Any problem to reverse the modbus master?


Thread Starter


We have a SCADA application in which we want to read/write data from a F&G system into 3 different RTUs and transmit the values to separate locations. The F&G system supports Modbus. Normally I would make the RTUs modbus masters and the F&G modbus slave.

However by standard modbus, we cannot have 3 modbus master. So instead, in reverse, why not make the RTUs as slaves and make the F&G read/write the data to the RTUs instead. Is there any possible problem with this arrangement.
The only question I have in answer to yours is what else is polling the RTUs? Don't get yourself into a problem with the RTUs having too many masters. Otherwise I see no fault in your idea.

Another solution is if you have communication between the RTUs have one RTU collect the values from the F&G and then distribute it to the other RTUs.

Good Luck

Automation Linse

Hmmm, would it be a problem if the Fire&Gas (I assume F&G is) cannot be Master, now wouldn't it. Perhaps your RTU and F&G product both can be either Master or slave; perhaps not. You are VERY lucky if all of your devices support Master operation. *MOST* Modbus products will ONLY support slave operation.

If you do need to have the 3 RTU-as-Master "share" the single F&G-as-Slave, there are MANY devices on the market that can help you. Calta and FieldServer are 2 vendors who sell conccntrators or arbitrator for this.

Or if you convert the entire system to TCP/IP using Modbus Ethernet-to-serial bridges you'll gain multi-master capability.

best regards
- LynnL, (experts in Ethernet-to-Serial)