HP Color LaserJet 5500 Technical Reference Manual page 201

Host computers #2 and #3 are connected to the printer via
an external printer sharing box that buffers data. If the
printer sharing box was not designed to support
bi-directional communication with the printer, then
applications on host computers #2 and #3 will not be able to
receive printer status readback information. However, since
the printer sharing box is connected to the printer via the
Bi-Tronics I/O interface, which does support bi-directional
communication, it is possible for printer sharing boxes to be
designed to support printer status readback. In fact, the
printer sharing box itself may be able to inject a "print job"
to query the printer for specific information.
External printer sharing boxes can be designed to support
sending data from the printer to the host computer.
However, the printer sharing box has to determine which
host computer should receive the printer's response. The
simplest solution for printer sharing boxes is to forward all
data received from the printer to all attached host
computers, and let the applications on each host computer
ignore any impertinent data, as described in the "Handling
Unexpected Printer Responses" section in this chapter.
The printer in this example is also attached directly to a
network via a network interface card installed into the
printer's MIO interface. Network servers #4 and #5 spool
print jobs from network clients #6 and #7. In this example,
the network clients can not interact directly with the
network-attached printer. The network spoolers can inject a
"print job" to query the printer and can expect to get
responses back. However, for this example, the network
operating system does not provide a mechanism which
allows applications running on a network client to query
the printer and get a response.
The printer's I/O card, the printer sharing box, and the
spoolers running on the network servers can query the
printer and expect to receive a response since those
components are connected directly to the printer (i.e. no
other printing system component except for the
Programming Tips 10-21