Modbus TCP and Modbus RTU carry the same requests over different wires. The register model, function codes and exceptions are identical. What differs is the framing, the addressing and — most importantly in practice — the timing.
| Modbus TCP | Modbus RTU | |
|---|---|---|
| Transport | Ethernet, TCP port 502 | RS-485 or RS-232 serial |
| Framing | MBAP header: transaction, protocol, length, unit | Address byte, then CRC-16 at the end |
| Error check | TCP's own checksum | CRC-16 in the frame |
| Addressing | IP address, plus a unit ID | Slave ID 1–247 on the line |
| Concurrency | Possible, but many devices dislike it | Strictly one request at a time |
| Typical failure | Connection refused, timeout | CRC error, silence, wrong baud |
The unit ID is not decoration
On serial the slave ID selects one device among many on a shared line. On TCP the IP address already identifies the device, so the unit ID is often 1 or 255 and ignored — except when the target is a gateway bridging TCP to a serial segment. Then the unit ID selects the downstream device, and getting it wrong produces exception 0B or a timeout.
Timing is where serial bites
- Every device on one RS-485 line must share baud rate, parity, data bits and stop bits. One mismatched device produces CRC errors for everyone.
- Frames are delimited by silence — a gap of about 3.5 character times. Polling too fast can run frames together.
- The line is half duplex. Only one device may transmit at a time, so a client must wait for the response before sending the next request.
- At 9600 baud a 20-register read takes a few tens of milliseconds on the wire alone. A 100-tag poll list cannot run at 100 ms.
What changes in your configuration
Moving a tag map between TCP and RTU changes the connection, not the tags. Register types, addresses, data types, byte order and scaling are unchanged. This is why a device definition captured from one transport can be reused on the other.
There is also Modbus ASCII
A third framing, readable as hex characters with an LRC checksum. It is slower and rarer, mostly found on older equipment. If you have the choice, RTU is the better serial framing.