A maximum common mode voltage rating of +7V provides good noise immunity from voltages induced
on the twisted pair lines. The signal ground line connection is necessary in order to keep the common
mode voltage within that range. The circuit may operate without the ground connection but may not be
reliable.
Parameter
Conditions
Min.
Max.
Driver Output Voltage (unloaded)
4V
6V
-4V
-6V
Driver Output Voltage (loaded)
TERM
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.
Note
You do not have to add a terminator resistor to your cables when you use the card. Termination
resistors for the RX
+
and RX
-
lines are provided on the card and are placed in the circuit when you
install the TERM jumpers. (See the Option Selection section 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.
RS485 Two-Wire Multidrop Network
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-COM-1S
19
