I have an MSS4 or SCS400 with all 4 ports configured identically and connected to identical serial devices. Ports 1 and 2 communicate with the attached serial devices but ports 3 and 4 don't. If I move the serial devices on ports 3 and 4 to ports 1 and 2 they start to work while the ones that were connected to ports 1 and 2 no longer work when connected to ports 3 an 4. I have checked all 4 serial cables and they are all wired correctly and work fine on ports 1 and 2. What could the problem be?

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Please check that you have parity set correctly on all 4 serial ports. Ports 1 and 2 will continue to pass data even if there are parity errors, ports 3 and 4 do not forward any data with parity errors.

I.e. if the serial devices are using Even parity and the ports on the MSS4/SCS400 are set to Odd parity, change the configuration of either the serial device's ports or the MSS4/SCS400 so the parity settings match.

Why? On the MSS4 and SCS400 the UARTs (Universal Asynchronous Receiver Transmitters) for ports 1 and 2 are part of the processor chip while the UARTs for ports 3 and 4 are external to the processor. The external UART chip handles parity errors differently than the processor's internal UARTs.

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[Originally Published On: 07/08/2005 09:36 AM]