Acces I/O products PCI-ICM-2S, Руководство пользователя

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Appendix A: Application Considerations
Balanced Differential Signals
The reason that RS422 and RS485 devices can drive long lines with more noise immunity 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
is from +6V to -6V allowing 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 may not be reliable.
Parameter Conditions Min. Max.
Driver Output Voltage (unloaded) 4V 6V
-4V -6V
Driver Output Voltage (loaded) LDO 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-1:
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. (The exception is when 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.)
Manual PCI-ICM-2S 18