GC Modbus problem

I am using a Prosoft MVI56 MCM to poll registers from an Omni Flow Computer and a Daniels 2350A controller. The communications has been working fine until last week. Using a modbus analyzer I see the MCM poll both devices. The Omni answers the polls fine. There are polls to the Daniels for registers starting at 7001, 7016, 3034, and 8200. The only poll the Daniels answers is for 3034 which is the last stream analyzed. Programming has not been changed in the MCM. One can poll all these registers using the Modbus test of Daniels Mon2000 but cannot poll registers in the 7001 and 8200 using Modscan. Modscan and the MCM seem to have the same problem.

Any answers would be appreciated. Thanks

Something causing noise in the cable? Any new highvoltage cable run on the same cabletray etc. ?

The RS485 2wire is a short distance from the MVI56 MCM to the Omni and 2350A. It is sheilded and in a control room environment. No detectable noise. The 2350A consistently answers the poll for the 3034 registers but not the others.

Doesn't the fist digit specify the modbus fn? Typically for INT's they either started with a 3 or a 4. A first digit of 3 was "read only" and 4 was read/write. If you wanted read/write access to the first INT register available then the modbus address was 4001. Similarily digitals started either with a 0 or a 1.

The Modbus header will be as follows (and is so for this communication)

|Slave address| |Func Code| |HiStartAddress| |LoStartAddress| |HowmanyRegisters| |CRC Error bits|

The return from the device would be similar. The message headers look fine but the only return that is sent is for reg3034. Using Mon2000 all of these registers will poll. Using modscan I have the same problem as I do with the MCM module. Each cannot poll any adresses other than the group near 3034. One other tid bit of info- You can unplug the 485 connector on the GC, plug it back in and it is likely to allow one poll of the 700X and 82xx registers. Its almost like the GC is not restarting the modbus afte this but 3034 is being polled every time with no problem.

geverett said:

Using modscan I have the same problem as I do with the MCM module. Each cannot poll any adresses other than the group near 3034. One other tid bit of info- You can unplug the 485 connector on the GC, plug it back in and it is likely to allow one poll of the 700X and 82xx registers. Its almost like the GC is not restarting the modbus afte this but 3034 is being polled every time with no problem.

Click to expand...

The Prosoft MCM and Modscan are both Modbus Masters, and confirm the failure of the Daniels to respond to a legitimately formatted query.

The fact that it has worked in the past, does not now, a 2nd master gets an identical failure as the primary master says that the Daniels is faulting internally.

Today I hooked my laptop to the back of the Daniels GC (using a 485 adapter) while it was being polled by the MVI56 MCM. I turned on the Daniels MON2000 and polled a couple of registers not of interest. It causes the GC to go ahead and answer the MVI56 MCM and update registers I am polling. It seems like the GC is locked up communications wise until something dislodges it. Daniel's Tech Support is slower than Molasses in liquid nitrogen.

I wanted to post this follow up on the problem I encountered. Maybe someone can add to this or some may learn from this.

I decided since one poll was being answered by the GC immediately following the poll of the Omni flow computer that I would interleave the polling all together. The MCM module now polls the Omni, polls the GC, polls the Omni, Polls the GC and so on and so on. Guess what? The polls are answered by the GC. I can only suppose that the GC is being made to listen by the Omni's poll answer. I am still working with Daniel's on this and maybe I can pass an answer on.

Thanks

interesting. Thanks for the uydate. Dan

Hi geverett,

Currently I attach single 2350A direct to two S600 floboss. I really want to know if you have any suggestion since your problem become mine now.

This communication are ok for 2 month. Then, after I do update program download to floboss and get warm start, comm. to 2350A totally lost. Just like that !

Respond "Addr_err" are counting up for "good tx" pooling in floboss display. Is there any experience to do Interleave in floboss modbus comm?