What Is the Modbus Transaction Identifier
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When working with Modbus TCP, it is important to understand how a Modbus TCP frame is structured and how each field is used during communication. One field that often raises questions is the Transaction Identifier. While it is not always visible to end users, it plays a role in how Modbus TCP messages are matched and validated between a client and a server.
This article explains what the Modbus Transaction Identifier is, where it appears in a Modbus TCP frame, and how it is used during message exchanges. The explanation is based strictly on the standard Modbus TCP frame structure and does not introduce any behavior beyond what is defined in common Modbus implementations.
Modbus TCP Frame Structure
A Modbus TCP message is composed of several fields that together define how the message is interpreted. When sending a Modbus TCP frame, the frame can be broken down into six main sections:
1) Transaction Identifier (2 bytes)
2) Protocol Identifier (2 bytes)
3) Length Field (2 bytes)
4) Unit Identifier (1 byte)
5) Function Code (1 byte)
6) Data Bytes (n bytes)
These fields appear in a fixed order at the beginning of every Modbus TCP message. Together, they define how the message should be routed, interpreted, and processed by the receiving device.
Fields Most Relevant to Modbus Data
In many practical Modbus TCP applications, the most important fields from a data perspective are the Unit Identifier, the Function Code, and the Data Bytes. These fields determine which device is being addressed, what operation is being requested, and what data is being read or written.
Some Modbus scanners and diagnostic tools focus primarily on these fields when polling devices and displaying information. In these cases, the Transaction Identifier may not be surfaced to the user or may be handled automatically by the software without requiring manual configuration.
Purpose of the Transaction Identifier
The Transaction Identifier is a two-byte value that is included at the start of every Modbus TCP request and echoed back in the corresponding response. Its primary purpose is to help the Modbus TCP client associate a received response with the original request that was sent.
This becomes particularly relevant when multiple Modbus TCP requests are issued in close succession. Because TCP is a stream-based protocol, responses may not always arrive in the same order as requests were sent. The Transaction Identifier provides a way to synchronize messages and ensure that each response is matched to the correct request.
How Scanners Use the Transaction Identifier
Some Modbus TCP scanners and client implementations actively check the Transaction Identifier when processing responses. In these cases, the scanner compares the Transaction Identifier in the response message to the value that was used in the original query.
If the Transaction Identifier in the response does not match the expected value, the scanner may treat the response as invalid. This can result in the response being discarded and the request eventually timing out, even if the rest of the Modbus message is correctly formatted.
Transaction Identifier and Message Validation
The use of the Transaction Identifier for validation is not about the content of the Modbus data itself, but about message synchronization. By checking this value, a Modbus TCP client can confirm that the response it received corresponds to the request it sent, rather than to an earlier or unrelated transaction.
This mechanism helps improve robustness in environments where multiple requests are in flight or where network delays could cause responses to arrive later than expected. It is one of the elements that distinguishes Modbus TCP from serial Modbus variants, which rely more heavily on timing and line discipline.
FAQ
What is the Modbus Transaction Identifier?
The Transaction Identifier is a two-byte field in a Modbus TCP frame used to match response messages with their corresponding request messages.
Is the Transaction Identifier required for Modbus TCP communication?
Yes. It is part of the Modbus TCP frame header and is included in both requests and responses, even if it is handled automatically by the client software.
Why do some Modbus scanners ignore the Transaction Identifier?
Some scanners focus primarily on the Unit Identifier, Function Code, and Data Bytes and may not expose the Transaction Identifier to the user, handling it internally instead.
What happens if the Transaction Identifier does not match?
If a client checks the Transaction Identifier and the value in the response does not match the request, the response may be considered invalid and the request may timeout.