MSG blocks (Modbus) in Micrologix 1400

Brien100

Member
Join Date
Apr 2006
Location
Michigan
Posts
12
I am reading Input status and Analog data from a Woodward device, my MSG blocks work but I had to place timers in my logic to make them all execute. I tried to set the first MSG with first pass or a bit at the end of the MSG blocks sequence, and use DN bits to execute the next MSG, Only one MSG would execute over and over without the timers to slow the logic...WHY???

Any ideas to clean this up a bit.

Thanks in advance.
 
Last edited:
Without seeing the logic I couldn't comment on how to change it, so ZIP the *.RSS file and post it (or at least the subroutine with the MSG instructions).

With Modbus, there's often an issue of inter-message delay, but we shouldn't look into that until we've seen the sequencing logic.

Remember that Modbus is a one-message-at-a-time protocol, unlike DF1 which can tell the difference between messages (because of the transaction sequence word). The MicroLogix doesn't buffer or stack Modbus master messages.
 
I use a method that Ken showed me some time ago. It used an add function each time a message is done (or errors). At the last message, use a move fuction instead of an add.

Staggered_MSG.jpg
 

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