Modbus RTU and Modbus TCP use the same data model — coils and registers read with the same function codes — but travel over different networks. Modbus RTU runs over serial RS-485 cabling, suits daisy-chained meters and field devices over long distances, and only allows one request on the bus at a time. Modbus TCP runs over Ethernet on port 502, is faster, supports multiple clients and fits existing IP networks. Gateways convert between the two, so many sites use both.
What they have in common
Both are versions of the same protocol, originally published by Modicon in 1979 and now maintained by the Modbus Organization. A client (historically “master”) sends a request to a server (“slave”), which replies with data. Data is organised into four tables:
| Table | Access | Typical use | Read function |
|---|---|---|---|
| Coils | Read/write, 1 bit | Outputs such as relays | 01 |
| Discrete inputs | Read-only, 1 bit | Status contacts | 02 |
| Input registers | Read-only, 16 bit | Measurements | 04 |
| Holding registers | Read/write, 16 bit | Measurements and settings | 03 |
Because the data model is identical, the register map in a meter or PLC manual applies to both variants.
Side-by-side comparison
| Modbus RTU | Modbus TCP | |
|---|---|---|
| Physical layer | Serial — usually RS-485, sometimes RS-232 | Ethernet (or any IP network) |
| Topology | Multi-drop daisy chain | Star / switched network |
| Distance | Up to about 1,200 m per segment at lower baud rates | 100 m per copper Ethernet run; unlimited with switches or fibre |
| Speed | Typically 9,600–115,200 bit/s | 10/100/1000 Mbit/s |
| Addressing | Slave ID 1–247 on the bus | IP address (plus unit ID when behind a gateway) |
| Concurrency | One request at a time per bus | Many clients and requests in parallel |
| Error checking | CRC-16 in each frame | Relies on TCP/IP checksums |
| Cost per device | Lower — common on meters and sensors | Higher, falling |
When to choose Modbus RTU
- Many meters in one distribution board or along a riser, wired on a single twisted pair.
- Devices that only offer RS-485 — still the case for many sub-meters, sensors and VSDs.
- Long runs through a plant where running Ethernet to each device is impractical.
When to choose Modbus TCP
- Devices already on an Ethernet network, such as PLCs, power analysers and building controllers.
- Faster polling or large amounts of data from each device.
- Several systems — SCADA, a gateway and an engineering laptop — that need to read the same device.
Mixing both with a gateway
A Modbus gateway sits between an RS-485 bus and the Ethernet network, exposing each serial device as a Modbus TCP unit ID. This is the most common pattern in practice: RTU for the field wiring, TCP for everything upstream.