HP PCL/PJL reference - Printer Job Language Technical Reference Manual

11-26 Programming Tips for Using PJL EN
device to properly route since the unsolicited status response may be
received by the printer sharing box hours after sending the data from
the host computer to the printer that enabled unsolicited status.
Manufacturers of printer sharing devices can choose from the
following options to solve this printer-to-host data routing problem.
1 A printer sharing device that is a uni-directional device and does
not support printer-to-host data transfer. Applications need to be
designed to work with uni-directional printer sharing devices.
2 A printer sharing device that examines the data received from the
printer and only forwards solicited responses to the host
computer.
A printer sharing device can track which of its I/O interfaces is
currently sending data to the printer, and thus know which I/O
interface to use when sending solicited printer responses. When
the printer sharing device sends data to the printer from a
different I/O interface, the printer sharing device must know to the
exact byte boundary what printer response data should be sent to
the previous I/O interface and what data should be sent to the
current I/O interface.
Before the printer sharing device sends the printer data from a
different I/O interface than the source of the current print job, the
printer sharing device can inject a PJL ECHO command. All
solicited printer responses received before the PJL ECHO
response should be sent to the previously active I/O interface. All
solicited printer responses received after the PJL ECHO
response should be sent to the currently active I/O interface. The
printer sharing device should consume the PJL ECHO response
which was a result of the PJL ECHO command injected by the
printer sharing device.
Applications that use unsolicited PJL device, job, page, or timed
status need to work properly with printer sharing devices that do
not support unsolicited printer-to-host responses.
3 A printer sharing device that can route solicited responses as
described in 2 above, plus send all unsolicited responses to all
attached host computers. Applications must be designed to
properly ignore unexpected printer status readback responses.
Ideas on how to design an application to ignore unexpected
printer status readback responses are described in the next
section.