HP Color LaserJet 5500 Technical Reference Manual page 205

print jobs the printer may process before processing the
application's query.
When processing a job, I/O switching paces off all I/O
interfaces except for the one which is providing the current
print job. However, it is possible for an application to get a
few bytes of data into the inactive I/O interface before the
printer paces off that I/O interface. If the application times
out while trying to send the initial UEL command after one
or more bytes of the command are sent, but before the
entire UEL command is sent, then the application can
proceed using one of the following choices: 1) continue
attempting to send the data to the printer, 2) remember
how much of the initial UEL command has been sent to the
printer and attempt to send the rest of the command at a
later time, or 3) stop trying to send the command, making
sure the next command the application sends to the printer
is the UEL command. This is done so the parser inside the
printer properly recognizes all commands following the
previously sent partial UEL command. The printer will
properly recognize the UEL command, even if the
command appears after a partial UEL command.
For example, assume the initial query is :
<ESC>%-12345X@PJL ECHO 08/27/92 09:53:46.5
~033288925
If the printer only accepts "<ESC>%-12", and if the
application later sends:
<ESC>%-12345X@PJL ECHO 08/27/92 09:57:46.5
~6202323802 <LF>
The printer accepts the query and responds with:
@PJL ECHO 08/27/92 09:57:46.5 6202323802<LF>
<FF>
One of the conditions I/O-switching printers use to
determine when to switch to another I/O interface is the idle
time of the I/O interface supplying the current print job. Idle
time is defined as the time elapsed since the I/O interface
Programming Tips 10-25