Printers are connected to computer systems in a variety of ways. Small desktop printers are usually connected directly to computer's USB port. Older printers are connected to a parallel or “printer” port. Some printers are directly connected to a network, making it easy for multiple computers share them. A few printers use a much less common serial port connection.
FreeBSD can communicate with all of these types of printers.
USB printers can be connected to any available USB port on the computer.
When FreeBSD detects a USB printer,
two device entries are created:
/dev/ulpt0 and
/dev/unlpt0. Data sent to either
device will be relayed to the printer. After each print
job, ulpt0 resets the
USB port. Resetting the port can cause
problems with some printers, so the
unlpt0 device is used instead.
unlpt0 does not reset the USB port at
all.
The parallel port device is
/dev/lpt0. This device appears
whether a printer is attached or not, it is not
autodetected.
Vendors have largely moved away from these “legacy” ports, and many computers no longer have them. Adapters can be used to connect a parallel printer to a USB port. With such an adapter, the printer can be treated as if it were actually a USB printer. Devices called print servers can also be used to connect parallel printers directly to a network.
Serial ports are another legacy port, rarely used for printers except in certain niche applications. Cables, connectors, and required wiring vary widely.
For serial ports built into a motherboard, the serial
device name is /dev/cuau0 or
/dev/cuau1. Serial
USB adapters can also be used, and
these will appear as
/dev/cuaU.0
Several communication parameters must be known to communicate with a serial printer. The most important are baud rate and parity. Values vary, but typical serial printers often use a baud rate of 9600 and no parity.
Network printers are connected directly to the local computer network.
The DNS hostname of the printer must be known. If the printer is assigned a dynamic address by DHCP, DNS should be dynamically updated so that the host name always has the correct IP address. Network printers are often given static IP addresses to avoid this problem.
Most network printers understand print jobs sent with
the LPD protocol. A print queue name
can also be specified. Some printers process data
differently depending on which queue is used. For
example, a raw queue prints the data
unchanged, while the text queue adds
carriage returns to plain text.
Many network printers can also print data sent directly to port 9100.
Wired network connections are usually the easiest to set up and give the fastest printing. For direct connection to the computer, USB is preferred for speed and simplicity. Parallel connections work but have limitations on cable length and speed. Serial connections are more difficult to configure. Cable wiring differs between models, and communication parameters like baud rate and parity bits must add to the complexity. Fortunately, serial printers are rare.
All FreeBSD documents are available for download at http://ftp.FreeBSD.org/pub/FreeBSD/doc/
Questions that are not answered by the
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