Compactlogix and Modbus

Hi Everyone

Ive got an AB 1769-L33ER (i read its modbus compatible) and im looking to communicate with a Chinese multi controller via Modbus.
Ive never gone down the Modbus path but from what I read, I can plug in an ethernet cable locally into the PLC and use 2 wires to communicate
With the multi controller.
Am I going about this the right way or am I better off getting a 3rd party device to make things easier?

Im not pulling a lot of data, just Disolved oxygen level and Ph, and the resolution doesn’t matter so it can be quite crude if needed.

Any help or advice greatly appreciated.

Are you trying to connect to a serial port through two wires of the ethernet port of your PLC? that will never work, they are two connections totally incompatible at the physical and electrical level.

There are two types of Modbus: ModbusTCP that works through Ethernet and ModbusRTU through serial port, they are two different protocols.

If your PLC supports ModbusTCP and you have to connect it with a Modbus RTU device, you need to use a ModbusTCP to ModbusRTU converter (gateway) or put a ModbusRTU master module in the PLC

I think you're misunderstanding the stuff you're reading. The CompactLogix is not directly "Modbus compatible" and Ethernet is fundamentally different than serial RS-232 or RS-485 signalling.

The best way to handle this is to buy a third-party network gateway that can be configured to poll data from your "multicontroller".

Read the multicontroller's documentation to figure out if it's Modbus/RTU or Modbus TCP (the Ethernet variant), and exactly what serial communication standard it uses (RS-232, 422, or 485).

There are a large number of A-B to Modbus network interfaces on the market. In my opinion the easiest ones to use that are also broadly available in the UK would be the HMS Fieldbus Communicator.

Prosoft makes an In chassis module that works well for modbus, downfall is that it is around $1000 +/-. I have used it several times and their support is pretty good if you have questions.

There are other manufactures but the prosoft module is probably the cleanest route.

Depending on your time frame Spectrum Controls will be releasing a product (early 2019) called the Gateway that will bridge Ethernet/IP (CompactLogix) to Modbus RTU (and/or Modbus/TCP). It's very simple to set up and does not require any messaging on the CompactLogix or Modbus RTU device side other than setting up a Tag in the CompactLogix to link the data to.

or you could get one of these...


https://www.automationdirect.com/ad...rnet_media_converters_-a-_gateways/mb-gateway

Scott (and the general readership)

It's easy to misunderstand the details of Ethernet-to-serial gateway or bridge devices and what protocols and applications they support.

There are many inexpensive Ethernet-to-serial gateway devices on the market which generally provide a virtual serial port to a PC. The PC runs a driver that intercepts serial port commands and transports them over an IP network to the device, which sends and receives them over the serial port, or the device is configured to accept raw TCP packets and convert/transmit them out the serial port.

These are not generally appropriate for use with PLCs, because most PLCs don't run a PC operating system that can host those virtual serial port drivers, or cannot easily send out raw packets that can be directly processed by those devices.

In addition, there is a category of Modbus/TCP to Modbus RTU serial converters. Because these protocols are well-defined and work well together, it's straightforward to use these with a PLC or other automation device that supports Modbus/TCP. The Automation Direct MB-GATEWAY device works like that.

CompactLogix and ControlLogix support Modbus/TCP only with a set of very complex sample code. I very strongly recommend against its use by novices. I'm an expert, and I don't use it because of the memory footprint.

A more limited set of devices on the market will actually convert from an EtherNet/IP explicit message or from an I/O frame with a specific data payload to Modbus RTU Serial. These are appropriate for use with ControlLogix and CompactLogix controllers.

The ones I have personally used include the HMS Communicator, Red Lion Data Station Plus, Digi One IAP, GridConnect EIP-MBS, and Real-Time Automation gateways.

The Real-Time Automation device is probably the easiest to use, and the Red Lion is probably the most flexible. If I needed to install just one today I would grab a new Red Lion DA10D DataStation, but if I had to recommend one to a novice user in the UK I would recommend the RTA gateway (for its ease of use) or the HMS Communicator (for its European availability).