Parameter
Conditions
Min.
Max.
Driver Output Voltage (unloaded)
4V
6V
-4V
-6V
Driver Output Voltage (loaded)
TERMIN
&TERMOUT
2V
jumpers in
-2V
Driver Output Resistance
50
Ω
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
4K
Ω
Table A-2:
RS422 Specification Summary
To prevent signal reflections in the cable and to improve noise rejection in both the RS422 and RS485
mode, the receiver end of the cable should be terminated with a resistance equal to the characteristic
impedance of the cable. (An exception to this is the case where 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.)
Note
You do not have to add a terminator resistor to your cables when you use the PCI-ICM-1S card.
Termination resistors for the RX+ and RX- lines are provided on the card and are placed in the circuit
when you install the TERMIN and TERMOUT jumpers. (See Chapter 3, Option Selection of this manual.)
RS485 Data Transmission
The RS485 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.
The following illustration shows a typical multidrop or party line network. Note that the transmission line is
terminated on both ends of the line but not at drop points in the middle of the line.
Manual PCI-ICM-1S
18
