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Modbus

What Are Modbus RTU and TCP?

The serial (RTU) and Ethernet (TCP) variants of Modbus, industry's most common open protocol, its register logic and common issues.

4 min readUpdated: 2026-07-05

Modbus is a simple, open protocol working on a master/slave (client/server) model. Modbus RTU runs over serial (RS-485), Modbus TCP over Ethernet.

Coil (0x)
Read/write bit (output)
Discrete Input (1x)
Read-only bit (input)
Holding Register (4x)
Read/write 16-bit
Register types
The most common RTU issues: wrong baud/parity, missing termination resistor (120 Ω) and slave-address conflicts.

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