Writing from FactoryTalk HMI Numeric Inout through Kepware to Modicon PLC (Scaling for write)

[TS_12889420]

I have a situation that I am trying to understand to see if this actually can be done. I have a FactoryTalk View application with a Numeric input that is scaled from -0.5 to 0.5 but it is writing directly to the Kepware Tag and is linked to a Modicon Register. I am not scaling the -0.5 to 0.5 in the hmi, I would like Kepware to perform that. So Ftalk input would be -0.5 to 0.5 and then Kepware would scale the -0.5 to 0.5 to 0 - 4095 and write 0 - 4095 to the Modicon PLC. Can this be done?

[pshashipreetham · Accepted solution]

Hi, 

Yes, this absolutely can be done in Kepware — and it's the right way to do it when you want to keep your HMI logic clean and offload the scaling to the OPC layer.

Kepware has a feature called Linear Scaling built into its Tag Configuration, which allows you to scale the raw PLC values (in your case 0–4095) to engineering values (e.g., -0.5 to 0.5and vice versa.

Here's how you can set it up:

1. Create the Tag in Kepware

• Browse or manually add the tag pointing to the correct Modicon register (e.g., 400001 or %MW1, etc.).

2. Enable Linear Scaling

In the tag configuration:

• Right-click the tag → Properties → Select the Scaling tab.

• Check Enable Linear Scaling.

3. Set Engineering Unit Range

• Scaled (Engineering) Range:

  • Low: -0.5

  • High: 0.5

4. Set Raw PLC Range

• Raw (Device) Range:

  • Low: 0

  • High: 4095

5. Direction of Scaling

• Verify that Scaling applies to both read and write operations.

  • This ensures that when FactoryTalk writes -0.5, Kepware scales and sends 0 to PLC.

  • When PLC returns 2048Kepware scales it back to 0.0 for FactoryTalk.

6. Bind this tag to your FactoryTalk HMI input

• Ensure your Numeric Input object in FactoryTalk is bound to this OPC tag.

• FactoryTalk will see only -0.5 to 0.5, unaware of the underlying 12-bit raw range.

Important Considerations:

• Ensure the tag datatype is INT or WORD (Modicon register type) that can handle 0–4095 properly.

• Double-check that Modicon doesn’t expect any special format (e.g., signed, BCD).

• If writing fails or scales incorrectly, try adjusting the tag’s data format (e.g., Unsigned Word vs Signed Word).

Thanks,