Appendix C - Electrical Interface
Sealevel Systems
PCIO
Page
16
RS-485
RS-485 is backwardly compatible with RS-422; however, it is optimized
for partyline or multi-drop applications. The output of the RS-422/485
driver is capable of being
Active
(enabled) or
Tri-State
(disabled). This
capability allows multiple ports to be connected in a multi-drop bus and
selectively polled. RS-485 allows cable lengths up to 4000 feet and data
rates up to 10 Megabits per second. The signal levels for RS-485 are the
same as those defined by RS-422. RS-485 has electrical characteristics
that allow for 32 drivers and 32 receivers to be connected to one line.
This interface is ideal for multi-drop or network environments. RS-485
tri-state driver (not dual-state) will allow the electrical presence of the
driver to be removed from the line. Only one driver may be active at a
time and the other driver(s) must be tri-stated. The output modem control
signal RTS controls the state of the driver. Some communication software
packages refer to RS-485 as RTS enable or RTS block mode transfer.
RS-485 can be cabled in two ways, two wire and four wire mode. Two
wire mode does not allow for full duplex communication, and requires
that data be transferred in only one direction at a time. For half-duplex
operation, the two transmit pins should be connected to the two receive
pins (Tx+ to Rx+ and Tx- to Rx-). Four wire mode allows full duplex data
transfers. RS-485 does not define a connector pin-out or a set of modem
control signals. RS-485 does not define a physical connector.
Centronics and Bi-Directional Parallel Ports
In the early days of the IBM PC a decision was made to use the popular
“Centronics” style interface as the interface of choice for the parallel
printer. Centronics was a prominent manufacturer of “daisy wheel”
printers and of the then new “dot matrix” style printers originally
designed for Mini and Mainframe computers. The only difference
between the PC interface and the Centronics interface was the use of a
DB-25 style connector on the PC side of the interface. This was done
because the designers felt that the Amphenol style interface common to
the Centronics printers would not fit well on a PC bracket. The choice of
a female connector was made to avoid confusion with the RS-232 port
which was male.
Summary of Contents for PCIO 4035
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