Experimental Physics and
Industrial Control System

Hi Mark,

 

I’ve no experience with the device, but on google I saw it mentioned that “Autoconfigure is convenient but not required - if you have the Offline file you can enter the speed/data/parity/error checking/Device Type values directly into the driver”. Are you able to try that, it may be part of the negotiation happens too quickly for the moxa?

 

Freddie

 

 

Hi Mark,

 

Thanks for the response.

 

Ø  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.

 

Normally I do that as well.  However, that can be done in this case.  In order to use the Allen-Bradley RSLogix progamming tools I need to use the Allen-Bradley DF1 driver which is configured in their RSLinx Light program on Windows.  That can only talk to Windows COM ports, not sockets, and it directly manipulates the baud, parity, stop bits, etc.  So I need to have the terminal server doing COM emulation.

 

Ø  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?

 

I am quite sure that it only uses 3 wires on the RS-232 connection, TxD, RxD and GND.  I know what pins those are on both the SLC5/03 and Moxa, with only a potential ambiguity of swapping TxD and RxD (e.g. null modem). I built two connectors, one is straight, one is null modem.  When using one of them it fails to negotiate the baud rate, which it does by sending a message and looking at the response.  With the other connector it negotiates the baud rate OK, so it must be basically communicating.  But it then fails to set the error correction.

 

I think I will need to get an RS-232 capture device and see how the communication differs when using a real serial port and using the Moxa.

 

Thanks,

Mark

 

 

-----Original Message-----
From: Tech-talk <tech-talk-bounces at aps.anl.gov> On Behalf Of Koennecke Mark via Tech-talk
Sent: Friday, February 17, 2023 1:27 AM
To: tech-talk at aps.anl.gov
Subject: Moxa Terminal Server question

 

 

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