A draft implementation of a multi-master strategy over RS485 "auto-tx" PHY for PJON protocol stack

PJON_,ultidrop_rs485_auto_tx_strategy

/* ThroughHardwareSerialRs485AutoTx enables PJON multi-master communication through the Serial port with auto-tx RS485 PHY.
   Work based on ThroughHardwareSerial by Giovanni Blu Mitolo and Fred Larsen  
   Copyright (c) 2016 by Zbigniew Zasieczny All rights reserved.

   With rs485 "auto-tx" PHY the hardware serial port reads everything sent to the bus
   The "self-reading" property could be used in this strategy to detect colisions at a byte-level in a similar way the CAN network works on a bit-level to silelntly drop-off lower priority nodes; feature not implemented yet

   A naive timeout-based collision avoidance is used: bus is assumed clear for tx within specified time window after empty rx buffer was detected. The receive(<timeout_us>) function must be called often - transimission would occur only within 1ms after last receive call on empty rx buffer; within this time window the update() function must be called to transmit packets.

 */

#include <Arduino.h>

#define THROUGH_HARDWARE_SERIAL_MAX_TIME 500 // Wait up to 5 ms to receive a byte
#define THROUGH_HARDWARE_SERIAL_MIN_CHANNEL_SILELNCE_WINDOW 100 // min 1 ms silence to start sending

class ThroughHardwareSerialRs485AutoTx {
  public:
    HardwareSerial *serial;

    /* Pass the Serial port where you want to operate with */

    void set_serial(HardwareSerial *serial_port) {
      serial = serial_port;
    };


    /* Returns the Serial object value i.e. if(Serial) */

    boolean can_start(uint8_t input_pin, uint8_t output_pin) {
      if (micros() - _last_clear_channel_ts < THROUGH_HARDWARE_SERIAL_MIN_CHANNEL_SILELNCE_WINDOW ) return false;
      return (serial != NULL) && *serial; // Check if pointer
    };


    /* Send a byte and wait for its transmission end */

    void send_byte(uint8_t b, uint8_t input_pin, uint8_t output_pin) {
      serial->write(b);
      serial->flush();
      //if (receive_byte(input_pin, output_pin) != b) return FAIL; // needs additional work with logic analyzer; sent byte should be avaialbe in rx buffer immediatelly but isn't
      //return 0; 
    };


    /* Try to receive a byte with a maximum waiting time */

    uint16_t receive_byte(uint8_t input_pin, uint8_t output_pin) {
      uint32_t time = micros();
      while(micros() - time < THROUGH_HARDWARE_SERIAL_MAX_TIME)
        if(serial->available() > 0)
          return (uint8_t)serial->read();
      _last_clear_channel_ts = micros();
      return FAIL;
    };


    /* Receive byte response */

    uint16_t receive_response(uint8_t input_pin, uint8_t output_pin) {
      return receive_byte(input_pin, output_pin);
    };


    /* Send byte response to the packet's transmitter */

    void send_response(uint8_t response, uint8_t input_pin, uint8_t output_pin) {
      send_byte(response, input_pin, output_pin);
    };

   private:
     long      _last_clear_channel_ts;
};