Electronic – Besides address is there other limit on max nodes via RS485 Modbus-RTU

modbusrs485

Since there is 1byte for nodes' address in Modbus frame, the limit should be around 256.But some say there is some limit on this scenario (RS485, Modbus-RTU) base on chips and wire.

Chips support 32 nodes:SN75176,SN75276,SN75179,SN75180,MAX485,MAX488,MAX490

Chips support 64 nodes: SN75LBC184

Chips support 128 nodes:MAX487,MAX1487

Chips support 256 nodes:MAX1482,MAX1483,MAX3080～MAX3089

Are there other limits on this?Like reserved addresses as routine?

Best Answer

Address 0 is reserved as a "broadcast" address where the master can write to the slaves, however the slaves are not supposed to respond to broadcast requests. Technically the Modbus Spec only allows slave addresses 1 to 247 (246 total slaves).

The address field of a message frame contains two characters (ASCII) or eight bits (RTU). Valid slave device addresses are in the range of 0 – 247 decimal. The individual slave devices are assigned addresses in the range of 1 – 247. A master addresses a slave by placing the slave address in the address field of the message. When the slave sends its response, it places its own address in this address field of the response to let the master know which slave is responding. Address 0 is used for the broadcast address, which all slave devices recognize. When Modbus protocol is used on higher level networks, broadcasts may not be allowed or may be replaced by other methods. For example, Modbus Plus uses a shared global database that can be updated with each token rotation.

The rest is up to the hardware specification by the device vendor. A Modbus device that complies with the spec is supposed to be able to be a part of a network of up to 246 slaves, so you should only be seeing devices that support larger networks. In reality though, devices may choose to use smaller/more power efficient/cheaper chips that only support 32 slaves (or less). I've seen industrial devices that don't support more than 32 slaves on one RS-485 network.

If you want to read them, the Modbus Implementation and Specifications are free on the internet. You should be aware though, speaking from an industrial equipment standpoint, there is "Modbus"-Modbus, and Modicon Modbus (or Modbus+). The differences are minor, but worth mentioning.

By the way, you should separate RS-485/RS-422 from Modbus RTU/ASCII. Modbus uses RS-485 (or RS-422, RS-232, Ethernet, Fiber, etc) as the wire to transmit the protocol. Just because a chip is RS-485 doesn't mean that it was designed with Modbus uses in mind. For example, Profibus also uses RS-485 as a network with a 126 device limitation.

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