Experimental Physics and
Industrial Control System

Koennecke Mark via Tech-talk <tech-talk at aps.anl.gov> · Fri, 17 Feb 2023 07:27:19 +0000

Hi Mark,

> Folks,

> 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

I have two ideas:

I never use the COM port emulation on the host system but configure the serial port on the terminal server 
and then talk to the TS/device through a socket connection. 

Then RS-232 is somewhat ill defined regarding pin assignments and such. I would try to compare the pin assignment 
form the SLC5/03 PLC with the pin assignments used by the Moxa TS. We routinely use special adapter plugs when 
connecting devices to the Moxa TS. May be you need an electronics person to provide you with a special plug?

Best Regards,

     Mark

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System