Modbus TCP is the TCP/IP version of the Modbus protocol — same 16-bit registers and function codes, but no RS485 bus and no CRC. The gateway uses it to expose Modbus meter readings to the AMI Platforms over the network, without the AMI Platforms needing to manage serial communication.
Why does «Modbus TCP» appear in the app?
When configuring the TCP–UART bridge on the gateway, the «Modbus TCP» option tells the gateway to read the meter via RS485 (using the serial Modbus protocol) and then convert those readings to Modbus TCP format before sending them to the AMI Platforms. In other words: the RS485 bus is still Modbus RTU — Modbus TCP is only the format used for communication toward the server.
How the conversion works
The gateway performs the conversion between the two formats transparently:
- Reads the meter via RS485 using Modbus RTU (frame: Address + Function code + Data + CRC-16).
- Extracts the function code, data and slave address.
- Builds a Modbus TCP frame: 7-byte MBAP header + PDU (no CRC).
- Exposes the response on the configured port (default 8090). The AMI Platforms connect and read as a standard Modbus TCP client.
This conversion avoids the AMI Platforms having to manage RS485 timings, frame silence and CRCs — it simplifies integration on the server side.
The MBAP header: what replaces the CRC
The structural difference between Modbus RTU and Modbus TCP lies in the header. Modbus TCP uses a 7-byte MBAP header instead of the RTU's 2-byte CRC-16:
| Field | Size | Function |
|---|---|---|
Transaction Identifier | 2 bytes | Sequence number to match requests and responses in multiplexed connections. |
Protocol Identifier | 2 bytes | Always 0x0000 for Modbus. Reserved for future extensions. |
Length | 2 bytes | Following bytes: Unit ID (1 byte) + PDU. |
Unit ID | 1 byte | Equivalent to the slave address in RTU. The gateway maps it from the meter's Modbus address. |
TCP handles packet integrity — that is why the RTU CRC-16 is not needed.
«TCP» vs «Modbus TCP» — which to choose
The gateway offers two different bridge modes:
- TCP mode — Forwards the meter's serial frame as-is (raw bytes) to the AMI Platforms. The AMI Platforms interpret those bytes directly — useful for DLMS/COSEM meters (Microstar, ISKRA, Landis+Gyr) whose central system already speaks DLMS.
- Modbus TCP mode — Converts the Modbus RTU frame to Modbus TCP format (MBAP header). The AMI Platforms read it as a standard Modbus TCP client. Required for Modbus meters like the Star DTS27.
Practical rule: DLMS meter → TCP mode. Modbus meter → Modbus TCP mode.
Which meters require Modbus TCP mode?
- Star DTS27 (China) — RS485 Modbus RTU meter. Use «Modbus TCP» mode at 9600 baud.
- Eastron SDM120 / SDM630 — Modbus RTU meters. Use «Modbus TCP» mode.
- Photovoltaic inverters with RS485 port (Huawei SUN2000, SMA, Fronius) — speak Modbus RTU. Use «Modbus TCP» mode.
- Any meter configured with Modbus protocol (not DLMS) uses «Modbus TCP» mode.