Manual LPCI-COM-8SM
21
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)
LD and LDGND 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 card. Termination
resistors for the RX+ and RX- lines are provided on the card and are placed in the circuit when
you install the Ch X - LD jumpers. (See the Option Selection section of this manual.)
