I need Help with a CompactLogix L35E

Can anyone please tell me if i can, and how to set a CompactLogix L35E PLC communication ports to Modbus protocol.

The CompactLogix 1769-L35E controller does not natively support Modbus on the serial or the Ethernet ports.

There is some complicated application sample logic available from RA Technical Support to implement Modbus RTU on the serial port. I do not recommend this for novice users.

To implement Modbus on a CompactLogix controller, I strongly suggest the use of a third party gateway device like a Digi IA One, a HMS Fieldbus or Real Time Automation gateway device, or a Prosoft module (chassis or standalone).

Thank you for the info.

do you know if i can use the Modbus poll software to poll some register for testing from the PLC DB9 to my computer or i have to go trough the Digi?

As I described above, if you want the CompactLogix to "speak" Modbus, you need to add Modbus to it in one of those ways.

If you want your computer to "speak" Modbus, you can use OPC drivers or something simple like ModScan32 or another Modbus simulator.

You have not specified if you want to use Modbus Serial (RTU or ASCII mode) of if you want to use Modbus/TCP over Ethernet.

You also have not specified if you want your CompactLogix to be a Modbus Master or a Modbus Slave.

If you can provide those sorts of details, I'm sure that you will get some additional advice.

i am using modbus serial RTU and modbus master

Are you using an HMI? Many HMI's support multiple protocols and could act as a converter.

yes i am using Ignition.

I am no Ignition expert, but from what I could see it looks like its all OPC based. They have a Modbus Ethernet driver, but not sure about support for serial devices. You could try to add a serial OPC device. If not, any OPC serial driver that supports Modbus should work. Once the data is in the HMI, then you'll probably need to write a script to transfer the variables over to the tags for the PLC. I have used Software Toolbox for OPC servers, but there may be less expensive alternatives out there. http://www.toolboxopc.com/

Take a step back: Ignition's OPC UA driver has a native ControlLogix driver that can browse and access all the tags in that CompactLogix controller.

Ignition works best over Ethernet. Using Ignition over Serial (Modbus RTU is always over a serial connection) would require a Modbus/TCP -> Modbus RTU converter like the Digi One IAP.

I very strongly recommend using the built-in ControlLogix driver to communicate from Ignition to CompactLogix.

alexFSC , what are you trying to communicate with using Modbus RTU?

could you add an SM2?

Thanks for the help so far, community. I work with alexFSC and I have a strong understanding of the current situation.

The software to communicate "Modbus" over the serial connection has been successfully installed in RSLogix 5000 and successfully downloaded to the CompactLogix 1769-L35E PLC.

We have downloaded ModScan32 and been able to successfully communicate directly through the serial connection from a laptop/computer to the PLC.

Ultimately, we will need to speak Modbus/TCP over Ethernet/Internet to communicate to the PLC. The CompactLogix PLC will be a Modbus Slave (if I got the terminology correct: what I meant was it will have values that our internet server will read, similar to the PLC being an OPC client, assuming I got *that* terminology correct).

We are trying to communicate with the PLC in Modbus to get the holding register values (amongst others) so we can display them over the internet.

Ken, we are now having problems communicating through the internet to Digi and, thus, to the PLC. You suggested there is a way to tell some Digi devices to act as a converter from Modbus TCP to Modbus RTU. I sure would like to see some help on how to do that.

For PLC GURU, yes, we could probably add an SM2 if I knew what it was.

adamFSC said:

We are trying to communicate with the PLC in Modbus to get the holding register values (amongst others) so we can display them over the internet.

Click to expand...

As Ken mentions, why are you not using EthernetIP? Why involve modbus at all?

PLC GURU said:

could you add an SM2?

Click to expand...

Ewww. 1769-SM2 is a PLC module, but not made by the PLC group. Its really a drive product that can be tricked into working on Modbus RTU.
Apparently, AB drives speak a special version of Modbus RTU (with addresses >128), so somebody decided to sell the 1769-SM2 for a modbus interface.
Read my tome on the AB Knowledgebase, and move with caution.

So the only option with the cellular link is RS232 with a Modbus protocol? Are you trying to use the ConnectPort Serial Wireless server? Also, do you have a PC based HMI connected to the Compact Logix running Inductive Automation's Ignition communicating over EIP?