Modbus/TCP
What is Modbus/TCP?
Modbus/TCP is the IP-based variant of the Modbus protocol where a client issues read/write requests to a server’s data model (coils and registers) over TCP port 502.
Examples
- An HMI reads holding registers (function 03) from a PLC over TCP/502 to display tank levels and temperatures.
- A maintenance tool writes a single coil (function 05) to start a motor, restricted to a specific engineering subnet.
Discover 🔎
Modbus/TCP is one of the most common, simple protocols in industrial networks. A client (master) sends a request and a server (device or gateway) replies. The protocol exposes a logical data model—coils, discrete inputs, input registers, and holding registers—addressed by function codes. Its simplicity and ubiquity make it easy to integrate—and easy to misuse. Classic Modbus has no built-in authentication or encryption, so network design and policy enforcement matter.
Summary 📝
Modbus/TCP is simple, fast to integrate, and widely supported—but it trusts the network. Treat it as sensitive control traffic: keep it inside OT, allowlist who can talk and what they can do, prefer read-only paths, and use DPI to police function codes and register ranges. Clear memory maps and staging tests prevent addressing and endianness surprises.
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