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Modbus Master basic info

1.1 Communication Parameters

Parameter RTU TCP Where you get it
Slave / Unit ID Required Device dependent; keep the device's required unit identifier Slave manual / configuration
IP address Not used Required Network configuration
TCP port Not used Usually 502 Slave manual / network configuration
Serial port Required Not used AG-702 device interface
Baudrate Required Not used Slave communication settings
Data bits Required Not used Slave communication settings
Parity Required Not used Slave communication settings
Stop bits Required Not used Slave communication settings
Function code Required per query Required per query Slave Modbus map
Start address Required per query Required per query Slave Modbus map
Quantity Required per query Required per query Slave Modbus map
Datatype / byte order Required for decoded values Required for decoded values Slave Modbus map

1.2 Modbus Function Codes Used by This Application

Function code Standard meaning How it appears in this UI
01 Read Coils 01 — Read/Write Coils
02 Read Discrete Inputs 02 — Read Discrete Inputs
03 Read Holding Registers 03 — Read/Write Holding Register
04 Read Input Registers 04 — Read Input Register

The application UI exposes these four query options. The Modbus specification defines 01/02/03/04 as read operations; write functions use other standard function codes. This guide does not add write configuration that the current UI does not expose.

2. Supported Hardware & Devices

This section separates OpenWrt gateways that can host Modbus software from field devices that can act as Modbus slaves and provide data to the gateway.

2.1 OpenWrt Modbus Gateways

The following Atreyo gateway models have public product information explicitly listing OpenWrt and Modbus support. This is a platform capability table; the exact Modbus Master UI in the supported-device section is verified in this guide only on AG-702.

Gateway OpenWrt Modbus capability Interfaces Application status
AG-702 OpenWrt 23.05 TCP slave, TCP master, RTU master, RTU gateway 2× Ethernet, RS-485, RS-232 Verified in this guide
AG-221 OpenWrt 23.05 TCP slave, TCP master, RTU master, RTU gateway 2× Ethernet, RS-485 Validate firmware/package
AG-207 OpenWrt 23.05 TCP slave, TCP master, RTU master, RTU gateway 1x Ethernet, 1x isolated RS-485 Validate firmware/package
AG-205 OpenWrt 23.05 TCP slave, TCP master, RTU master, RTU gateway 2× Ethernet, Wi-Fi Validate firmware/package
AG-707 OpenWrt 23.05 TCP slave, TCP master, RTU master, RTU gateway 2× Ethernet, 2× isolated RS-485 Validate firmware/package
AG-743 OpenWrt TCP slave, TCP master, RTU master, RTU gateway 4× Ethernet, RS-485 Validate firmware/package
AG-104 OpenWrt TCP slave, TCP master, RTU master, RTU gateway 1x Ethernet, 1x isolated RS-485 Validate firmware/package

2.2 Atreyo Products That Can Act as Modbus RTU Slaves

These products can be placed on the Gateway RS-485 bus as field devices. The Gateway acts as Modbus Master and reads the data exposed by each device according to its own Modbus documentation.

Product Interface Role / protocol Example data for AG-702
Thermolog V3-T RS-485 Modbus RTU slave Temperature registers
Thermolog V3-TH RS-485 Modbus RTU slave Temperature / humidity registers
AMB-4I-4O RS-485 Modbus RTU I/O module; all Modbus masters Input status and relay control
AMB-8I-4O RS-485 Modbus RTU / IEC-60870-5-101 Input status and relay control
AMB-12I-4O RS-485 Modbus RTU I/O module; all Modbus masters Input status and relay control
AMB-8I-AC RS-485 Modbus RTU I/O module; all Modbus masters Input status
AMB-16I-AC RS-485 Modbus RTU I/O module; all Modbus masters Input status
ADI-524 RS-485 Modbus RTU Input status

Physical connection: These RTU slave devices connect to the Gateway RS-485 port and share the RS-485 bus. They do not connect to the Gateway Ethernet/TCP port.

3. Deployment Scenarios

These diagrams show how the AG-702 can collect data from multiple RS-485 Modbus RTU slaves and multiple Modbus TCP/IP slave devices.

3.1 AG-702 Mixed TCP/IP & RS-485 Deployment

modbus-tcp-ip-ethernet.png

In this example the AG-702 is the central Modbus Master. Five RTU slaves share one RS-485 daisy-chain connected directly to the AG-702 RS-485 port: three slaves are shown on the left and two on the right. Four separate TCP/IP slaves connect through the Ethernet network on the AG-702 LAN side.

RS-485 side — five RTU slaves
  • All five devices connect to the AG-702 RS-485 port.
  • Three slaves are shown on the left and two on the right.
  • All five share one RS-485 daisy-chain.
  • Each slave has a unique Slave ID.
Ethernet side — four TCP/IP slaves
  • TCP/IP Slave A, B, C and D are slave devices.
  • They connect to the AG-702 LAN through the same Ethernet network.
  • Each slave has its own reachable IP address.
  • They are not gateways in this example.
Device group Physical connection Protocol Addressing
RTU Slave 1–5 AG-702 RS-485 → shared A/B bus Modbus RTU Slave IDs 1–5
TCP/IP Slave A–D AG-702 LAN → Ethernet network → slave Modbus TCP IP address + device/unit identifier as required

Important: RTU slaves are connected to the AG-702 RS-485 port. TCP/IP slaves are connected through the AG-702 Ethernet/LAN port. These are two separate physical communication paths.

3.2 RS-485 Daisy-Chain Wiring — Multi-Slave Field Example

A single AG-702 RS-485 port can serve multiple Modbus RTU slave devices on one shared two-wire bus. Connect A (D+) to A (D+) and B (D−) to B (D−) throughout the chain, assign a unique Slave ID to every device, and install a 120 Ω termination resistor between A and B only at the two physical ends of the bus.

RS485-daisy-chain-wiring.png

Important: RS-485 is a shared two-wire bus. Keep the topology physically ordered as a daisy-chain, keep drop cables short, connect A to A and B to B consistently, and place 120 Ω termination resistors only at the two physical ends of the bus.