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Arun Opto 22

Question about Multiple Masters on Modbus-TCP

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On Ethernet/Modbus-TCP, if a device is a Master, can it get polled by another Master? i.e. Can it act as a slave as well. Thanks, Arun

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Hello, No. The protocol is master/slave.

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See other answers here: http://www.plctalk.net/qanda/showthread.php?t=92475 I recall someone used a multiport Modbus/TCP slave, like the Red Lion DataStation Plus (DS+), so that master 1 could write to DS+ register set A and read from DS+ register set B while master 2 could write to DS+ register set B and read from DS+ register set A. Edited by DanW

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Hi, He is the same poster. I was originally considered creating a long response. It really all comes down to the protocol. Masters cannot access masters and slaves cannot access slaves. Regardless of what I do in the "device", multiple sockets, message threading, internal pipes, etc., to appear as a device supports both, it is an illusion. The master side of the protocol will not respond to any commands. The OP original question, "...a device is a Master..." started the long discussion on the other site. "Devices" can do many things and appear to be magic. :) The question is can the protocol support it, hence, my short answer. My2c. Edited by Mark-

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Hi Arun, In some cases you cannot but here is a ICP DAS USA solution; xp_8149_atom_ce6 can act like master/slave depends on your configuration though. http://www.icpdas-usa.com/xp_8149_atom_ce6.html?r=Kaan Thank you,

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