HP 9500n HP PCL/PJL reference - Printer Job Language Technical Reference Manua - Page 218
Old Printer Status Readback Responses, Response Data Buffer Overflow
UPC - 808736429672
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To keep the I/O interface from timing out, the application can either send data to the printer more frequently than the timeout setting or can cause the printer to use a different setting by sending a PJL JOB command. When I/O-switching printers receive the PJL JOB command, the printer uses an I/O timeout value equal to ten times the TIMEOUT environment variable setting or five minutes, whichever is greater. (If the application sends the PJL JOB command, the application should send a PJL EOJ command at the end of the job.) Old Printer Status Readback Responses Applications need to be designed to handle receiving printer status readback responses that were generated because of an action performed by a different application. For example, a word processing application may send a query to the printer, but because the user turned off the host computer, the application never got a chance to retrieve the response from the printer before the host computer was turned off. When another application, or even the same application at a later time, queries the printer, the first response is the response to the word processing application's query. The currently executing application should accept old printer status readback responses and discard them until the expected response is received. Applications should include unique information as part of the ECHO command, such as the current time and date and a random number, so that the application can resynchronize with printer status readback responses. There is also a chance the PJL ECHO response will never by sent by the printer, due to overflowing of the printer's response data buffer as discussed in the following section. Response Data Buffer Overflow The printer contains a limited amount of memory. It is possible for an application to fill all available printer memory with printer status readback responses that have not yet been sent to the host computer, simply by sending many queries and never accepting the responses from the printer. Also, if an application enables unsolicited status and never accepts responses, the printer's response data buffer can overflow. The printer limits the amount of memory allotted to hold responses, and when that memory is full (and the host has not recently accepted any responses), the printer discards all future responses until the host accepts the currently queued responses in the printer. 11-24 Programming Tips for Using PJL EN