RTA's Blog
The One Teeny Error That Sabotages Modbus to ControlLogix Integration Projects
Jan
Summary: When Modbus data appears incorrect in a ControlLogix or CompactLogix PLC, the cause is rarely communication or wiring issues. In most cases, the issue is byte and word ordering differences between Modbus devices and Logix PLC memory structures. Modbus registers are often interpreted using big-endian formats, while Allen-Bradley PLCs typically use little-endian representations. This mismatch can silently corrupt values even when data transfer appears successful. This article explains how endian differences, byte swapping and word swapping affect Modbus integrations, why simple fixes often fail and how to correctly map Modbus registers into Logix PLC tags without trial-and-error debugging.
The Problem: About once a month or so, our Enginerds get a frantic call from a Control Engineer moving Modbus data to a ControlLogix or CompactLogix PLC. It’s always the same message:
“I see data moving but the values are completely wrong!”
The problem is easy to understand – the fix, not so much.
Why Modbus Integrations Fail
The one big reason that Modbus integration with Logix PLCs fail is that Modbus data is commonly stored in memory as big-endian, while Logix PLCs tend to be little-endian.
Endian is the order of bytes in memory. Big-endian (most significant byte first) is commonly (though now always) how Modbus devices store data. Little-endian (least significant byte first) is commonly how PLCs store data.
When two bytes of big-endian with a value of 01 02 (1×256 + 2) are transferred to a little-endian system, that same word is interpreted by the little-endian system as 513 (2×256 + 1).
The big red flag is that you can’t count on a particular implementation of a Modbus system to be big-endian or little-endian. You’ll have to check.
Why Simple Byte-Swapping isn’t Always the Answer?
Modbus devices only support bits (coils) and 16-bit unsigned integers (registers), but Modbus device applications often treat Modbus registers as more sophisticated types. Modbus applications accomplish that by overlaying 16-bit unsigned integers with other types.
How do Modbus Applications Overlay Modbus Registers?
- By using two registers as signed integers – while technically, the value in a Modbus register is unsigned, a Modbus application might treat it as a signed integer
- By using two registers as doubles – a Modbus application might treat two Modbus registers as a double. The most significant word (MSW) would commonly be the first register (but not always)
- Byte using four registers as floating point – a Modbus application might treat four registers as a floating point value.
In the last two cases, transferring that data to another system means that not only do the bytes need to be swapped, but the words need to be swapped too:
| Original Modbus | BYTE A | BYTE B | BYTE C | BYTE D |
|---|---|---|---|---|
| Byte Swapped | BYTE B | BYTE A | BYTE D | BYTE C |
| Word Swapped | BYTE C | BYTE D | BYTE A | BYTE B |
| Both (“The Fix”) | BYTE D | BYTE C | BYTE B | BYTE A |
Yes, It’s a Nightmare for the Control Engineer.
How to Improve Modbus to PLC Data Conversions?
Three important actions to implement Modbus to ControlLogix and CompactLogix data conversions:
- Locate the Modbus documentation for the specific revision of your Modbus device.
- Identify if your Modbus device is big-endian or little-endian
- Identify the Modbus registers to map and the data type of those registers used by the Modbus application
- Identify the destination tag in the PLC data table for that data
- Verify the endian of your PLC
- Configure the conversion by activating byte-swapping, word-swapping or both as needed
Some Common Questions:
Because Modbus devices and Logix PLCs often use different byte and word ordering, causing values to be misinterpreted.
Modbus devices commonly use big-endian, but little-endian Modbus devices do exist.
No, floating point and multi-register values often require both byte and word swapping.
Check the vendor documentation or evaluate your Mobus device with one of the common tools listed here: https://www.andivi.com/top-modbus-sensor-testing-tools/.
Use a gateway that explicitly supports configurable byte and word swapping and allows value validation before deployment.
What Modbus Gateways are Available from Real Time Automation?
RTA gateways are specifically designed to perform byte swapping and word swapping needed when moving Modbus registers to a Rockwell PLC data table. When a control engineer selects one or more Modbus registers to transfer to a PLC tag, byte swap, word swap or both can be selected.
| If your PLC is… | And your Modbus device(s) are… | You need gateway | Operating as |
|---|---|---|---|
| Control/CompactLogix | Modbus RTU Slave Devices | 460ETCMM | EtherNet/IP Adapter |
| Control/CompactLogix | Modbus RTU Master | 460ETCMRS | EtherNet/IP Adapter |
| Control/CompactLogix | Modbus TCP Client Device | 460ETCMC | EtherNet/IP Adapter |
| Control/CompactLogix | Modbus TCP Server Devices | 460ETCMS | EtherNet/IP Adapter |
Ready to tackle a Modbus to ControlLogix and CompactLogix data conversion project? Learn more by contacting an Enginerd application engineer at 262-436-9299 or solutions@rtautomation.com.
