RS485/RS422 signal line direction determination method - Database & Sql Blog Articles

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When using RS485 or RS422 communication protocols, it's crucial to ensure correct wiring. If the signal lines are reversed, the logical "1" and "0" will be inverted. In such cases, if a converter is involved, the receiving end may constantly show a signal. To troubleshoot, disconnect the converter from the device and perform the following tests:

a. For RS485 interfaces with built-in pull-up resistors, use a multimeter to measure the DC voltage between the D+(A) and D-(B) terminals. The voltage should be greater than 1V. The polarity of the measured voltage can help identify the direction: the terminal with positive voltage is D+(A), and the one with negative voltage is D-(B).

b. For RS485 networks without pull-up resistors, the voltage at both ends may be only a few hundred millivolts. These can still be measured with a multimeter set to a low voltage range. Again, determine the signal direction based on the polarity as described in part (a).

c. For RS422 networks, which operate in a full-duplex mode, you can use A and B modes to check the signal direction. This helps in identifying whether the signal is being transmitted correctly through the network.

Proper configuration and testing of these communication interfaces are essential to avoid data corruption and ensure reliable transmission. Always double-check the wiring and use appropriate tools like multimeters for accurate measurements.