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RS485 Modbus, Limitation on Number Of Slaves
Want to know the Theoritical & Practical Mimitation on RS485 Modbus Communication Length & Number OF Slaves a Master Can Handle.

Want to know the Theoritical & Practical Mimitation on RS485 Modbus Communication Length & Number OF Slaves a Master Can Handle.

By M. Harisman - Integra Teknik on 11 November, 2002 - 5:25 pm

The limitation will come from the RS-485 network, and not from Modbus protocol run on it.

The maximum Nodes on one RS-485 network is 32. Maximum distance will depend on the applied speed as well as the cable type.

The maximum Modbus Slaves per one multi-drop network is 255.

For complete overview on RS-485 interface, you could visit :
"http://www.rs-485.com/specs.htm":http://www.rs-485.com/specs.htm
"http://www.embeddedsys.com/subpages/resources/images/documents/microsys_art_RS485.pdf":http://www.embeddedsys.com/subpages/resources/images/documents/microsys_art_RS485.pdf

For technical resource on Modbus protocol, you could visit :
"http://www.modbus.org":http://www.modbus.org

M. Harisman harisman(AT)integrateknik.com

By Mike McCafferty on 14 November, 2002 - 6:07 pm

For MODBUS, VALID slave addresses are 0-247, where 0 is for broadcast.

The maximum also depends on the PHYSICAL network.

For RS485 networks, depending on the design, a "typical" maximum is 32 nodes, however, with proper design this can be increased. Also, buffer transceivers can be used to extend the network.

-Mike

You may have up to 256 RS-485 nodes with 1/8 impediance transceivers. Many manufactures produce these today (try Maxim or TI).

Michael