Experimental Physics and
Industrial Control System

I have an Allen-Bradley SLC5/03 PLC.  I use an RS-232 connection from a Windows PC to the PLC for programming it.  This works fine when I connect to a real serial port on the PC.  However, that is inconvenient because that is over 50 meters from the PC, and I have 2 PLCs and only 1 serial port, so I need an A/B switch.  I would like to use a multi-port Moxa terminal server instead.  I have installed the Moxa software on the PC that makes COM2 appear as a Windows COM port, using one of the serial ports on the terminal server.

I then try to configure the Allen-Bradley RSLinux software to use COM2 with their DF1 protocol using the "Auto-Configure" option.  This "almost" works.  If I disconnect the serial cable from the Moxa to the PLC and run Auto-Configure it fails to find the matching baud rate and parity.  This is expected of course, since it is not communicating at all.  If I connect the cable it does correctly find the baud rate and parity, so it is communicating OK.  However it then reports "Failed to dine error checking mode!".  There are 2 choices for error checking, BCC, and CRC.  Both fail.

I don't understand this, since I think the BCC and CRC are both done in software on the PC end.   If the Moxa is providing sufficient emulation of a COM port to allow the baud rate and parity to be configured OK, why it is failing to configure the error checking?

Any ideas are welcome!

Thanks,
Mark

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