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Resources · Interactive Guide
Same registers, same function codes — different wire. What actually changes between serial RTU and Ethernet TCP, when to use which, and how to bridge the two without touching your devices.
Modbus is one protocol with two transports. RTU sends compact binary frames over a serial RS-485 bus with a CRC checksum, and one master polls each slave in turn. TCP wraps the exact same request in a small MBAP header and sends it over ordinary Ethernet, where many clients can talk to a device at once. The registers, function codes and data are identical — which is why a converter can translate between them transparently.
Ask a meter for register 40001 and the question itself — function code + address + count (the PDU) — is byte-for-byte the same. Only the envelope differs: RTU adds a slave ID and a CRC; TCP adds an MBAP header and lets the network handle integrity.
Modbus RTU frame
the frame crawls the serial bus · delimited by 3.5 chars of silence
Modbus TCP frame
same packet on Ethernet — notice it travels quicker · no CRC needed
The grey PDU cells are identical in both frames — the coloured cells are just the envelope. Below each frame, the packet travels its wire: amber on the serial bus, blue on Ethernet.
On an RTU bus only the master speaks first — it polls slave 1, waits, polls slave 2, waits. Nobody else may ask. On TCP, the device is just a network node: SCADA, an HMI and a historian can all query it at the same moment. That single difference drives most upgrade decisions.
RTU · polled bus
ask → wait for reply → ask the next — one conversation at a time
TCP · switched network
three requests leave together — the meter answers all of them
| Modbus RTU | Modbus TCP | |
|---|---|---|
| Physical layer | RS-485 / RS-232 twisted pair | Ethernet (RJ45), port 502 |
| Framing | Silence-delimited binary + CRC-16 | MBAP header, TCP handles integrity |
| Who can ask | One master polls the bus | Many clients, simultaneously |
| Addressing | Slave ID 1–247 on the wire | IP address (+ Unit ID for bridges) |
| Topology | Daisy-chain bus, 120 Ω ends | Star, via switches / routers |
| Speed | 2400 – 115200 bps | 10 / 100 Mbps + |
| Distance | Up to ~1200 m per bus segment | 100 m per segment — unlimited via network |
| Registers & FCs | Coils, registers, FC 01–16 | Identical — same data model |
The last row is the one that matters: because the data model never changes, moving between RTU and TCP is a transport swap, not a re-engineering project.
In practice most plants run both: serial devices on the floor, Ethernet upstairs. That is exactly what the bridge below is for.
A protocol converter translates live in both directions — your serial meters appear as Modbus TCP devices on the LAN, or your TCP SCADA reaches an RS-485 bus. Because the PDU is identical, the translation is transparent: no firmware changes, no register remapping.
One request, crossing live: blue TCP frame in from SCADA, amber RTU frame out to the meter — and the reply swaps back. Only the envelope changes.