Software

NOTE: For RPI PLC Version 4 you must use ttySC2 instead of ttySC0 and ttySC3 instead of ttySC1. 

Once the hardware configuration is done, let's configure the software. The industrial Raspberry Pi PLC is configured by default to work with the RS485 communication. If you are using a different image than the default, please follow the instructions of the Section 15.2 (32-bit version) or 15.3 (64-bit version) of the Raspberry PLC Family User Manual.

The Raspberry Pi industrial PLC uses the serial ports ttySC0 and ttySC1 to communicate. In this post, you will see how to communicate with an M-Duino Family PLC in order to send and receive data. First of all, you will do an initial test to check the RS485 in your Programmable Logical Controller.

Verification Test

In the following test, you are going to use the two RS485 channels to communicate with each other, connecting A+ pin with the other A+ and B- to the B- pin of the same PLC. After connecting the wires, you will create a new bash script.

nano Test_RS485.sh 

First of all, you will initialize both serial ports to the same baud rate of 115200. 

#!/bin/bash

stty 115200 -F /dev/ttySC0 raw -echo #will be used to send data
stty 115200 -F /dev/ttySC1 raw -echo #will be used to receive data

All data received from the /dev/ttySC1 will be stored in the temporary file rs485.txt. You will send an "OK" message from the /dev/ttySC0, if everything is connected properly, it will be sent to the serial /dev/ttySC1 and written at rs485.txt. The resulting message will be stored in the RESULT variable for future verification. Be sure to kill the cat process, otherwise, it will run in the background.

cat /dev/ttySC1 > /tmp/rs485.txt &
sleep 1
echo "OK" > /dev/ttySC0
RESULT=$(cat /tmp/rs485.txt)
sudo killall cat > /dev/null

Finally, you will check if there is any data in the variable RESULT. If the communication has been successful, you will print on the terminal the received message "OK". Otherwise, a message error will be displayed.

if [ -n "${RESULT}" ] ; then
        echo rs485 true "${RESULT}"
else
        echo rs485 false "RS-485 cannot read from /dev/ttySC1"
fi

// Include Industrial Shields libraries
#include <RS485.h>
//// IMPORTANT: check switches configuration
////////////////////////////////////////////////////////////////////////////////////////////////////
void setup() {
  // Begin serial port
  Serial.begin(9600);
  // Begin RS485 port
  RS485.begin(115200);
}
////////////////////////////////////////////////////////////////////////////////////////////////////

////////////////////////////////////////////////////////////////////////////////////////////////////
void loop() {
  // Wait bytes in the serial port
  if (Serial.available()) {
    byte tx = Serial.read();

    // Echo the byte to the serial port again
    Serial.write(tx);

    // And send it to the RS-485 port
    RS485.write(tx);
  }
}
////////////////////////////////////////////////////////////////////////////////////////////////////

#!/bin/bash

stty 115200 -F /dev/ttySC0 raw -echo #will be used to send data

end=$((SECONDS+5))

while [ $SECONDS -lt $end ]; do
	cat /dev/ttySC0 > /tmp/rs485.txt &
	sleep 1
	RESULT=$(cat /tmp/rs485.txt)
	if [ -n "${RESULT}" ] ; then
		echo "${RESULT}"
		break
	fi
done

sudo killall cat > /dev/null

  int Period = 1000; //Periode d'enviament de dades en ms
  int unsigned long Time;
  String rcv_message;

////////////////////////////////////////////////////////////////////////////////////////////////////
void loop() {
  //Print received byte when available
  if (RS485.available()) {
    rcv_message = RS485.readString();
    Serial.println(rcv_message);
  }
}
////////////////////////////////////////////////////////////////////////////////////////////////////

#!/bin/bash

stty 115200 -F /dev/ttySC0 raw -echo #will be used to send data

echo "Hello World!" > /dev/ttySC0