SoftRS485/SoftRS485.cpp

/*
    Library for Software Serial over RS485 connection
    Created by Stepan Richter, November 2023
*/

#include "Arduino.h"
#include "SoftRS485.h"

//#define DEBUG
#define MSG_LEN 64

// Pins
int _RO, _RE, _DE, _DI;
long _pulse_len = 100;

// Receive vars
boolean _recv_bit = false;
long _last_flip; // this is the time when the bus switched to logical zero
byte _recv_msg[MSG_LEN];
int _recv_pos = 0;
int _recv_idx = 0;
boolean _avail = false;

void interrupt485(){
    _recv_bit = !digitalRead(_RO); // HIGH level = logical zero and vice versa
    long now = micros();
    long diff = now - _last_flip;
    _last_flip = now;
    if (diff > 20*_pulse_len){ // we received a start bit
#ifdef DEBUG
        Serial.print(F("State changed to ")); Serial.println(_recv_bit);
        Serial.println(F("received start bit"));
#endif
        _recv_pos = 0;
        _recv_idx = 0;
        _last_flip+=_pulse_len;
    } else {
        long count = diff/_pulse_len;
        for (int i=0; i<count; i++){
            if (_recv_idx == 0) {
                _recv_msg[_recv_pos] = (!_recv_bit);
            } else {
                _recv_msg[_recv_pos] |= (!_recv_bit)<<_recv_idx;
            }
            _recv_idx++;
            if (_recv_idx == 8){
                _recv_idx = 0;
                if (_recv_msg[_recv_pos] == 0x00){
                    _avail = _recv_pos != 0;
                    return;
                }
                _recv_pos++;
            }
        }
    }
}

void init485(int RO, int nRE, int DE, int DI){
    _RO=RO;
    _RE=nRE;
    _DE=DE;
    _DI=DI;

    pinMode(_RO,INPUT);

    pinMode(_RE,OUTPUT);
    digitalWrite(_RE,LOW); // enable reading

    pinMode(_DE,OUTPUT);
    digitalWrite(_DE,LOW); // disable writing

    pinMode(_DI,OUTPUT);
    digitalWrite(_DI,LOW); // output line = LOW

    attachInterrupt(digitalPinToInterrupt(_RO),interrupt485,CHANGE);

#ifdef DEBUG
    Serial.begin(115200);
    Serial.print(F("RO = "));
    Serial.println(_RO);
    Serial.print(F("^RE = "));
    Serial.println(_RE);
    Serial.print(F("DE = "));
    Serial.println(_DE);
    Serial.print(F("DI = "));
    Serial.println(_DI);
#endif
    _last_flip = micros();
}

void speed485(long baudrate){
#ifndef DEBUG
    _pulse_len = 1e6 / baudrate;
#else
    _pulse_len = 1000;
    Serial.print(F("Set pulse_len to ")); Serial.print(_pulse_len); Serial.println(F("µs"));
#endif
}

boolean available485(){
    return _avail;
}

String get485message(){
    _avail = false;
    return String((char*)_recv_msg);
}

boolean writeBit(boolean bit){
    digitalWrite(_DI,!bit);
    delayMicroseconds(5);
    if (_recv_bit != bit){ // collision!
       digitalWrite(_DE,LOW);
#ifdef DEBUG
       Serial.println(F("Collision!"));
#endif
       return false;
    }
    delayMicroseconds(_pulse_len-5);
    return true;
}

boolean send485(char message[]){
#ifdef DEBUG
    Serial.print(F("preparing to send "));
    Serial.print(message);
    Serial.print(F(": line state = "));
    Serial.print(_recv_bit);
    Serial.print(F(" / last_flip = "));
    Serial.print(_last_flip);
    Serial.print(F(" / micros = "));
    Serial.println(micros());
#endif
    while (_recv_bit || (micros() - _last_flip) < (20 * _pulse_len)) {
        delayMicroseconds(20*_pulse_len);
    }
#ifdef DEBUG
    Serial.println(F("enabling driver…"));
#endif
    digitalWrite(_DE,HIGH); // enable driver
    if (!writeBit(HIGH)) return false;
    int pos = -1;
    boolean good = true;
    do {
        pos++;
#ifdef DEBUG
        Serial.print(F("sending character ")); Serial.println(message[pos]);
#endif
        good = writeBit(message[pos] & 0x01)
            && writeBit(message[pos] & 0x02)
            && writeBit(message[pos] & 0x04)
            && writeBit(message[pos] & 0x08)
            && writeBit(message[pos] & 0x10)
            && writeBit(message[pos] & 0x20)
            && writeBit(message[pos] & 0x40)
            && writeBit(message[pos] & 0x80);
    } while (good && message[pos]);
    if (good) writeBit(1);
    digitalWrite(_DE,LOW); // disable driver
#ifdef DEBUG
    Serial.println(F("disabled driver…"));
#endif
    return good;
}