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What is Modbus?

• PLC's and SCADA are Industrial computers that control factories and factory equipment.
• Modbus is a communication protocol used to communicate with PLC and SCADA.
• Modbus protocol can be used with 2 physical Interfaces
  1. RS485/RS232 Serial (Modbus-RTU)
  2. Ethernet/Wifi (Modbus-TCP)

Modbus Registers

• PLC's and SCADA store the data for monitor and control into 16-bit memory blocks called as registers.
• Each register has its unique address that allows us to read or write into the registers, called as Register address.
• There are 4 different types of registers in a PLC with Modbus.
  1. Coil (Discrete Output)
  2. Discrete Input (or Status Input)
  3. Holding Register
  4. Input Register

List of Modbus features implemented in Shunya stack

With Shunya stack you can make IoT device to act as a Modbus client i.e you can

1. Read data stored in PLC/SCADA.
2. Write data stored in PLC/SCADA.

Using Modbus Shunya stack

Requirements to use Modbus Shunya stack

1. Shunya OS installed (supported Arm devices) or Shunya OS docker container (X86 based windows/linux devices)
2. PLC/SCADA with Modbus support.

Steps to use MQTT Shunya stack

1. Set PLC Modbus settings in Shunya.
2. Read data stored in PLC/SCADA registers via Modbus.
3. Change values in PLC/SCADA registers via Modbus.

note

Run the steps given below inside Shunya OS installed (supported Arm devices) or Shunya OS docker container (X86 based windows/linux devices) terminals.

Step 1: Set PLC Modbus settings in Shunya

Set PLC Modbus details, by editing the config file /etc/shunya/config.json inside Shunya OS.

A simple configuration should contain these parameters.

Config	Description
modbusType	Modbus is available in 2 variants RTU and TCP. Select the mode that you want to communicate.
modbusServerIp	Modbus server IP address (only for Modbus TCP type)
modbusSerialBaudrate	Modbus server Port (only for Modbus TCP type)
modbusSerialDevice	Modbus server Device node (only for Modbus RTU type)
modbusSerialBaudrate	Modbus server Baud rate (only for Modbus RTU type)

Example configuration:

Modbus over Ethernet/Wifi

Writing the below json in /etc/shunya/config.json file, will tell the Shunya stack to connect to PLC over Ethernet/Wifi interface with IP 192.168.0.101 and port 5059.

"modbusTcpSettings": {

"modbusType": "tcp",

"modbusServerIp": "192.168.0.101",

"modbusServerPort": "5059",

},

Modbus over Serial

Writing the below json in /etc/shunya/config.json file, will tell the Shunya stack to connect to PLC over serial interface with device /dev/ttyS1 and with baudrate 9600.

"modbusRtuSettings": {

"modbusType": "rtu",

"modbusSerialDevice": "/dev/ttyS1",

"modbusSerialBaudrate": 9600,

},

Step 2: Read data stored in PLC/SCADA registers via Modbus.

Let's take an example use case: Say we need to

• Read status of PLC coil and check if it is on/off.
• Read data from the analog sensor connected to PLC.

Steps are :

1. Get Modbus adresses for PLC registers which store the data that we want to read, these Modbus addresses are available on the PLC's software.

   For example:

   1. PLC coil status is stored in coil register at address 0x01
   2. Analog sensor data is stored in holding register at address 0x02

   Name	Register Type	Register Address	Data type of the value stored in the register
   PLC coil	Coil register	0x01	int16
   Analog sensor	Holding Register	0x02	int16

2. Start with an ready to use template for reading data stored in PLC/SCADA via Modbus

   git clone https://gitlab.iotiot.in/repo-public/examples.git

   cd examples/simple-examples/read-modbus-registers

3. Open the read-modbus-registers.c in an text editor and modify as per your use case.

4. For our example, we need to first read PLC coil status, add the highlighted code in your main() function

   /* Create the modbus instance */

   modbusObj plc = newModbusClient("modbusTcpSettings");

   /* Connect to the Modbus Slave/Server*/

   modbusConnect(&plc);

   /* --- Read the values from the PLC --- */

   int status = modbusRead(&plc, MODBUS_READ_COILS, 0x01); /* Read Modbus Coil register at 0x01 */

   /* Disconnect from the Modbus server */

   modbusDisconnect(&plc);

5. Read Analog sensor data stored in PLC at 0x02 address, add the highlighted code in your main() function

   /* Create the modbus instance */

   modbusObj plc = newModbusClient("modbusTcpSettings");

   /* Connect to the Modbus Slave/Server*/

   modbusConnect(&plc);

   /* --- Read the values from the PLC --- */

   int status = modbusRead(&plc, MODBUS_READ_COILS, 0x01); /* Read Modbus Coil register at 0x01 */

   uint16_t sensorValue = modbusRead(&plc, MODBUS_READ_HOLDING_REG, 0x02); /* Read Modbus Holding register at 0x02 */

   /* Disconnect from the Modbus server */

   modbusDisconnect(&plc);

6. Once you are done editing, save and compile the code , by running

   mkdir build && cd build

   cmake ../

   make

7. Run the code

   sudo ./read-modbus-registers

8. More examples to read data from PLC via Modbus using shunya stack

   Read Coil Registers

   For reading data from PLC coil register, change code in step 4. with the example given below.

   /* --- Read the values from the PLC --- */

   uint16_t value1 = modbusRead(&plc, MODBUS_READ_COILS, 0x01); /* Read Modbus Coil register 0x01 */

   Read Input Registers

   For reading data from PLC Input register, change code in step 4. with the example given below.

   uint16_t value5 = modbusRead(&plc, MODBUS_READ_INPUT_REG, 0x06); /* Read Modbus Holding register 0x06 */

   Read Holding Register (Integer)

   For reading integer data from PLC Holding register, change code in step 4. with the example given below.

   uint16_t value3 = modbusRead(&plc, MODBUS_READ_HOLDING_REG, 0x03); /* Read Modbus Holding register 0x03 */

   Read Holding Register (Float)

   For reading floating point data from PLC Holding register, change code in step 4. with the example given below.

   float value4 = modbusReadFloat(&plc, MODBUS_READ_HOLDING_REG, 0x04); /* Read Modbus Holding register 0x04 */

Step 3: Change values in PLC/SCADA registers via Modbus.

Let's take an example use case: Say we need to

• Change status of PLC coil to on.

Steps are :

1. Get Modbus adresses for PLC registers which store the data that we want to change, these Modbus addresses are available on the PLC's software.

   For example:

   1. PLC coil status is stored in coil register at address 0x01

   Name	Register Type	Register Address	Data type of the value stored in the register
   PLC coil	Coil register	0x01	int16

2. Start with an ready to use template for reading data stored in PLC/SCADA via Modbus

   git clone https://gitlab.iotiot.in/repo-public/examples.git

   cd examples/simple-examples/write-modbus-registers

3. Open the write-modbus.c in an text editor and modify as per your use case.

4. For our example, we need to change PLC coil status to on, add the highlighted code in your main() function

   /* Create the modbus instance */

   modbusObj plc = newModbusClient("modbusTcpSettings");

   /* Connect to the Modbus Slave/Server*/

   modbusConnect(&plc);

   /* --- Write the values to the PLC --- */

   modbusWrite(&plc, MODBUS_WRITE_COIL, 0x01, 1); /* Write value 1 to Modbus Coil register */

   /* Disconnect from the Modbus server */

   modbusDisconnect(&plc);

5. Once you are done editing, save and compile the code , by running

   mkdir build && cd build

   cmake ../

   make

6. Run the code

   sudo ./write-modbus

7. More examples to change data stored in PLC via Modbus using shunya stack

   Read Coil Registers

   For changing data in PLC coil register, change code in step 4. with the example given below.

   /* --- Write the values to the PLC --- */

   modbusWrite(&plc, MODBUS_WRITE_COIL, 0x01, 1); /* Write value 1 to Modbus Coil register */

   Read Input Registers

   For changing data in PLC Input register, change code in step 4. with the example given below.

   /* --- Write the values to the PLC --- */

   modbusWrite(&plc, MODBUS_WRITE_REG, 0x02, 1); /* Write value 1 to Modbus Input register */

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