Understanding Modbus TCP/IP serial servers

I need to get a CompactLogix PLC to talk to an oil temperature controller which has a Modbus serial interface. Since the PLC has no serial port I would like to use an Ethernet-serial server.

I have various Moxa and Brainboxes Ethernet-serial RS232/485 devices to hand but I notice that the various vendors supply Modbus-specific devices.

Q1. What do the Modbus versions of these do that an RS485 won't?
Q2. What would be the difference in configuration in the PLC if both can be used?



Many thanks.

RS485 is an electrical standard, not a communication protocol.
Modbus is a communication protocol.

So if the bogstandard devices servers don't 'talk' modbus, it doesn't work very well.

The one that are modbus specific(or have modbus support) will understand how to grok Modbus and make it sane for you.

Transistor said:

...
I have various Moxa and Brainboxes Ethernet-serial RS232/485 devices to hand but I notice that the various vendors supply Modbus-specific devices.
...

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If what you want is to convert Modbus TCP into Modbus RTU (serial) you need a specific Modbus converter since both protocols are different.

Moxa MB3170. Job done.

Q1. What do the Modbus versions of these do that an RS485 won't?

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They're aware of the Modbus/TCP protocol. They will unpack the Modbus commands from the Modbus/TCP packet, convert them to Modbus RTU serial format, and send them to the target device that's designated in the Unit ID, receive and parse the response, and convert that to a Modbus/TCP response.

Ordinary "terminal servers" don't know about the Modbus/TCP protocol, so the client has to encapsulate Modbus RTU in a TCP frame and send that to the device for it to go out over the wire "raw" without interpretation. That's a little harder and less common.

Q2. What would be the difference in configuration in the PLC if both can be used?

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Modbus/TCP is ugly but do-able in the ControlLogix or CompactLogix (no offense to Control_Conn, an expert !). Encapsulated Modbus RTU would be almost a start-from-scratch matter.

Moxa MB3170. Job done.

Click to expand...

I would actually recommend the Moxa MGate 5103, or a similar device, instead. It functions as an easy-to-configure-using-Studio-5000 EtherNet/IP Adapter on the Ethernet side, and as a Modbus RTU Master on the serial side.

There are other EtherNet/IP to Modbus converters on the market, with a linear graph of ease-of-use versus price that goes from GridConnect EIP-485 to Prosoft PLX31-EIP-MBS, with datapoints labeled Hilscher and RTA and Red Lion and HMS.

Normal Serial to Ethernet adapters don't interpret the serial signals, and so Serial Modbus frames may be broken up in middle of the frame to create an Ethernet Packet.



The GC-NET485-MB is the perfect small compact solution


https://www.gridconnect.com/products/net485-mb-modbus-rs485-adapter

I consider that GridConnect to be a little tricky to learn and configure, but it's an extremely good value for money.

I happen to have one in my box of test-bench parts that I'm re-commissioning in our shop, so if you choose it (or want an explanation or example) let me know.

There are a lot of communication "Gateways" out there with varying levels of complexity.
The Spectrum Controls Universal Industrial Gateway is in that class however it is very simply to setup and use. It configures with a web browser and does not require any PLC programming other than making sure the data files / tags exist and are reachable from the outside. One device will interface with multiple protocols so you could do Modbus TCP/IP to Modbus RTU as well as EtherNet/IP to Modbus RTU and you can do it at the same time on the same Gateway.

https://www.spectrumcontrols.com/products/universal-gateway/

Thanks, all. I'm going to try the MOXA MB3170.