HP PCL/PJL reference (PCL 5 Printer Language) - Technical Reference Manual Part I

EN Status Readback Programming Hints 16-27
Status Readback Programming Hints
The following hints can assist in using the status readback feature.
z PCL status readback is useful during the development of
applications. Status readback allows you to determine that fonts,
macros, user-defined patterns, and symbol set resources you
have downloaded were accepted by the printer.
z If the printer does not contain sufficient memory to accept a
downloaded entity, the printer discards the data. Status readback
can be used to determine if the printer accepted a downloaded
entity.
z Status responses are directed to the printer’s I/O port from which
the request is received. If the status is not read and the printer
switches to another I/O port, the status response is not directed
to that port. The status response returns if the printer switches
back to the original port (see note on following page).
z Your application should work correctly when an unexpected
status response is received. For example, when requesting a
PCL status, it is possible that PJL could return an unsolicited
status response, if PJL is enabled.
• All PCL status responses begin with the “PCL” header and end
with the <FF> control code. When reading PCL status
responses, your application should be able to read all the data
between the “PCL” header and the <FF> control code. It
should ignore any other status response syntax.
• Lines within the PCL status response begin with a specific
keyword (those described in this chapter) and end with the
<CR> and <LF> control codes. Future printers may support
new keywords in the PCL status response. Your application
should be designed to ignore those lines which it does not
understand.
z The first PCL status readback command an application should
send is the PCL Echo command. A random number should be
generated for the value field each time the command is sent. Your
application should ignore all printer status readback data until the
PCL Echo status readback response is received, echoing the
number the application sent.
z To clear any possible unread status responses from previous
applications, an application, upon starting up, may want to read
any pending responses until they are cleared from the printer.