Acksys MI400 Series Technical Documentation Manual page 73

PARTICULARITIES OF RS422A AND RS485 TRANSMISSION LINES
TERMINATION RESISTOR:
The line termination resistor for the receiver in RS422A mode (100 Ohms) and for the
transmitter/receiver in RS485 mode (120 Ohms) reduces reflections generated in a
long high-speed line that may interfere with reception.
The termination resistor is not required in the following case:
Environment free from interference and if the distance and speed are within 1,000
metres at 9,600 bits/sec or 100 metres at 122 Kbits/sec.
BIAS:
Bias of the line is necessary in order to have a stable state:
− in RS485 mode on transition from transmission to reception,
− in RS422A mode if several transmitters are on the bus and consequently the high
impedance state must be used.
Only one bias per line is necessary.
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