Modbus RTU 485

Node and slave are virtually the same as far as the network is concerned. Node is a network point and is is typically used in describing a device on the network that is simply placing information on it when polled by the master. Slave is usually used in describing a device at a node that performs an action of some sort in response to commands from a master. So if I am looking at a meter, it can be referred to as a node, if I’m looking at a VFD, it’s a slave (node). The 32 node limit applies either way.

They are two different communications protocols, RS-485 defines the electrical signals, Modbus defines the data that travels via the those signals, and how it is managed by the software.

Node is a generic term for a connection point to the communication network. A Master is a node that can initiate a request for, and accept, data from a slave. A slave is usually a node that responds to data/status requests from a Master, but can also broadcast data for the Master to process.

There are a number of Modbus tutorials on the web, use your favorite search engine to find and read them.

Thanks for all the info.

I am still confused. in several posts above people have defined node and salve as the same thing

Maybe a better question is.... How are they different?

And to help fill in the blanks let me explain what im doing

I have a field device for modbus RTU

i could possibly need to talk to several hundred of the device

The device will act as a "slave" meaning it has registers and an address and i will send it a poll command and wait for its data to come back.

Can i have 32 of these things or 254

That is what i dont understand

When deciding how many devices can be on a single Modbus line, you need to know the baud rate, time between reads, how much data is being transferred per read, and other factors like line length. This can be used to calculate the theoretical throughput of the Modbus line.

Ensure all devices on a single line use the same settings (baud rate, stop bits, parity, etc).

The master will have its own maximum number of devices which can be added: older models may only take 32, most newer ones can cope with 256. The decision has to be made as to how many of the devices you have can be written to and read from within the cycle. I use power meters which have (the maximum) 255 bytes read from them each cycle. As the cycle is 10s, and there are propagation delays on long wires, we only connect 10 of these per line. We have run more, but occasionally lost some data as the acquisition loop ran again before all data was collected.

Above, I have used 'devices' instead of node or slave.

The limit will be set by the address space. For instance, an 8-bit address space is limited to 255 addresses. Again, this limit is going to be defined by the hardware used to implement Modbus over an RS-485 bus structure.

As stated previously, a node is just a connection to the bus. It could be a slave device, i.e. an RTU (remote terminal unit) or a master. The master issues commands for the RTU's to respond with data. The slaves (RTU) are set to listen for a command with their specific address in the command.

If you really need hundreds of remote units, it is probably a good idea to investigate what it would take to implement your control/data management with an Ethernet based communications system.

Good luck with your quest.

...and if you find you want to limit the number of dependants, RU-486 can be very effective, however, time is of the essence.

The screen shot below is from the Modbus spec which says there's a maximum of 247 (RTU) slave addresses:

So, according to the standard, a master should be able to address any number of slaves from slave #1 through slave #247.

Caveats:

Modbus is an open standard and no regulatory 'foundation' with Modbus bureaucrats/police to enforce the Modbus standards. Modbus has only the device vendor to stand behind whatever the implementation is. And implementations vary all over the map. Some vendors pay strict attention to the standard, other vendors don't.

I've run into some Masters that will address #248-#255. Will the slave accept an address greater than 247? Depends the slave implementation of Modbus. Maybe, maybe not. You need try it to see.

The limitations of the physical network as to the total number of unit-loads any slave or master can drive is what limits the number slave nodes on a multidrop network.
Once a network is 'filled' to the maximum number of slaves, a second or third or fourth network will need to be used to communicate with additional slaves.