CRC required for Modbus slave request

[bluffit]

This is my first post to the group. It's also the first day of trying to integrate my PLC (A DirectLogic 06) with a PT100 RTU heat sensor monitor: http://www.contrel.it/eng/ctt8.php over Modbus RTU. ( See manual attached ).

Having dug into the manual, I am now very concerned that my expensive investment in this analog data capture unit may have been misplaced. The manual for its modbus communications indicates that I have to create a CRC of the data I'm sending. However, the DirectLogic 06 PLC has no CRC function, or from what I can see no advanced math/string library or even bitwise operations.

Am hoping that this is all my misundertanding, but any help would be much appreciated.

On a final note, I'm a software guy, and hardware terrifies. me. I hope this won't be another of "those" experiences! sigh.

 

[Andybr]

If you are simply polling repeatedly for the same information do not try to calculate the CRC in the PLC. Just work it out yourself and then hard code the data request into your PLC programme. Checking the CRC for the received data is not normally required unless there are serious consequences if bad data is received since the data is normally updated frequently and you can check for "silly" values using much simpler techniques.

 

[nettogrisen]

I'm not very experienced in AD's products, but it seems strange that you have to write the protocol yourself. From this manual it seems that Modbus RTU is already implemented int the controller

http://www.automationdirect.com/static/manuals/d006userm/appxk.pdf

 

[bluffit]

Thanks guys. It seems the PLC should implement this for me so it might have been a case of panicing uncessarily. One day I hope to overcome my HW phobia! Really appreciate the prompt replies!

 

[bluffit]

Further to this, I've realised getting my first Modbus program actually talking and working is going to be harder than first thought. There's a bunch of stuff like just familiarising myself with the DL06, checking registers, stepping through code, and writing Ladder logic, that I'm simply a total newbie at.

If there's anyone who'd like an hours work helping get me started, please feel free to get in touch. I could give you a Remote Desktop connection into a workstation with DirectSoft5 running on it. Once I can read registers, I should be away.

Feel free to drop me an email - andrew at bluffit.co.nz. cheers