Automation D 260 master to AB 1400 slave

I have an Automation Direct 260 master PlC trying to communicate with a Micrologix 1400 slave using a Transnet 900 serial radio modem. I am using Modbus MRX and MWX instructions in the AD. Setting it up on the bench connected with a hard wire cable it works fine but when I connect it using the radio modem it will execute only the first instruction in the ladder logic but not the one after it. Meaning it will write data from the AD to the AB if MWX is first in the ladder logix but will not read any data in the AB. If I put the MRX first it will read data and not write.

This is on our SCADA system so the AD has 60 sites it is polling using a counter incremented by SP116 (special relay on when comm. with another device) in series with an "= equal to) contact in each rung with the counter value. This ensures that only one rung executes at a time.

I have tried separating the rungs and slowing down the radio poll but nothing seems to help. Hoping someone has run into something like this before.

Since the hard cable worked, I would look at the modems. Is there any Hardware handshake lines that need to be disabled or jumpered?

Problem solved, the counter used to increment the polling in the ladder logic needed to be increased by one. The counter reset value has to be one number larger than the last rungs "equal to" contact number. Meaning the last "equal to" contact was 126 and the reset counter value had to be 127, otherwise the last rung was not being read. I knew this to be the case but overlooked it. Old man syndrome catching up with me.

Have you tried the Iboxes? They work really well with Modbus Coms. Also on the next project, try the DoMore. It will blow you away after working with the 260.

No I have not thanks for the tip. I have used some of the iboxes but not much. My programming needs have not been very extensive but are getting more and more lately.

Good Morning,
When using the MRX or MWX IBOX set each Exception Responce Buffer to a different register this will aid in detecting communication errors. Here may not of helped but could help in future.

Tom