Balanced Differential Signals
The reason that RS422 and RS485 devices can drive longer lines with more noise immunity than
RS232 devices is that a balanced differential drive method is used. In a balanced differential system,
the voltage produced by the driver appears across a pair of wires. A balanced line driver will produce a
differential voltage from +2 to +6 volts across its output terminals. A balanced line driver can also have
an input "enable" signal that connects the driver to its output terminals. If the "enable signal is OFF, the
driver is disconnected from the transmission line. This disconnected or disabled condition is usually
referred to as the "tristate" condition and represents a high impedance. RS485 drivers must have this
control capability. RS422 drivers may have this control but it is not always required.
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. The maximum operating voltage range from +6V to -6V,
allows for voltage attenuation that can occur on long transmission cables.
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 might 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 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 Tx+/Rx+ and Tx-/Rx- lines are provided on the card and are placed in the circuit when
you install the LD jumpers. (See the Option Selection section of this manual.)
