Xerox 4520 User Manual page 266

Note
C-2 4 5 2 0 / 4 5 2 0 m p U s e r ' s G u i d e
The 4520/4520mp printers provide a parallel port, serial port,
and three additional network interface port options, all of
which may be active at the same time. Automatic switching
among all active I/O (input/output) ports is supported.
Port Polling Without a Hard Disk Installed
To coordinate automatic switching among the ports, the
printer monitors the parallel, serial, and network interface
ports for incoming data. When a print job is detected on one
port, the printer sends a busy signal to the other ports and
queues the print job into memory for processing. This is
called port polling. Note that while all ports may be
simultaneously active, only one port at a time is able to
receive a print job.
When print jobs are pending on all five ports (parallel, serial,
Ethernet, LocalTalk, and Token Ring), the port polling
sequence is:
1 Parallel
2 Serial
3 Ethernet
4 Token Ring
5 Back to Parallel
LocalTalk is not a part of this sequence. When a print job is
detected on the LocalTalk port, the print job is put in the
queue immediately after any current job finishes.