Emerson SiteLink-12E - Manual (Page 19)

15
A
PPENDIX
A - C
OMMUNICATION
P
ROTOCOL AND
W
IRING
C
ONSIDERATIONS
A.1 Differences Between RS-485 and RS-422
Third-party manufacturers sometimes specify a device as either RS-485 or RS-422.
RS-485 and RS-422 terminology is interchangeable, but there is at least one wiring difference: Each
can have two-wire or four-wire connections, but an RS-422 connection needs four wires for two-way
communication.
An RS-422 four-wire network actually consists of two RS-422 two-wire, one-directional networks com-
municating between the same devices.
The one-directional networks must be used because an RS-422 transmitter cannot turn off automati-
cally after transmission. An RS-422 transmitter that is On—even when not sending data—will pre-
vent another transmitter from effectively sending data.
An RS-485 transmitter, by contrast, can turn On and Off between transmissions. This makes a
two-wire connection preferable for many reasons, including the fact that two-wire networking allows
for daisy-chaining with no need to designate any devices as master, as four-wire requires.
The only advantage of four-wire over two-wire is that four-wire allows full duplex communications.
However, few protocols need full duplex communications, meaning there is little practical reason to
run four-wire RS-485. There are voltage and driver load differences between RS-485 and RS-422, but
the differences are negligible in normal use.
Liebert recommends using a two-wire connection unless:
• you are communicating with a device that uses RS-422 or
• the third-party device does not support two-wire RS-485.
Use of the two terms—RS-422 and RS-485—can be confusing in engineering drawings. When draw-
ings include Liebert SiteLink-E wiring details, Liebert suggests labeling any occurrences of RS-422 as
“RS-422 (RS-485 four-wire).”
A.2 Termination
You can reduce reflections that cause communication and data errors on RS-485/RS-422 networks by
terminating a data cable with a value equal to its characteristic impedance. Although termination is
often unnecessary on networks where the baud rates are slow or the cables are short, termination
becomes important as the baud rate increases.
Resistors acting as terminators typically have 120-130 ohms, although twisted-pair cable impedances
can be as low as 100 ohms. Liebert recommends 120 ohm terminating resistors on RS-485/RS-422 net-
works. You must apply a value that closely matches the cable impedance as near as possible to the
ends of the network segment.
NOTE
Some third-party manufacturers provide a resistor within a device, using a jumper to disable
the termination option if termination is not required. If you are using one of these devices,
make sure that only devices that require termination are set to have termination.