Arduino Modbus connection to a C-more HMI

January 4, 2013

Using an Arduino to collect information from an array of sensors is an economical alternative to using a PLC when the data collection is not critical. In this post I will explain the basics of connecting the hardware of an Arduino to a C-More touch panel from Automation direct. I will then show a basic setting to allow MODBUS communication to work between the two devices.

Hardware you will need:

C-more touch screen:

C-more Comm cable:

Arduino, in this case I used a Duemilanove:

RS232 to TTL converter:

Parts to make a rj12 to db9 connection. I actually used a serial cable cut in half and spliced into a RJ45 jack. You may want to consider a part like this:

Software you will need:

C-more programming software:

The Arduino IDE, I used version 1.01. At the time of writing this 1.03 is available.

The MODBUS library that I got to work with the C-more.

*note this library does not work with Arduino IDE prior to the 1.00 release.

Setting up the hardware.

Create the connector to convert the db9 connection to the RJ12 connection.

DB9 connections required are pin 2, 3,and 5. Pin 7 and 8 will not be used.

To make the RJ45 jack as I did connect make these connections.

DB9 — RJ45

Pin 2 – B4

Pin 3 – B5

Pin 5 – B2

When plugging in the RJ12 to the RJ45, be sure to center the plug in the jack.

###Edit adding Photo to show the intermediary cable.

DB9 to RJ45 hacked cable. Ideally should be DB9 to RJ12.
DB9 to RJ45 hacked cable. Ideally should be DB9 to RJ12.
Here the EA-2CBL RJ12 plug is plugged into the center of the RJ45 jack.

Here the EA-2CBL RJ12 plug is plugged into the center of the RJ45 jack.

( of course you can create your own DB9 to DB15 cable instead )


DB9 |–| DB15

Pin 2 – Pin 3

Pin 3 – Pin 2

Pin 5 – Pin 5

Connecting the Arduino with the RS232 to TTL DTE board

Place jumpers on the RTS and CTS connections at the top of the board. clip_image004

Then make these following connections:

Arduino |—| DTE

5V – VCC

Gnd – GND

Digital pin 0 – RX (Use a jumper wire or place a switch in the middle of this connection.)

Digital pin 1 – TX

Do not connect the RTS or CTS pins from the board to anything.

Configure the Software.

For the Arduino:

Place the MOBUS library into the Arduino IDE library. See this for more information on how to do that:

You can also copy the MODBUSslave example into the example folder.

Launch the Arduino IDE.

Open the MODBUSslave example. ( from the example menu or by navigating to it in the MODBUS library folder.

Now connect to your board. Remove the jumper to the RX pin. ( or turn off the switch if you used one) This will allow the computer to communicate with the Arduino. If you forget to do this, the DTE board will interfere with the upload of the sketch.

Upload the Example to the board.

Once complete, reconnect RX to D0.

C-more setup.

In the C-more software, Start a new project for the panel you have.

Go to Setup -> Panel Manager

On the com port, select the DEV001. Now would be a good time to rename this to ARDUINO.clip_image006

Set PLC Protocol to Modicon Modbus RTU

Select direct connection

Set PLC SLAVE number to 1.

Baud Rate 9600

Parity None

Stop bit 1

Control RTS No

Require CTS No

Select RS 485 No

Byte Order High Byte, Low Byte

Word Order Low Word, High Word

Character Order in register Char1, Char2

Registers per Message 32

Coils per message 64

Register Write Function Code 06

Coil Write Function Code 05

Time out 30

Poll time 3

Click ok.

Let’s place a Numeric Display on the screen,

In the Data Display tag open the tag database to create a new tag.

Click on Add


Select ARDUINO for the device name, Give the tag a name like ReadOnlyRandom.

Set it to Unsigned INT 16

Select memory type 3

Enter an address of 1

Click ADD, Select the new tag Click OK. (notice that the PLC address is shown as 30001)

Click OK again to assign the tag to the display.

Save and send the project to the C-more.

Test it!

Connect the C-more to the Arduino using the com cable.

You should see LEDs on the DTE board flicker and a random number updating on the C-more screen. Congratulations, you have gotten past the biggest hurdle to getting a C-more to talk to an Arduino.

If you do not see this, check that you have RX and Digital pin 0 connected. Also try swapping the RX and TX connections.

Writing to an Arduino

Ok so now you can read from an Arduino, but how about writing to it.

Continuing with the example, let’s add a Numeric Entry in the same fashion as we did with the Numeric Display.

But this time when you add the new tag, Name it WriteableNumber with a memory type of 4 and an address of 9.


Save and transfer the project to the screen.

You should now see a 9 in the new entry.

Change the 9 to a different number. Now you have written to the Arduino.

To make use the register in the Arduino, update the example to use regBank.get(40009).

So in this post I covered how to setup the hardware to between a C-more touch screen HMI, and an Arduino. I also showed how to load the MODBUS library and example onto the Arduino. Lastly I showed how to configure the settings on the C-more to make use of the MODBUS RTU protocol in conjunction with the supported features of the Arduino.

The MODBUS library I used is not the only one available on the web, but it is the first one I got to work with a C-more.

Remember, when you are uploading to the Arduino, you must remove the RX connection.

In the next post, I will share a derived library that takes advantage of SoftSerial. This will allow You to connect the DTE board to Pins 2 and 3 instead of 0 and 1. There by allowing you to upload to the Arduino while remaining connected to the C-more.

Lastly I will share some methods to make it easy to read and write discrete bits using whole registers, instead of coils.

Please let me know what you think of this post. I welcome questions and feedback.