Skip to main content

Modbus Master basic info

1.6 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

1.7 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.

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 AG-702.

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 Ethernet, RS-485 Validate firmware/package AG-205 OpenWrt 23.05 Modbus TCP; product page lists TCP master/slave and RTU functions 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 Modbus TCP/IP and RTU 4× Ethernet, RS-485 Validate firmware/package

Application scope: AG-702 is the reference platform for the Modbus Master application documented in this guide. For the other gateways, verify that the same package, drivers and UI are available in the installed firmware.

2.2 Atreyo Products That Can Act as Modbus RTU Slaves

These products can be placed on the AG-702 RS-485 bus as field devices. The AG-702 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 slave Temperature registers Thermolog V3-H RS-485 Modbus slave Temperature / humidity registers AMB-4I-4O RS-485 Modbus RTU I/O module; all Modbus masters Input and relay status/control AMB-8I-4O RS-485 Modbus RTU / IEC-60870-5-101 Input and relay information AMB-12I-4O RS-485 Modbus RTU I/O module; all Modbus masters Input and relay information

Physical connection: These RTU slave devices connect to the AG-702 RS-485 port and share the RS-485 bus. They do not connect to the AG-702 Ethernet/TCP port.

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

AG-702 Modbus Master with four Modbus TCP IP slaves and five Modbus RTU slaves

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.

      Proper RS-485 two-wire daisy-chain wiring with AG-702 Modbus Master, five Modbus RTU slaves, A D plus and B D minus wiring, and 120 ohm termination resistors at both physical bus ends

      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.