COMM: micrologix 1400 and modbus rtu woes.

Wed May 20 08:24:42 EDT 2009

==> Automation List post by M Griffin...
To view or reply to this thread: http://www.control.com/thread/1242764816#1242796874

------------------------------------------------------------------------------------
Re: COMM: micrologix 1400 and modbus rtu woes.
------------------------------------------------------------------------------------
Is the register really 40001, or is it actually holding register 1? Some people like to use numeric prefixes to indicate the address type, where "4" stands for holding register. It's like using "I" for inputs and "Q" for outputs on IEC style PLCs. 

The prefix isn't actually part of the address though. It also isn't actually part of Modbus as there is nothing in the specification that allows it. It's just something that some people unfortunately like to do when writing documentation, and it has probably caused more confusion than anything else. 

An exception code of "3" should indicate an incorrect number of registers. However, you are only asking for 1 register. I suspect that the AB PLC is returning an incorrect error code where it should be returning a "2" for an address error.

With Modbus, the type of register is selected by using the correct function code, and function code 3 is for reading holding registers. I would suggest trying it again with register address 1 instead of 40001. 

------------------------------------------------------------------------------------

==> A Note from the Sender of this Message...
You are receiving this message because a subscription was entered for your address
for the Control.com Automation List.  To unsubscribe, send an email to us at 
forum at control.com requesting your removal from the subscription list.

This email was sent by: 
Control.com, Nerds in Control, LLC,
116 Brigham Hill Rd., N. Grafton, MA 01536, USA.