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davidt

Modbus master

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Hello I want to know if i can use two Masters in a Modbus network with 10 slaves , so that each one of the two masters can work independently . Thanks

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i actually dont have ur answer but can u explain me briefly how modbus works? thanks

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http://www.csimn.com/CSI_pages/Modbus101.html

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The short answer is 'no', if the network is RS-485 using Modbus RTU. The long is an iffy maybe. The reason for 'no' is there is no means of preventing 'data collision' on the network. An RTU network has no means of 'moderating' two different masters. An example of errors could be both masters attempting to poll simultaneously so neither wins, or Master 2 could poll when a slave was responding Master 1 and wipe out the response and its own poll. So RTU networks on RS-485 are limited to a single master. Schneider owns the proprietary, licensed "Modbus Plus" protocol, which uses token sharing to regulate which master has control of the network, so multiple masters can reside on a Modbus Plus network, but only one can speak at a time, determined by who has possession of the token. At one time, I saw notice that Miille Engineering (Houston, TX) might have a gateway that will monitor and regulate the messaging from 2 masters on an RTU 485 network, but you'd have to call and inquire about details. Some people have attempted a workaround that uses serial servers on each Modbus RTU device that convert the native RS-485 to ethernet, because an ethernet switch does a form of regulating its network by store and forward. The issue they run into is that a lot of industrial devices that are Modbus slaves are not full blown PCs with full blown ethernet stacks and they can choke if they get polled once by one master, and then again by another master before they've had a chance to reply. So that's not a guaranteed solution.

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I have used several of the Mille Products with good success. I know thay have a board that does what your asking for too. I think. Edited by a062549

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