ML1100 Modbus error

Ive got an ML1100 setup as Modbus RTU Master, through a B&B RS232-485 converter. I have 3 power transducers (Multitek M550), on the network, along with a Hitachi SJ200 -HFU75 VFD.
I have 7 MSG instructions to move data as needed. One each MSG instruction to read 32 elements from each transducer. The other 4 messages are for the VFD control, 2 writes and 2 reads. In normal operation, the VFD MSG'ing is working fine, it communicates and I have control. However, the three transducers will not communicate. When I have only one of the transducers (remove all other messaging, I can see the transducer. As soon as I add another transducer MSG, it will stop communicating with all transducers. Ive tried to "cascade" the MSG instructions, but as soon as the cascade gets to an error'ed MSG, it stops.
I can hook up to the 232/485 converter, and using Multitek software, talk to each transducer with no issue, and I verified the ID#, Baud, Stop Bits and Parity are all correct.
Ive noticed that im getting an error 37 "Message timedout in local processor". It will clear when the MSG is re-enabled, but then error out shortly after. This happens on all the trasducer MSG instructions.

I have a few questions;
-Is there a limit to the number of MSG instructions I can have in Logix500/ML100?
-Can they be executed in order at the same time (by rung scan)?

Im scratching my head, as it doesnt make sense why its not working. I can only think that the timeout error means something with my MSG instructions isnt correct.

My original program is attached. The REV_A is an example of my original MSG instructions, but it would have all 7. REV_D is my current attempt to "cascade" the messaging, one at a time. Any insight or comments would be greatly appreciated!

Thanks
NH

is the RS485 network terminated correctly at the ends (and only at the ends)?

There is no termination resistor installed on either end - the wire length is 3 feet from the PLC to the end device.

nhicks said:

I can hook up to the 232/485 converter, and using Multitek software, talk to each transducer with no issue, and I verified the ID#, Baud, Stop Bits and Parity are all correct.
NH

Click to expand...

Just to clarify whats above. Does this mean that you have communicated from your laptop/pc to ALL transducers at the same time ? Eliminating any cabling issues ?

In the past I have used an application called "Simply Modbus" to help setup Modbus comms (funny how a standard comes in sooo many slightly different variations). it displays some details on response time etc. it may be worth a try. The demo will let get online to test for a shortwhile.

As a followup - after checking the knowledgebase, we changed the Handshaking to None(485 network), and added a 50ms Pretransmit delay. We also put the 4 VFD commands on a 4 second timer, and the 3 xD's on there own 2 second timer. The VFD is talking like a champ, but the transducers are having issues. They are still returning an error 37, but occasionally, a message will make it through just fine.
Im trying to figure how to run the 3 xD MSG commands, and when they are done tx/rx, THEN move to the 4 VFD commands. Not pull the plug at the end of the 2 second timer.
Any help with this would be appreciated!

NH