HP Color LaserJet 5500 Technical Reference Manual page 206

received the last data byte. If the application queries the
printer and waits for a response, it is possible for I/O
interface idle time to exceed the set timeout value (the
TIMEOUT environment variable). When the printer detects
the current I/O interface timed out, the printer allows I/O
switching and treats any data received over the I/O
interface as unrelated to the data previously received over
that interface.
To keep the I/O interface from timing out, the application
can either send data to the printer more frequently than
the timeout setting or can cause the printer to use a
different setting by sending a PJL JOB command. When
I/O-switching printers receive the PJL JOB command, the
printer uses an I/O timeout value equal to ten times the
TIMEOUT environment variable setting or five minutes,
whichever is greater. (If the application sends the PJL JOB
command, the application should send a PJL EOJ
command at the end of the job.)
Old Printer Status
Applications need to be designed to handle receiving printer
status readback responses that were generated because of
Readback Responses
an action performed by a different application. For
example, a word processing application may send a query to
the printer, but because the user turned off the host
computer, the application never got a chance to retrieve the
response from the printer before the host computer was
turned off. When another application, or even the same
application at a later time, queries the printer, the first
response is the response to the word processing
application's query.
The currently executing application should accept old
printer status readback responses and discard them until
the expected response is received. Applications should
include unique information as part of the ECHO command,
such as the current time and date and a random number, so
that the application can resynchronize with printer status
readback responses.
10-26 Programming Tips