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what is + and - connection pins to PM9C over Modbus RS485 port.

Goals and Symptoms

PM9C: What are the positive (+) and negative (-) connection pins on the RS485 port for Modbus communications?.

Facts and Changes

In telecommunications, EIA-485, also known as RS-485 (Recommended Standard 485) is a standard for serial binary data signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-terminating Equipment).
The EIA-485 signaling specification states that signal A (TxD-/RxD-) is the inverting or '-' pin and signal B (TxD+/RxD+) is the non-inverting or '+' pin. However this is in conflict with the A/B naming used by a number of differential transceiver manufacturers such as Texas Instruments, (as seen in their application handbook on EIA-422/485 communications: A=non-inverting, B=inverting); Intersil (as seen in their data sheet for the ISL4489 transceiver); Maxim (as seen in their data sheet for the MAX483 transceiver), etc... they may be incorrect according to EIA, but their practice is in widespread use. Therefore, care must be taken when using A/B naming.

Causes and Fixes

For Schneider Electric Powerlogic PM9C meter, D0 (A/A') is defined as the inverting '-' pin and D1 (B/B') as the '+' pin. Unfortunately the labeling error was found by the end of 2007 for version 2.01, in some PM9C meters, signals D0 (A/A') & D1 (B/B') were inverted. It is unable to identify more accurately which units had been affected. For version 2.01, the first terminal (the far left on bottom) should be labeled as D1 (B/B'), otherwise it may be necessary to swap the wires.

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