HP Color LaserJet 5500 Technical Reference Manual page 209

know which I/O interface to use when sending solicited
printer responses. When the printer sharing device sends
data to the printer from a different I/O interface, the
printer sharing device must know to the exact byte
boundary what printer response data should be sent to
the previous I/O interface and what data should be sent
to the current I/O interface.
Before the printer sharing device sends the printer data
from a different I/O interface than the source of the cur-
rent print job, the printer sharing device can inject a PJL
ECHO command. All solicited printer responses received
before the PJL ECHO response should be sent to the pre-
viously active I/O interface. All solicited printer re-
sponses received after the PJL ECHO response should be
sent to the currently active I/O interface. The printer
sharing device should consume the PJL ECHO response
which was a result of the PJL ECHO command injected
by the printer sharing device.
Applications that use unsolicited PJL device, job, page, or
timed status need to work properly with printer sharing
devices that do not support unsolicited printer-to-host re-
sponses.
3. A printer sharing device that can route solicited re-
sponses as described in 2 above, plus send all unsolicited
responses to all attached host computers. Applications
must be designed to properly ignore unexpected printer
status readback responses. Ideas on how to design an ap-
plication to ignore unexpected printer status readback re-
sponses are described in the next section.
Programming Tips 10-29