ASCII Comms issue

[ThePLCPeon 0]

Hello,

I am having a recurring issue with a system using ascii to tranmist weighing data. The system will work but every few days there will be a random erroneous value input as the current weight reading. The plc then seems to "freeze" and can only be reset by going online and clearing the erroneous readings from the strings as well a the string reply from the terminal. This is all new to me and I think the problem lies with the structure of "write, read and extract" I think I may be picking up part of the reply string I do not want.

The following is the reply string I recieved "   0.0    27.0kg     1   54732>^M^J<000010-01-0007:08   01    2  " what this should be is " <000010-01-0007:08   01    2     0.0    27.0kg     1   54732>^M^J " The strign seems to have been split in two, this is a problem as to extract the weight I am counting "x" characters along the string and then extracting what i need. I need that string to be sent unfragmented as I think this is my problem.

I have a attached an image for the current instructions I am using to receive the string, I write a command to the terminal, check the buffer and the read in the string. I then extract data from the string.

Any help is greatly appreciated.

 

 

Edited 18 Sep 2017 by ThePLCPeon

[vds 8]

Hello,

It's been a long time since I've done anything ASCII related, but here are a few comments:

1. Try synchronising your reads and writes more logically in a sequence. Currently your trigger to Write is completely disassociated with the ACB and ARD. This might cause some timing issues if a write and a read execute simultaneously.

You could try something like:

Step 1 - issue AWT instruction.

Step 2 - wait for AWT.DN bit + ?2? second delay (delay long enough for device to reply).

Step 3 - perform ACB instruction

Step 4 - upon ACB.DN, check buffer length >= 62 and perform the ARD instruction.

Step 5 - Go back to Step 1

2. If you know your response should start with a particular pattern, you can check for that pattern using additional ASCII functions such as AEX etc - Refer to chapter 10 - http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1747-rm001_-en-p.pdf

 

Good luck!

 

[ThePLCPeon 0]

Thank you for your help I will give this a try!

[ThePLCPeon 0]

Hello again, I have made changes to the code to what I think is a more logical sequence of events. My only question now is, I want to wait until the buffer is clear before I perform my write instruction again. I am not sure how to do this, Is the a clear buffer "done bit" I can use and will that work? Or should I check the buffer for characters again before I go back to my write instruction. Here is a picutre of the current structure. Also just FYI the timer T4:14 Is flip flopping with another 1s timer so atm the write instruction will be perforemed every 2s regardless of anything else.

 

[vds 8]

Hello,

Please refer to attached sample code - sorry I don't have RS500 with me so have prepared sample in RS5000.

Hopefully it is descriptive enough - is uses a state machine to shuffle along the different steps, then returns to the beginning.

Regards,

vds

 

ASCII.pdf

Edited 22 Sep 2017 by vds
wrong info

[vds 8]

Oops typo on rung 5, EQU instruction should be comparing for 4 not 3...

[ThePLCPeon 0]

Yep I thought that was the case, this was a big help thanks again.