU  23  sets 
     
       
         available capacity= TSUBTP+BTSUBR   
       
     
     When “reception DMA address RAD&lt;transfer DMA address TAD,” the CPU  23  sets 
     
       
         available capacity= TSUBR   
       
     
     Next, a second embodiment of the invention will be discussed. 
     The general configuration of the embodiment is similar to that shown in FIG.  1 . In the second embodiment, a host  1  determines the video transfer start timing and sends the determined start timing to a printer  3 . Generally, the host  1  has a high CPU processing capability as compared with the printer  3 , thus if the host  1  determines the start timing, the start timing can be set more accurately than that if the printer  3  determines the start timing. When the available capacity of a reception buffer is reduced to a predetermined amount (for example, one band) before the specified start timing comes from the host  1 , the printer  3  starts video transfer spontaneously at the point in time, thereby avoiding a situation in which the reception buffer becomes full and the printer  3  is stalled before the specified start timing comes from the host  1 . If the video transfer is started earlier than the specified start timing from the host  1 , actually printing can be executed in some cases, thus error occurrence can be minimized. When the video transfer start timing specified by the host  1  is timing earlier than reception completion of one page of data, if an underrun error occurs, the host  1  first changes the start timing to the timing following reception completion of one page of data and sends the new timing to the printer  3 , then resends the data. If an underrun error again occurs, then the host  1  resends the data with the resolution dropped (for example, from 600 dpi to 300 dpi) to the printer  3 . The reception buffer of the printer  3  need not have a capability of storing one page of the original high-resolution data, but has a capacity capable of storing one page of the data with the resolution dropped. 
     FIG. 9 is a flowchart of control performed by the host  1  for each page when the host  1  sends image data to the printer  3  in the second embodiment. 
     Here, only differences from the control flow of the host in the first embodiment previously described with reference to FIG. 2 will be discussed; parts not described are the same as those in the flow in FIG.  2 . 
     The host  1  transmits host information (namely, information concerning image data) to the printer  3  at step S 43 . The host information contains the video transfer start timing (what&#39;th band) in addition to the resolution of the image data (300 or 600 dpi) and the total number of bands of the image data. The host  1  determines the start timing so as to satisfy expression (3) previously described in the first embodiment, namely, the condition 
     
       
           M≧P&gt;T (α− x/y )  (3) 
       
     
     where x is the data reception speed from the host  1  [bytes/sec], y is the video transfer speed [bytes/sec], α is data compression ratio, T is the total number of data bytes of one page [bytes], M is the reception buffer size [bytes], and P is the size of data stored in the reception buffer at the time at which video transfer is started [bytes]. The data reception speed x varies with the type of image data transmission port (parallel, USB, Ethernet, etc.,) or the communication mode of the port (if parallel is applied, Compatibility, ECP, etc.,). Thus, when determining the start timing, the host  1  changes the start timing in response to the port type or the communication mode. For example, if the reception speed x is high as in parallel ECP, early start timing is set; if the reception speed x is not stable as in network communication, late start timing is set (for example, so as to start video transfer after reception of all of one page of data). The printer  3  previously sends the communication mode established between the host  1  and the printer  3  as printer information to the host  1  as required at step S 42  (for example, if the printer  3  does not inform the host  1  of parallel communication mode (Compatibility or ECP), the host  1  is not informed, thus the printer  3  informs the host  1  of the parallel communication mode). 
     The printer  3  performs register setting and page setting of buffer clear, etc., based on the host information sent from the host  1  at step S 44 . At the time, the start timing contained in the host information is also stored in a register in the printer  3 . After this, the printer  3  stores each page of data sent from the host  1 , and starts video transfer as a rule if the start timing specified from the host  1  for each page is reached (namely, upon reception of the number of bands as the start timing specified from the host  1 ). 
     The printer  3  sets a print success/failure flag each time print of one page is complete. The host  1  sees the print success/failure flag contained in the printer status gotten from the printer  3  at step S 46 . If the flag indicates “success,” the host  1  deletes the page of the image data from the host  1 ; if the flag indicates “failure,” the host  1  goes to error handling at step S 47  and retransmits the image data starting at the failing page. 
     FIG. 10 shows a flow of the error handling. 
     First, the error contents are checked at step S 51 . If the error cause is a paper jam, simply the data is retransmitted starting at the failing page. On the other hand, if the error cause is an underrun error, then the video transfer start timing specified for the printer  3  at the error occurrence time is checked at step S 52 . If a premature start (namely, start of video transfer before reception completion of all of one page of data) is specified, the start timing is changed to “start of video transfer after reception completion of all of one page of data” at step S 53 , the post-changed start timing is again specified for the printer  3  at step S 44  in FIG. 9, and the data is retransmitted starting at the failing page at step S 49  in FIG.  9 . If the printer is already instructed to “start video transfer after reception completion of all of one page of data” at the error occurrence time, the resolution is dropped (for example, from 600 dpi to 500 dpi), the failing page of data is again prepared from the beginning at step S 53 , the printer  3  is informed of the dropped resolution at step S 44  in FIG. 9, and as the failing page, the data with the resolution dropped is retransmitted at step S 49  in FIG.  9 . The data retransmission may be automatically executed by the host  1 . Alternatively, the user may be informed of error occurrence and be requested to specify whether or not the data is to be resent and specify whether or not the resolution is to be dropped if the data is to be resent. 
     FIG. 11 shows an operation flow of the printer  3  when the printer  3  receives data from the host  1 . 
     Here, only differences from the control flow of the printer in the first embodiment previously described with reference to FIG. 3 will be discussed; parts not described are the same as those in the flow in FIG.  3 . 
     At step S 65 , the printer  3  receives host information (resolution, and the total number of bands, video transfer timing, etc.,) from the host  1  and performs register setting concerning data reception and page setting of buffer clear, etc., based on the received host information. 
     Subsequently, the printer  3  enters a process for receiving one band of image data from the host  1  on a flow as shown in FIG. 4 at step S 66 . While receiving data in the reception buffer from the host  1 , the printer  3  checks whether or not as many bands as the number of bands as the start timing specified from the host  1  has been received and whether or not the available capacity of the reception buffer becomes less than one band at step S 67 . If reception of as many bands as the specified number of bands is complete, the printer  3  can start video transfer at step S 68 . If the number of bands specified from the host  1  is the total number of bands of one page, namely, the printer  3  is instructed to “start video transfer after reception completion of all of one page of data,” the printer  3  returns to step S 61  immediately after video transfer is started at step S 71 , and enters the data reception operation of the next page. 
     If there is a situation in which the data compression rate is poor unexpectedly or the original data is fairly larger than the reception buffer size, the available capacity of the reception buffer may becomes less than one band before as many bands as the number of bands specified from the host  1  is received. In this case, at the instant at which the available capacity of the reception buffer may becomes less than one band, the printer  3  starts video transfer. In this case, the probability that an underrun error may occur becomes high, but normal printing can be executed to no small extent because the start timing specified by the host  1  contains a sufficiently large margin, etc. 
     FIG. 12 shows a flow of control of video transfer performed by the printer  3 . 
     Here, only differences from the video transfer flow in the first embodiment previously described with reference to FIG. 5 will be discussed; parts not described are the same as those in the flow in FIG.  5 . 
     First, whether or not video transfer is executable is checked based on whether or not as many bands as the number of bands specified from the host  1  or more bands have been received at step S 71 . If as many bands as the specified number of bands or more bands have been received, one-band video transfer is executed in a sequence similar to that in the flow in FIG. 5 at steps S 72  to S 75 . The printer engine state is checked at step S 76 . If a paper jam occurs, a paper jam flag is set at step S 77 . If the host  1  detects a paper jam error, it resends the data without changing the parameter as shown in FIG.  10 . 
     While the preferred embodiments of the invention have been described, such description is for illustrative purposes only, and it is to be understood that the invention is not limited to the embodiments thereof. Therefore, many apparently widely different embodiments of the invention may be made without departing from the spirit and scope thereof.