I would like to understand how Modbus over TCP/IP overcome the 247 addressing restriction and how the request/response polling is different from MODBUS over serial?

 

Modbus TCP Slaves (Servers) each have a unique IP address. Therefore they can all have the same "Slave ID" so the maximum number of Slave IDs being 247 is no longer a limitation.

The RTU message format is modified for TCP by adding a 6 byte MBAP header at the start and removing the 2 byte CRC from the end. It is explained in detail at www.simplymodbus.ca/TCP.htm

Mark
www.simplymodbus.ca

 

Modbus TCP uses IP addresses an thus is not limited.

For example you may use a TCP/RS485 Modbus gateway like:
http://www.ucs.co.uk/index.php?pid=948

On the RS485 side of the gateway you may have 254 slaves.
But you may have several such gateways connected to ethernet. Thus an unlimited number of Modbus slaves can be connected.

 

For application where the master Station communicate with ..say 500 MODBUS slave units, does this mean All 500 communication channels are active at the same time? how would single Master Station manage request/response (polling)?

 

Modbus does not define a protocol for anything other than Master/Slave communications. One PLC must be the master and all others are slaves. If polling 500 slaves cannot be completed in a required cycle, then you must find another way to configure the network. You might consider dividing the network with a hierarchy of PLCs each controlling fewer slave points, and a master PLC to just accumulate data from the networked PLCs.

Dick Caro
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Richard H. Caro, Certified Automation Professional, CEO,
CMC Associates,
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Is this for Modbus/RTU over RS-485 or Modbus/TCP over Ethernet? The practical limit isn't going to be due to the Modbus protocol. The limit is going to be the media the data is carried over.

For Modbus/RTU over RS-485 the RS-485 wiring and chips have electrical limitations of 32 nodes per network (special chips and cables can extend this somewhat). You can also use RS-485 repeaters to string multiple RS-485 segments together, but if you have too many nodes on the same network you run into the problem that speed limitations caused by the electrical characteristics of RS-485 make the whole system too slow to be practical in most applications.

For Modbus/TCP over Ethernet you could theoretically have billions of slaves. TCP/IP is responsible for getting things where they need to go and the Modbus messages just go along for the ride. The practical limit is going to be the software and hardware limitations of your client (master) and servers (slaves).

In practical terms, Modbus defines what the messages look like. You need to look at what actual hardware and software is sold by whatever vendor you are using to see what they can offer you. Don't expect to just stick generic hardware together and expect to get the "theoretical maximum" performance out of it.

Any system design is going to be a compromise. If you don't understand the vendor's specs, then talk to their sales or technical reps and ask them to explain it to you. That's what these guys get paid for.

 

Yes, all 500 might be active at once ... but you might find the SCADA/HOST hardware has limitations - few consumer-class devices expect to have 500 active TCP/IP sockets open at the same time.

The one limitation here is that Modbus/TCP does NOT overcome the old RS-485 half-duplex behavior. So if you have 10 RS-485 Modbus serial device on one "Ethernet to RS-485 device", then they must still be polled one by one, so the 10 still run in series, not concurrently. The Ethernet device will hide that from you, so you can issue 10 requests at once, and they will slowly 'reply' over time. Holding 10 TCP sockets open to such a device wastes resources, as using a single socket to poll all ten works just as fast.