Modbus RS485 node limit

DLMUK · Oct 22, 2015

Hi,

I am looking at a modbus RS485 network with 40 slave devices in. What I would like to do is place all 40 slaves on the same network but... not address all of them simultaneously, due to hardware limitations I would be addressing them in groups of 8.

So my question is, I know the "limit" is 32 nodes per network based on a unit load of 1, but is this 32 active nodes, or just 32 nodes full stop?

Thanks,

tragically1969 · Oct 22, 2015

DLMUK said:
Hi,

I am looking at a modbus RS485 network with 40 slave devices in. What I would like to do is place all 40 slaves on the same network but... not address all of them simultaneously, due to hardware limitations I would be addressing them in groups of 8.

So my question is, I know the "limit" is 32 nodes per network based on a unit load of 1, but is this 32 active nodes, or just 32 nodes full stop?

Thanks,

That's the design limit according to the RS485 standard, so its 31 slaves and 1 master hence 32 nodes, I have seen chains with 70+ on there all working fine.

Not quite sure what you mean by addressing in groups of 8 though, if they are on the chain they are on the chain full stop.

DLMUK · Oct 22, 2015

Modbus addresses the nodes on the network. Each node has an address.

Therefore if a broadcast message requested information from nodes 1-8 then nodes 1-8 would respond with the information requested. If you have 9 nodes on the network node 9 would not respond. It would look at the broadcast message but not respond.

According to the modbus standard...

"...A RS485 system may implement a larger number of devices. Some devices allow the implementation of a RS485-MODBUS serial line
with more than 32 devices, without repeater.
In this case these MODBUS devices must be documented to say how many of such devices are authorized without repeater..."

tragically1969 · Oct 22, 2015

DLMUK said:
Modbus addresses the nodes on the network. Each node has an address.

Therefore if a broadcast message requested information from nodes 1-8 then nodes 1-8 would respond with the information requested. If you have 9 nodes on the network node 9 would not respond. It would look at the broadcast message but not respond.

According to the modbus standard...

"...A RS485 system may implement a larger number of devices. Some devices allow the implementation of a RS485-MODBUS serial line
with more than 32 devices, without repeater.
In this case these MODBUS devices must be documented to say how many of such devices are authorized without repeater..."

OK, that's fine but are you using broadcast, if so why ? Do your slaves allow authorisation without repeater ?

The limit is in the electronics of the slaves nothing to do with the addressing or how and when you request data from them.

Normally you would be addressing slaves by ID directly, if you have 70 slaves all on the chain and powered then that's 70 slaves regardless of how you address them.....

DLMUK · Oct 22, 2015

tragically1969 said:
OK, that's fine but are you using broadcast, if so why

Because thats how Modbus works.

tragically1969 said:
Normally you would be addressing slaves by ID directly, if you have 70 slaves all on the chain and powered then that's 70 slaves regardless of how you address them.....

My question is, surely an inactive slave is a smaller load on the master than an active slave?

The PLC I am using (S7-314) cannot have more than 8 active connections established at once, therefore it will need to cycle through the nodes in groups of 8.

tragically1969 · Oct 22, 2015

DLMUK said:
Because thats how Modbus works.

My question is, surely an inactive slave is a smaller load on the master than an active slave?

The PLC I am using (S7-314) cannot have more than 8 active connections established at once, therefore it will need to cycle through the nodes in groups of 8.

Not if its in Unicast mode it isn't.....

Not sure what you area asking really then in that case, seems you know all the answers.

DLMUK · Oct 22, 2015

tragically1969 said:
Not if its in Unicast mode it isn't.....

Not sure what you area asking really then in that case, seems you know all the answers.

I don't know all of the answers, hence why i'm asking. My opinion is that an inactive slave would be less load on the network... but I stand to be corrected.

I think my misunderstanding and using the phrase "broadcast" was a poor choice of words. I'm not actually broadcasting with an address 0, that was misinformation.

I suppose my system polls through each node one by one, but in groups of 8. Perhaps the fact that it is doing it in groups of 8 is irrelevant.

tragically1969 · Oct 22, 2015

DLMUK said:
I don't know all of the answers, hence why i'm asking. My opinion is that an inactive slave would be less load on the network... but I stand to be corrected.

I think my misunderstanding and using the phrase "broadcast" was a poor choice of words. I'm not actually broadcasting with an address 0, that was misinformation.

I suppose my system polls through each node one by one, but in groups of 8. Perhaps the fact that it is doing it in groups of 8 is irrelevant.

As I said before the limitation is the electronics, specifically the impedance added to the network by the transceivers, if you have 41 slaves on a network there is 41 slaves regardless if you are communicating to them or not, they all have to be active to receive/send a message.

Some slaves will have lower impedance hence >32 slaves, there is only one way of sorting this and that's bench test with the same amount of slaves and cabling as you would have in the real world. Either that or talk to the manufacturer of the slave devices and see what their design limitation is.

DLMUK · Oct 22, 2015

tragically1969 said:
As I said before the limitation is the electronics, specifically the impedance added to the network by the transceivers, if you have 41 slaves on a network there is 41 slaves regardless if you are communicating to them or not, they all have to be active to receive/send a message.

Some slaves will have lower impedance hence >32 slaves, there is only one way of sorting this and that's bench test with the same amount of slaves and cabling as you would have in the real world. Either that or talk to the manufacturer of the slave devices and see what their design limitation is.

Ok - i will contact the supplier first. Thanks for your help.

Pudden · Oct 23, 2015

Hi guys,

One way to overcome the limitations of 31 slaves is to use a Modbus concentrator, such as Anybus Communicator for Modbus RTU:
http://www.anybus.com/support/support.asp?PID=317&ProductType=Anybus Communicator

It can act as a Modbus slave on one side and Modbus RTU master on the other, meaning you can connect theoretically 31 slaves to the gateway...
However, the update time for each slave on a Modbus RTU network with 31 slaves will be in the range of a few seconds..

//Patrick L

KalleOlsen · Oct 23, 2015

DLMUK said:
Ok - i will contact the supplier first. Thanks for your help.

Or you could use an RS485 repeater. Google it, and you get a lot of hits.

Kalle

Modbus RS485 node limit

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