mt7688-rs485-webservices

RS485 Modbus RTU Control for Taiwan Hitachi Air Conditioner on MTK Linkit Smart 7688 Duo

Getting Started

You need Taiwan Hitachi air conditioners with RS485 signal transmitters. However, this project should be able to control any Modbus RTU via RS485 with little modification.

Prerequisites

This build this project, you need:

1. MTK Linkit Smart 7688 Duo
   • https://labs.mediatek.com/en/platform/linkit-smart-7688
2. RS-485 module for Arduino (MAX485)
   • http://yourduino.com/sunshop//index.php?l=product_detail&p=323
   • https://www.taiwaniot.com.tw/shop/module-sensor/%E7%94%A8%E6%88%B6%E4%BB%8B%E9%9D%A2%E8%BC%B8%E5%85%A5%E8%BC%B8%E5%87%BA/ttl-to-rs485rs422-max485-module/
3. RC-RO1XT - Hitachi RS485 Controller

Wiring:

Name	MAX485 Pin	7688 Pin	Desc
5V	VCC	5V
GND	GND	GND
RS485 A	A		connect to RS485 bus
RS485 B	B		connect to RS485 bus
MAX485_TX	DI	D17
MAX485_DE	DE	D15
MAX485_RE	RE	D16
MAX485_RX	RO	D14

To be able to compile the Arduino code, you need:

1. Download the whole project in zip format from here
2. In Arduino, Sketch -> Include Library -> Add .zip library and add the zip file.

Control Modbus on Arduino side.

By using this library in our Arduino side code, we contrl the Modbus devices by SoftwareSerial.

Communicate Arduino side by serial port.

We talk to the Arduino side by open serial port /dev/ttyS0.

The OpenWRT side is basically a web service taking REST call and send the request to Arduino side.

Final Results

MT7688 Control Box

RS485 Wiring

Finish

Control Cable

Operation

The basic operation is to control the Modbus devices by web service calls.

The Modbus devices takes four parameters:

1. Slave Address
   • In Modbus protocol topography requires each units on the bus to have a unique id (an integer value).
   • This id is set (by jumper) in the RC-RO1XT, the Hitachi RS485 Controller.
2. Function Code
   • 0x03 -> Read
   • 0x06 -> Write
3. Resgister Address
4. Resgister Value

Register Chart

Resgister Address	Item	Resgister Value
0x00	Current Temperature	Celsius (Read Only)
0x20	Power	0: OFF 1: ON
0x21	Operational Mode	0: Cooling 1: Dehumidify 2: Fan Only 3: Auto 4: Heating
0x22	Fan Speed	5: Auto 4: High 3: Mid 1: Low 7: Quiet
0x23	Target Temperature	Range 16℃ - 32℃
0x24	Sleep Mode	0: OFF Others: minutes in sleep mode (1~1440)
0x25	Fuzzy Mode	0: OFF 1: ON
0x29	Auto On/Off Timer	0: OFF Others: minutes (1~1440)
0x2A	Air Flow Up/Down	0: OFF Others: angle
0x2B	Air Flow Left/Right	0: OFF Others: angle
0x2D	Remote Control On/Off	0: Enable 1: Disable
0x2E	Beeper On/Off	0: Enable 1: Disable

Sample REST Calls

Read current temperature on unit 2

Request

curl -X GET http://<ip-address>:8080/2/0x00

Response

{ "value": 24 }

Read current power status on unit 2

Request

curl -X GET http://<ip-address>:8080/2/0x20

Response

{ "value": 0 }

Turn unit 2 On.

curl -X PUT -H "Content-Type: application/json" -d "{\"value\": 1}" http://<ip-address>:8080/2/0x20

Reference, Example, and Source:

1. https://github.com/4-20ma/ModbusMaster
2. https://github.com/openopen114/Arduino_Modbus_viaRS485/blob/master/ArduinoCode/ModbusMaster_viaRS485/ModbusMaster_viaRS485.ino

Authors

Yu-Hua Chang

License

MIT