Modbus nodes vs slaves?

I'm trying to find out the maximum number of slaves a modbus RS485 network can support. I saw this:

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.

What's a modbus node? I thought it was only masters and slaves...
I thought the max number of slaves is 247 (?). Here it says 255...

The link to where I pulled that above quote:
http://modbus.control.com/thread/1026160877

Please advise. Thanks.

This is going to vary by manufacturer. RS-485 is supposed to be limited to 32 nodes, but the Modbus protocol is not subject to that limit. A node is a master or a slave or a client or a server depending on which maker you are reading from and whether you are talking about RTU, TCP or mixing terminology and limitations as many manufacturers have chosen to do.

Being a 40 year old open technology, Modbus has been flexed and stretched and varied greatly, mostly for the betterment of the end user once you get past the initial commissioning and figuring out these types of details for whatever specific brands you are integrating.

The "Slave ID" address byte value in a Modbus serial network frame can be 0 (broadcast) or 1-247.

Addresses 248-255 are reserved for "future expansion of the protocol" and should not be used.

The number of physical devices you can put on an RS-485 network is an electrical limitation. The traditional limit is 32 devices actually wired in parallel to the RS485 bus, but as noted some modern transceivers can handle more.

Some serial networks use multiple serial ports in order to handle more than whatever the limit is on one serial network - usually stated to be 32 devices but sometimes 32 is an optimistic number given the devices and the network.

Suppose 32 is a useable number, then one could have
serial network 01 cover slaves 1-32
serial network 02 cover slaves 33-64
serial network 03 cover slaves 65-96

A node may support multiple slaves. For instance a motion controller could use the slave number as an axis number so one controller/node can support 8 axes/slaves.

Ken Roach said:

The "Slave ID" address byte value in a Modbus serial network frame can be 0 (broadcast) or 1-247.

Addresses 248-255 are reserved for "future expansion of the protocol" and should not be used.

The number of physical devices you can put on an RS-485 network is an electrical limitation. The traditional limit is 32 devices actually wired in parallel to the RS485 bus, but as noted some modern transceivers can handle more.

Click to expand...

Schneider use 255 hardcoded into the TCP/IP slave of their Ethernet Meters, it wont respond on anything else....go figure !

Hi,

I think that Schneider recommends up to 8 Modbus RTU slaves/network (RS485). This because the update time will get worse, the more slaves that are connected.
If update time is not of importance, 31 slaves/network is ok.
If you have one Modbus RTU master and alot of slaves that needs to be connected to the master, one may consider Modbus RTU concentrators.
A Modbus RTU concentrator, is a device that acts as Modbus RTU slave towards the Main master and acts as a Modbus RTU master towards Modbus RTU slaves.

I work for HMS and we have a Modbus RTU concentrator called Anybus Communicator for Modbus RTU:
https://www.anybus.com/products/gat...cator/detail/anybus-communicator---modbus-rtu

My 2 cents

Patrick L

Pudden said:

Hi,

I think that Schneider recommends up to 8 Modbus RTU slaves/network (RS485). This because the update time will get worse, the more slaves that are connected.
If update time is not of importance, 31 slaves/network is ok.
If you have one Modbus RTU master and alot of slaves that needs to be connected to the master, one may consider Modbus RTU concentrators.
A Modbus RTU concentrator, is a device that acts as Modbus RTU slave towards the Main master and acts as a Modbus RTU master towards Modbus RTU slaves.

I work for HMS and we have a Modbus RTU concentrator called Anybus Communicator for Modbus RTU:
https://www.anybus.com/products/gat...cator/detail/anybus-communicator---modbus-rtu

My 2 cents

Patrick L

Click to expand...

Never seen Schneider recommend 8 on anything, we use Schneider metering exclusively and the only thing we see a documented limit on is the MCCB NSX comms units at 12 but that's got more to do with the way the power and comms is arranged than data

Its also very depended on the device and amount of data being read, for example 10 x slaves reading 1 value is a very different to 10 x slaves reading 1000 values.

I have seen up to 60 PM710 meters (not our install) working very happily on one Modbus RTU chain

Never used any of your products and i am sure you know them well but i can't see how your linked device will be of any help in this example

And to the OP, be careful you don't confuse physical slaves (nodes) with addresses, as above, maximum physical slaves (nodes) 32, maximum number of addresses 247