Standalone PLC


Thread Starter

ghazanfar farooq

How can I make a standalone PLC, let's say any brand, communicate with a HMI using MODBUS protocol?
Very general question. To give a more specific answer, we need to know the brand of PLC and HMI you have. However, generally speaking... your PLC should be capable of communicating via the Modbus protocol, either natively, or through an interface module. The HMI would also need drivers for Modbus, or some kind of DDE or OPC server which can communicate to your PLC via Modbus.

Jerry Miille

You just need to give it a Modbus communications port to connect up to the HMI. A general purpose converter that can do this easily for many brands of PLC is the MARC Omnii-Comm module. It can directly connect to Allen-Bradley, Square D, Toshiba, GE, Reliance and other PLCs with one serial port and can be set up to have a Modbus Slave port for connection to the HMI. You can see a data sheet at this address or contact me directly at [email protected] for more information.

Jerry Miille

Rainer Lehrig

We have exactly done that, using Linux/Windows and a TwidoPLC from Schneider Electric. The TwidoPLC is connected to the serial port of the computer. The protocol used is ModbusRTU.

In the computer we have a SharedMemory. A daemon writes all inputs from the Modbus to SharedMemory. The daemon also reads from a Mailbox. Output telegrams to the Modbus may be send to this Mailbox.

The HMI reads the SharedMemory and writes to the Mailbox.

For more:

You may contact me from there or write to our Forum.
Ask a general question... I'd say you need to "take some steps" to make sure your PLC can talk Modbus, which may include programming! Just kidding, but you really need to be much more specific with your question. What brand PLC? How much data and what mix of register (integer) and coil
(discrete) values? What is the purpose - control or data collection? Does the HMI talk Modbus RTU or ASCII? etc., etc.

Steve Myres, PE
Automation Solutions
(480) 813-1145

Ghazanfar Farooq

I am really grateful to you for the answer you provided me. However, I am a little confused, what is Daemon??? Also I visited the website you mentioned but I could not find any relevant information. I will be grateful if you can give me some more information on it.

Ghazanfar Farooq

I am grateful to you for providing me with the solution. To be specific, I am using Wonderware's Intouch 8.0 for HMI, the server that I am using is my own, as far as the PLC is concerned I am still undecided as to which one I should purchase. The PLCs which have built in MODBUS capability are very expensive and my budget wont permit me to go for them. So that is why I decided to use a Standalone PLC, and then somehow make it communicate with the HMI but the question is HOW???? I will be grateful to you if you can provide me with more suggestions, such as which PLC i should go for (my budget for PLC is around $500).

Steve Myres, PE's DL250 has a serial port which can act as Modbus master or slave (RTU only), and a complete system with I/O might cost less that $500, depending on signal count. I've used them many, many times as Modbus slaves and they are extremely reliable. As a lower cost solution, I believe the DL06 series may also support Modbus comms natively.

Rainer Lehrig

A daemon is a term frequently used on Unix. It is equivalent to a Service on Windows.
- A program running in the background and doing some service for you
- In this case Modbus->SharedMemory and Mailbox->Modbus

Some Documentation is included on
Documentation->Booklet->Modbus using rllib

Actually you no longer have to write the daemon by your own, because it is generated from a small IniFile.

For more information, contact me in our Forum.

Automation Linse

Interestingly enough, most small/micro PLC can now be set or loaded to speak Modbus for free (Momentum, Twikdo, AB/MicroLogix12/1500, Siemens S7-2??, GE/VersaMax, Omron/???, PLCDirect). I always get a kick out of Rockwell guys saying "Oh, we don't get many Modbus requests anymore" - yet every new firmware release for MicroLogix 1200/1500 come with NEW Modbus features. I guess they have extra time over there ;^)

However, in general few larger PLC do - so you'll find you can BUY a new PLC & select one that includes Modbus, but just taking any existing PLC and very likely it will NOT speak Modbus.

Bob Peterson

My take on this is that the people marketing the smaller PLCs are probably positioning them for use as RTUs, as most RTUs support Modbus, but think there is no real need in the big boys, or are afraid it will cut into the license fees from prosoft for their Modbus interfaces.

Bob Peterson