Manual 104i-COM-8SM
21
Table A-1:
RS-422 Specification Summary
Parameter
Conditions
Min.
Max.
Driver Output Voltage (unloaded)
4V
-4V
6V
-6V
Driver Output Voltage (loaded)
120Ω Termination 2V
-2V
Driver Output Resistance
50Ω
Driver Output Short-Circuit Current
±150 mA
Driver Output Rise Time
10% unit interval
Receiver Sensitivity
±200 mV
Receiver Common Mode Voltage Range
±7V
Receiver Input Resistance
4KΩ
To prevent signal reflections in the cable and to improve noise rejection in both the RS422
and RS-485 mode, the receiver end of the cable should be terminated with a resistance
equal to the characteristic impedance of the cable. (The exception is when the line is driven
by an RS422 driver that is never “tristated” or disconnected from the line. In this case, the
driver provides a low internal impedance that terminates the line at that end.)
RS-485 DATA TRANSMISSION
The RS-485 Standard allows a balanced transmission line to be shared in a party-line mode.
As many as 32 driver/receiver pairs can share a two-wire party line network. Many
characteristics of the drivers and receivers are the same as in the RS422 Standard. One
difference is that the common mode voltage limit is extended and is +12V to -7V. Since any
driver can be disconnected (or tristated) from the line, it must withstand this common mode
voltage range while in the tristate condition.
