Modbus RS-485 with Teensy 4.0

[adi1893]

Hi,

I need to use a teensy 4.0 as a Modbus Server(Slave) to send and receive integer and bit values to a touchscreen HMI or a PLC for process diagnostics and control. I am using the ArduinoModbus library and wanted tried using the code mentioned in the following page by @PaulStoffregen :
https://forum.pjrc.com/threads/37018-Modbus-RTU/page2

#include <ArduinoRS485.h>
#include <ArduinoModbus.h>

bool clientStarted = 0;

void setup() {
  clientStarted = ModbusRTUClient.begin(57600);
  while (!clientStarted) {
    Serial.println("Client Error");
  }
}

void loop() {
  ModbusRTUClient.beginTransmission(15, HOLDING_REGISTERS, 0x100, 6);
  ModbusRTUClient.write(100);
  ModbusRTUClient.write(101);
  ModbusRTUClient.write(102);
  ModbusRTUClient.write(103);
  ModbusRTUClient.write(104);
  ModbusRTUClient.write(105);

  if (!ModbusRTUClient.endTransmission()) {
    Serial.println ("failed! ");
    Serial.println (ModbusRTUClient.lastError());
  }
  else{
    Serial.println("Success");
  }
}

After compiling and downloading the code, and connecting an oscilloscope to various Teensy pins, I can see some modbus like data on Pin 1 of the Teensy 4.0 (standby state is high at 3v3). I can also see Pins 19 and 20 both go from 0V to 3V3 for the duration of the time that Pin 1 is sending data. I am also hoping that I can use the Server commands in place of the Client commands to use the Teensy as a Slave (code attached is for Teensy as a Master if I am reading it correctly),

I am assuming that the code will work with transmitting the required data but before I can test it with ModScan or an actual HMI or PLC, I need to wire up a test system.

Please help me with how I should wire this system. (Teensy 4.0 - maybe some level converter to step up 3v3 to 5v - connection to Data+ and Data- terminals on the HMI).

Are the Teensy Pins configured as under:
Pin 0 (Rx) - RO
Pin 1 (Tx) - DI
Pin 19 and 20 - DE and RE?

If so, what components should I use to convert the 4 wire signals to a 2 wire RS485 communication that can be connected to a standard HMI or PLC

Thanks!

 

[joepasquariello]

Think of your project as having two main parts. One is software to support the Modbus protocol, which is what ArduinoModbus does. The other is to convert the Serial1 RX/TX signals on pins 0,1 to RS-485 levels, and if you google "RS-485 breakout", you will find many.