RS422I-PA Manual
Page A-2
Manual 00650-519-1
A balanced differential line receiver senses the voltage state of the transmission line across the two
signal input lines. If the differential input voltage is greater than +200 mV, the receiver will
provide a specific logic state on its output. If the differential voltage input is less than -200 mV,
the receiver will provide the opposite logic state on its output. A maximum operating voltage
range is from +6V to -6V allows for voltage attenuation that can occur on long transmission
cables.
The maximum common mode voltage rating 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
S
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
S
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.)
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.
