KTI Networks KSC-240, Installation Manual, Page: 32 / 43

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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 RS-422 RS-485
Mode of operation Differential Differential
Unidirectional Full Duplex
Multipoint Multipoint
Allowed no. of Tx and Rx 1 Tx, 10 Rx 32 Tx, 32 Rx
Maximum cable length 4000ft length 4000ft length
Maximum data rate 10Mbps 10Mbps
Minimum Tx driver output range +/- 2V +/- 1.5V
Maximum Tx driver output range +/- 5V +/- 5V
Maximum Tx short-circuit current 150mA 250mA
Tx load impedance 100 54
Rx input sensitivity +/- 200mV +/- 200mV
Maximum Rx input resistance 4k 12k
Rx input voltage range +/- 7V -7V to +12V
Rx logic high >200mV >200mV
Rx logic low <200mV <200mV