HP Color LaserJet 5500 Technical Reference Manual page 210

Application Design
The previous pages have described various printing system
topologies and how components in those topologies can
Ideas
operate so each printer system component can receive
printer status readback responses. This section discusses
application design ideas that handle the complexities
involved with using printer status readback information.
The obvious design approach of querying the printer and
waiting for an answer to that query is sure to lead to
failure. A busy printer, old printer responses, overflowed
printer response buffer, and a uni-directional printing
environment all cause the obvious design approach to be
inadequate. A better design approach is to treat printer
responses as events that cause the application to take some
action. Solicited and unsolicited printer status readback
queries/responses are discussed separately.
Whenever the application sends a solicited query to the
printer, the application should add the query to the
"outstanding solicited query" list. Also, if the application
can not proceed until the response is received, then a
watchdog timer should be set so that some event will be
guaranteed to occur in case the application does not receive
a timely response. Whenever a solicited response is
received, the response should be compared to the list of
outstanding solicited queries. If there is no match, then the
solicited response should be discarded. If there is a match,
then the query should be removed from the outstanding
solicited query list, any associated watchdog timer should
be disabled, and the application should process the
response. If the application was blocked waiting for the
response, then the application should become unblocked.
If the application does not need an unsolicited status
response, then the application can discard the response.
If the application ever receives the unsolicited PJL "printer
to host data buffer overflow" message, then the application
should resend all queries in the "outstanding solicited
query" list. When the application resends the outstanding
solicited queries, it is possible that the application will
receive two responses (one generated because of the initial
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