HP Color LaserJet 5500 Technical Reference Manual page 204

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
I/O switching occurs when a printer contains more than one
I/O interface and accepts data from any interface, based on
Readback and
a first-come first-served basis. Some PJL printers do not
Printer I/O Switching
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
10-24 Programming Tips