RTU is a timed serial frame
A Modbus RTU Application Data Unit contains server address, function and data, followed by CRC16. Frame boundaries depend on serial timing, so baud rate, parity, stop bits, adapter behavior, turnaround, and silent intervals are part of the protocol path.
TCP adds an MBAP header
Modbus TCP places the PDU after a seven-byte MBAP header containing transaction ID, protocol ID, length, and Unit ID. TCP provides ordered delivery and checksum handling; Modbus therefore omits the RTU CRC. The transaction ID pairs concurrent requests and responses.
Gateways combine both failure domains
A TCP-to-RTU gateway terminates the network request and performs a downstream serial transaction. An open TCP port proves only the first hop. Unit ID, serial framing, timing, CRC, and the downstream server can still fail.
- 0A usually indicates an unavailable gateway path
- 0B indicates that the downstream target did not respond
- A timeout without a Modbus exception may originate on either hop
- Preserve the TCP request and gateway route configuration together
Choose from the real topology
Use RTU for a serial device or multidrop RS-485 bus. Use TCP when the server or gateway is reached over Ethernet. Use Modbus TCP Security when the deployment requires TLS identity and encryption. Do not choose the transport from a register table; choose it from the physical and network route.