USB-COMI PRO / USB-2COMI PRO USER’S MANUAL
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6
PROPER WIRING FOR RS-422/485 OPERATION
This section will provide proper wiring information about RS-422 and RS-485 data
communication. It is necessary to have the basic knowledge in order to avoid or find
errors in data transmission. Failures in cabling are responsible for most transmission
problems.
6.1
RS-422 & RS-485 Transmission Technique
The RS-422 and RS-485 use the same balanced transmission method. Signals are not
transmitted as voltage on a single wire, like in RS-232. Instead, two wires are used;
when one carries high voltage, the other one carries low voltage. The signal is defined
by the difference in voltage between those two wires. This hardens the transmission
against noise. Usually twisted pair cables are used, which further reduces the
sensitivity for noise.
To make sure the signals meet the common voltage range, the GND of sender and
receiver must be connected somehow. To ensure the signals are in the valid voltage
range and the differential voltage can be correctly sensed by the receiver, the GND
lines of the transmitter and receiver must be connected.
6.2
RS-422 Signals Connected
The following diagram shows how RS-422 signals are connected.
USB-COMi PRO
USB-2COMi PRO
