# Modbus Master Modbus Master is an application that allows full operation of devices connected to the gateway over Modbus RTU and Modbus TCP/IP. Also, the application allows sending data to the server via MQTT, TCP/IP JSON and saving to storage. --- ### Configuration of Modbus Master To open Modbus master, go to **Modbus > Modbus Master** Add Slave Device in Modbus Slave Devices [![ag-221-web-modbusmaster-start.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-start.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-start.png) Enter a slave device name of your choice & Select communication protocol. [![AG-221-web-modbusmaster-advanceSettingsAlert.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/TEaag-221-web-modbusmaster-advancesettingsalert.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/TEaag-221-web-modbusmaster-advancesettingsalert.png) #### Modbus RTU Need to enter the serial port configuration according to the slave device. In AG-221 is for RS485 **dev/ttyS1**. **/dev/ttyS1 = RS485** [![ag-221-web-modbusmaster-serial.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-serial.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-serial.png) Change the baud-rate according to your slave device requirement. In Modbus RTU 9600 baud-rate is most common used. [![AG-221-web-modbusmaster-baudrate.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-baudrate.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-baudrate.png) Data bits are used to represent each character or data unit in a communication protocol. Select the appropriate setting. [![AG-221-web-modbusmaster-databit.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-databit.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-databit.png) Parity is an error-checking mechanism to detect data transmission errors. Most devices use the **None** option. [![AG-221-web-modbusmaster-parity.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/xbBag-221-web-modbusmaster-parity.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/xbBag-221-web-modbusmaster-parity.png) A stop bit signals the end of a data frame, helping the receiver recognize when one byte is complete. - 1 Stop Bit: For stable connections and higher speed. - 2 Stop Bits: For increased reliability or when devices need more processing time. Select the option according to the slave device. [![AG-221-web-modbusmaster-stopbit.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-stopbit.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-stopbit.png) #### Modbus TCP/IP In Modbus TCP/IP, only the IP address and port number (typically 502) are required for communication. [![AG-221-web-modbusmaster-tcp.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-tcp.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-tcp.png) ### Modbus slave

After entering these parameters, the next configuration steps are the same for both Modbus TCP and Modbus RTU.

You should enter the slave ID (1-255), polling interval, and request timeout according to your requirements. [![ag-221-web-modbusmaster-slaveidpollingrequest.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-slaveidpollingrequest.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-slaveidpollingrequest.png) --- ##### Query Configuration To make a Query, click on **ADD QUERY** [![AG-221-web-modbusmaster-rtu2.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-rtu2.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-rtu2.png) Enter the **Function code**, **Start Address**, Register/Coil number as per the Slave documentation or instruction. Here 1-50 register/coil quantity is supported. [![AG-221-web-modbusmaster-Query1.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-query1.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-query1.png) ##### Parameter Configuration Enter and select the option as per your requirement. [![AG-221-web-modbusmaster-parameter2.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-parameter2.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-parameter2.png) [![AG-221-web-modbusmaster-parameter1.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-parameter1.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-parameter1.png) [![AG-221-web-modbusmaster-parameter.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-parameter.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-parameter.png) Click on **SAVE** after entering and selecting the Parameters. This is how we can configure Modbus and view the details. And for another slave do the same process by click on **ADD SLAVE DEVICE.** [![ag-221-web-modbusmaster-save-modbusconfig.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-save-modbusconfig.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-save-modbusconfig.png) --- ### Data output configuration Click on Output configuration. There are two option: 1. Save Data to File. 2. Send Data to Server. We can choose both options and then the data is both sent to the server and saved locally. [![ag-221-web-modbusmaster-outputconfig.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-outputconfig.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-outputconfig.png) If Save Data to File is selected, it is necessary to specify the folder path. #### Send data to server In Send Data to Server. There are 2 option: - ATRA-DATA - 3rd-PARTY-PLATFORM Atra-Data is Atreyo's system for quick and easy presentation of data in the cloud. In the case of 3rd-PARTY-PLATFORM, we can choose any data platform that communicates using one of the selected ports: HTTP, MQTT or TCP/IP. ##### ATRA-DATA ATRA-DATA. In This Data is send direct to the ATRA server without any aditional configuration. [![ag-221-web-modbusmaster-outputconfig1.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-outputconfig1.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-outputconfig1.png) ##### 3rd-PARTY-PLATFORM. There are some parameter required to select protocol – HTTP, MQTT, TCP/IP. [![ag-221-web-modbusmaster-outputconfig-select.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-outputconfig-select.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-outputconfig-select.png) ##### HTTP(s) Enter the URL For HTTP Post and Header. Where able to Change default header or add new one if needed. [![ag-221-web-modbusmaster-outputconfig2.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-outputconfig2.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-outputconfig2.png) ##### Certificates To enhance security for data transmission, you can use **certificate-based TLS** along with **credentials**. **Certificate-based TLS (Transport Layer Security)** require **CA(Certificate Authorities)** file, **Client Certificate** and **Private key** Credentials often require a username and password for authentication and access control in secure systems. [![ag-221-web-modbusmaster-outputconfig-certificates.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-outputconfig-certificates.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-outputconfig-certificates.png) ##### MQTT Enter the Server **Hostname/IP**, **Server Port**, **Publish Topic** and **Subscribe Topic**. Select **QoS level** as per requirement and Client ID it is Auto-generate. [![ag-221-web-modbusmaster-outputconfig-mqtt-cert.png](https://docs.atreyo.in/uploads/images/gallery/2025-12/scaled-1680-/ag-221-web-modbusmaster-outputconfig-mqtt-cert.png)](https://docs.atreyo.in/uploads/images/gallery/2025-12/ag-221-web-modbusmaster-outputconfig-mqtt-cert.png) QoS (Quality of Service) level - QoS 0 (At most once): Fast, no acknowledgment, possible message loss. - QoS 1 (At least once): Acknowledged, possible duplicates. - QoS 2 (Exactly once): Highest reliability, no duplicates, uses a four-step handshake.