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

11-22 Programming Tips for Using PJL EN
However, under some conditions the application will send, or attempt
to send, a PJL ECHO command and not receive a timely response
because of one of two reasons: the printer may not be accepting data
(due to the printer processing a print job from a different I/O interface
or the printer being off-line), or the printing system does not support
printer status readback to the application. If the application does not
receive a response within three seconds, then the application can
assume printer status readback is currently unavailable.
In some printing environments, the application may be able to
distinguish between the host operating system not being able to send
the query, and the query being sent without receiving a response in a
reasonable amount of time. Application developers should not
assume that because a query was sent and a response was not
received in a timely fashion, that the printer system does not support
bi-directional communication. Printer sharing devices may buffer the
query and the response may be available, however the data buffered
before the query may take more than three seconds to process,
delaying the response by more than three seconds.
The application should notify the user that printer status readback is
currently unavailable and query the user to determine how to
proceed. The common choices are: 1) printer busy, continue retrying
until printer status readback is available, 2) printer busy on a long
printer job, temporarily do not use printer status readback, or 3)
printer status readback unavailable due to host-printer configuration,
do not use printer status readback. After obtaining the user's
response, the application can continue as appropriate.
Printer Status Readback and Printer I/O Switching
I/O switching occurs when a printer contains more than one I/O
interface and accepts data from any interface, based on a first-come
first-served basis. Some PJL printers do not support I/O switching.
I/O switching adds complexity to printing solutions taking advantage
of printer status readback. If the printer is currently processing a print
job from a different I/O interface, applications querying the printer will
have to wait for the printer to respond to the query, (or proceed
without using printer status readback). The amount of time until the
printer provides the response is dependent on the length of the
current print job and the length of any other print jobs the printer may
process before processing the application's query.