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Advice for Modbus Connection
Integrating a bus to another Modbus

Hi!

I'm a trainee in a trouble and i need your advice.

I have a task to integrate a bus to another modbus. Is connection like this possible? Connection which is already used is 2-wire RS-458:


SLAVE1---SLAVE2---(SPLITTER/REPEATER/HUB???)--- SLAVE3---MASTER1(HMI)
xxxxxxxxxxxxxxxxxxxxxxxxxxx |
xxxxxxxxxxxxxxxxxxxxxxxxxxx |-----MASTER2 (WHICH ONLY READS SLAVE1/2)

I hope this isn't too stupid question.

No, connection like that is not possible, unless the device labeled splitter/repeater/hub is a specialized, custom programmed device designed to control the 485 bus traffic. The published Rule is that Modbus RTU over RS-485 can have one and only one master.

Without a specialized bus controller, the messages from different masters and/or the slaves will collide with one another on the 485 bus so the messages will not get through and the system live in comm error land and nothing will communicate with anything else.

One approach is if Master 2 can also be a slave, which I'll call Master2/slave. Then Master 1 can communicate (read data from) with slave 1 and slave 2 and write any needed data to master2/slave.

Or vice versa, because HMI can frequently be a Modbus slave, too, where Master 2 communicates to all three slave and writes whatever data master1/slave/HMI needs.

Okay! Thank you for your answer!

We have a product that will allow for up to 3 modbus masters using serial or 2 serial masters and Modbus TCP.

http://www.miille.com/366-10p-xyz.htm

The messages are processed in the order they are received buffered and sent or timed out depending on slave response. For your application the part number would be 366-103-222.

As mentioned by David it is not possible to have multiple masters on a modbus RTU network without a specialized module. You could use a module like the TSH series from ICP DAS. https://www.icpdas-usa.com/documents/WhitePaper-Multiple-Modbus-RTU-Masters-Sharing-One-Slave.pdf

If Master1 and Master2 have an ethernet port with Modbus TCP capability you can also use a modbus TCP to RTU gateway. https://www.moxa.com/en/products/industrial-edge-connectivity/protocol-gateways/modbus-tcp-gateways/mgate-mb3170-mb3270-series