HP 1100 HP PCL/PJL reference - Printer Job Language Technical Reference Manual - Page 215
Printer Status Readback Usage, Determining If Printer Status Readback is Available
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Printer Status Readback Usage Applications use printer status readback for two primary purposes (although there are other uses): printer driver configuration and monitoring device status. Since printer configuration does not change often (i.e., the amount of installed printer memory does not change frequently) using printer status readback to assist in initial configuration (or when the printer's configuration changes) is straightforward. If the application determines printer status readback is unavailable (discussed in the "Determining If Printer Status Readback is Available" section), the application can ask the user for the information instead of querying the printer. The other primary use for printer status readback is to allow applications to monitor the state of the printer. In particular, applications can inform the user when the printer requires intervention (e.g. fill paper trays). If the application enables unsolicited PJL device status, the printer will inform the application whenever the printer's status changes. Since the printer informs the application using unsolicited status, even when the printer is processing a print job from a different I/O interface, the application gets timely printer status information. If printer status readback is unavailable, the application will not be able to provide the user with current printer state information. Another use for printer status readback is to determine what volatile resources (e.g. soft fonts) are available in the printer right before a print job is sent to the printer. If an application optimizes the printer job using printer status readback (i.e., does not send soft fonts if the printer already has the font), then if printer status readback is unavailable, the application should assume worst case (i.e., the font is not in the printer) when generating print jobs. In summary, the application should be designed to take advantage of printer status readback, if available, but still function if it is unavailable. Determining If Printer Status Readback is Available If the application sends a PJL ECHO command (preceded by the Universal Exit Language command, of course) to the printer and receives the expected response, then the application can assume bi-directional communication with the printer is available and supported solicited status queries will be answered in a timely fashion. EN Programming Tips for Using PJL 11-21