Serial Comms using Allen Bradley 1769-L32E

Using the native serial port on the PLC I am connecting a Fairbanks Scale.

I have set the serial settings for the Fairbanks as follow

9600-8-N-1, it'll output CR as the end of line, and it uses STX and ETX indicators, I have connected it directly to my laptop and I could see the correct information being spit out on to the hyperterminal

Now I have connected the serial cable of the scale directly to the native port of the PLC.

I have set the serial port options on the PLC as the pictures attached.




WHen I use ASCII function block provided by AB, eg ARL, ABL, I am getting no characters returned. e.g. it's reading no character?

Hi,

I am assuming that you are using RS232...
The first thing I would check is the cable, have you connected RX and TX correctly? Rx on one device should go to Tx on the other and vice versa.
What length on the cable do you have? Using standard cable, I think that you are limited to 15 meters.
Thirdly, connect a PC to the PLC (hyperterminal) and send some telegrams, to ensure that your PLC program is correct...

BR

Patrick L

The very first step in diagnosing serial communication problems for me is to get everything working to a terminal emulator on a PC. Realterm or Hyperterminal, or something similar.

The same cable should even work. Once it's working on the PC, then get back to the PLC.

For scales, make sure the scale is set up to do what you want as well. I generally prefer to use the "Request/Answer" method of reading scales, but if I'm being lazy, I'll set them to just continuously retransmit the canned data packet and parse it in the PLC. That method doesn't require the PLC to do anything to initiate the comms.

Look at the Real Time Automation 435. It is a serial to Ethernet adapter. In 10 minutes you can have the PLC communication running and not tie up you serial port.
rdrast has given great solid advice. I like the "ask reply" or "request reply" it does call for more work on your side but. If you just parse out the constant data you have to assume when the data us good. So if you get a snapshot of the weight and the scale hasn't settled yet you will get junk data.

The scale does output correctly on to the PC's hyperterminal. However when I plug the scale into the PLC, there is no lights flashing on the PLC to indicate RS-232 activity. All the settings seem correct to me.

Allenz,

Did you ever resolve the issue? I am facing setup of a similar system and would like to hear how you resolved the problem.

Yes it was resolved, turns out the ground wasn't connected properly. I had to re-solder the ground pin again. Everything came right afterwards.