Making Rs-485/422 Connection - KTI Networks KSC-240 Installation Manual

3 Making RS-485/422 Connection

EIA-485 [TIA-485] Balanced (differential) interface; defines the Physical
layer, signaling protocol is not defined. EIA-485 specifies bidirectional,
half-duplex data transmission. Up to 32 transmitters and 32 receivers may
be interconnected in any combination, including one driver and multiple
receivers (multi-drop), or one receiver and multiple drivers.
EIA/TIA-422 define a Balanced (differential) interface; specifying a single,
unidirectional driver with multiple receivers (up to 32). RS-422 will sup-
port Point-to-Point, Multi-Drop topology, but not Multi-Point [EIA485].
EIA-485 devices may be used in 422 circuits, but EIA-422 may not be
used in 485 circuits (because of the lack of an Enable line).
The published TIA/EIA 485 and RS-422 standards define only the electri-
cal characteristics of the drivers and receivers as listed below. They did
not standardize such things such as cables and connectors, pinouts, bus
arbitration, signaling protocols, or physical wiring topology. Many dif-
ferent implementations have come into use and they are often incompat-
ible with each other.
Characteristics
Mode of operation
Allowed no. of Tx and Rx
Maximum cable length
Maximum data rate
Minimum Tx driver output range +/- 2V
Maximum Tx driver output range +/- 5V
Maximum Tx short-circuit current 150mA
Tx load impedance
Rx input sensitivity
Maximum Rx input resistance
Rx input voltage range
Rx logic high
Rx logic low
RS-422
Differential
Unidirectional
Multipoint
1 Tx, 10 Rx
4000ft length
10Mbps
100
+/- 200mV
4k
+/- 7V
>200mV
<200mV
-32-
RS-485
Differential
Full Duplex
Multipoint
32 Tx, 32 Rx
4000ft length
10Mbps
+/- 1.5V
+/- 5V
250mA
54
+/- 200mV
12k
-7V to +12V
>200mV
<200mV