RS-485 Library

RS-485 is a multipoint communication which allows multiple devices to be connected to a single bus. The mikroC PRO for PIC provides a set of library routines for comfortable work with RS485 system using Master/Slave architecture. Master and Slave devices interchange packets of information. Each of these packets contains synchronization bytes, CRC byte, address byte and the data. Each Slave has unique address and receives only packets addressed to it. The Slave can never initiate communication.

It is the user’s responsibility to ensure that only one device transmits via 485 bus at a time.

The RS-485 routines require the UART module. Pins of UART need to be attached to RS-485 interface transceiver, such as LTC485 or similar (see schematic at the bottom of this page).

Library constants:
  Important :

Library Dependency Tree

RS485 Library Dependency Tree

External dependencies of RS-485 Library

The following variable must be defined in all projects using RS-485 Library: Description : Example :
extern sfr sbit RS485_rxtx_pin; Control RS-485 Transmit/Receive operation mode sbit RS485_rxtx_pin at RC2_bit;
extern sfr sbit RS485_rxtx_pin_direction; Direction of the RS-485 Transmit/Receive pin sbit RS485_rxtx_pin_direction at TRISC2_bit;

Library Routines

RS485Master_Init

Prototype

void RS485Master_Init();

Returns

Nothing.

Description

Initializes MCU as a Master for RS-485 communication.

Requires Global variables :
  • RS485_rxtx_pin - this pin is connected to RE/DE input of RS-485 transceiver(see schematic at the bottom of this page). RE/DE signal controls RS-485 transceiver operation mode.
  • RS485_rxtx_pin_direction - direction of the RS-485 Transmit/Receive pin.
must be defined before using this function.


UART HW module needs to be initialized. See UARTx_Init.

Example
// RS485 module pinout
sbit RS485_rxtx_pin at RC2_bit;  // transmit/receive control set to PORTC.B2

// Pin direction
sbit RS485_rxtx_pin_direction at TRISC2_bit;   // RxTx pin direction set as output
...
UART1_Init(9600);                           // initialize UART1 module
RS485Master_Init();                         // intialize MCU as a Master for RS-485 communication

RS485Master_Receive

Prototype

void RS485Master_Receive(char *data_buffer);

Returns

Nothing.

Description

Receives messages from Slaves. Messages are multi-byte, so this routine must be called for each byte received.

Parameters :

  • data_buffer: 7 byte buffer for storing received data, in the following manner:

  • data[0..2]: message content
  • data_buffer[3]: number of message bytes received, 1–3
  • data_buffer[4]: is set to 255 when message is received
  • data_buffer[5]: is set to 255 if error has occurred
  • data_buffer[6]: address of the Slave which sent the message

The function automatically adjusts data[4] and data[5] upon every received message. These flags need to be cleared by software.

Requires

MCU must be initialized as a Master for RS-485 communication. See RS485Master_Init.

Example
char msg[8];
...
RS485Master_Receive(msg);

RS485Master_Send

Prototype

void RS485Master_Send(char *data_buffer, char datalen, char Slave_address);

Returns

Nothing.

Description

Sends message to Slave(s). Message format can be found at the bottom of this page.

Parameters :

  • data_buffer: data to be sent
  • datalen: number of bytes for transmition. Valid values: 0...3.
  • Slave_address: Slave(s) address

Requires

MCU must be initialized as a Master for RS-485 communication. See RS485Master_Init.

It is the user’s responsibility to ensure (by protocol) that only one device sends data via 485 bus at a time.

Example
char msg[8];
...
// send 3 bytes of data to Slave with address 0x12
RS485Master_Send(msg, 3, 0x12);

RS485Slave_Init

Prototype

void RS485Slave_Init(char Slave_address);

Returns

Nothing.

Description

Initializes MCU as a Slave for RS-485 communication.

Parameters :

  • Slave_address: Slave address

Requires Global variables :
  • RS485_rxtx_pin - this pin is connected to RE/DE input of RS-485 transceiver(see schematic at the bottom of this page). RE/DE signal controls RS-485 transceiver operation mode. Valid values: 1 (for transmitting) and 0 (for receiving
  • RS485_rxtx_pin_direction - direction of the RS-485 Transmit/Receive pin.
must be defined before using this function.


UART HW module needs to be initialized. See UARTx_Init.

Example

Initialize MCU as a Slave with address 160:

// RS485 module pinout
sbit RS485_rxtx_pin at RC2_bit;     // transmit/receive control set to PORTC.B2

// Pin direction
sbit RS485_rxtx_pin_direction at TRISC2_bit;   // RxTx pin direction set as output
...
UART1_Init(9600);                   // initialize UART1 module
RS485Slave_Init(160);                // intialize MCU as a Slave for RS-485 communication with address 160

RS485Slave_Receive

Prototype

void RS485Slave_Receive(char *data_buffer);

Returns

Nothing.

Description

Receives messages from Master. If Slave address and Message address field don't match then the message will be discarded. Messages are multi-byte, so this routine must be called for each byte received.

Parameters :

  • data_buffer: 6 byte buffer for storing received data, in the following manner:

  • data[0..2]: message content
  • data[3]: number of message bytes received, 1–3
  • data[4]: is set to 255 when message is received
  • data[5]: is set to 255 if error has occurred

The function automatically adjusts data[4] and data[5] upon every received message. These flags need to be cleared by software.

Requires

MCU must be initialized as a Slave for RS-485 communication. See RS485Slave_Init.

Example
char msg[8];
...
RS485Slave_Read(msg);

RS485Slave_Send

Prototype

void RS485Slave_Send(char *data_buffer, char datalen);

Returns

Nothing.

Description

Sends message to Master. Message format can be found at the bottom of this page.

Parameters :

  • data_buffer: data to be sent
  • datalen: number of bytes for transmition. Valid values: 0...3.

Requires

MCU must be initialized as a Slave for RS-485 communication. See RS485Slave_Init. It is the user’s responsibility to ensure (by protocol) that only one device sends data via 485 bus at a time.

Example
char msg[8];
...
// send 2 bytes of data to the Master
RS485Slave_Send(msg, 2);

Library Example

This is a simple demonstration of RS485 Library routines usage.

Master sends message to Slave with address 160 and waits for a response. The Slave accepts data, increments it and sends it back to the Master. Master then does the same and sends incremented data back to Slave, etc.

Master displays received data on PORTB, while error on receive (0xAA) and number of consecutive unsuccessful retries are displayed on PORTD. Slave displays received data on PORTB, while error on receive (0xAA) is displayed on PORTD. Hardware configurations in this example are made for the EasyPIC6 board and 16F887.

RS485 Master code:

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char dat[10];                          // buffer for receving/sending messages
char i,j;

sbit  rs485_rxtx_pin  at RC2_bit;               // set transcieve pin
sbit  rs485_rxtx_pin_direction at TRISC2_bit;   // set transcieve pin direction

// Interrupt routine
void interrupt() {
  RS485Master_Receive(dat);
}

void main(){
  long cnt = 0;

  ANSEL  = 0;                          // Configure AN pins as digital I/O
  ANSELH = 0;
  C1ON_bit = 0;                        // Disable comparators
  C2ON_bit = 0;
  
  PORTB  = 0;
  PORTD  = 0;
  TRISB  = 0;
  TRISD  = 0;


  UART1_Init(9600);                    // initialize UART1 module
  Delay_ms(100);

  RS485Master_Init();                  // initialize MCU as Master
  dat[0] = 0xAA;
  dat[1] = 0xF0;
  dat[2] = 0x0F;
  dat[4] = 0;                          // ensure that message received flag is 0
  dat[5] = 0;                          // ensure that error flag is 0
  dat[6] = 0;

  RS485Master_Send(dat,1,160);


  RCIE_bit = 1;                        // enable interrupt on UART1 receive
  TXIE_bit = 0;                        // disable interrupt on UART1 transmit
  PEIE_bit = 1;                        // enable peripheral interrupts
  GIE_bit = 1;                         // enable all interrupts

  while (1){
                                       // upon completed valid message receiving
                                       //   data[4] is set to 255
    cnt++;
    if (dat[5])  {                     // if an error detected, signal it
      PORTD = 0xAA;                    //   by setting portd to 0xAA
    }
    if (dat[4]) {                      // if message received successfully
      cnt = 0;
      dat[4] = 0;                      // clear message received flag
      j = dat[3];
      for (i = 1; i <= dat[3]; i++) {  // show data on PORTB
        PORTB = dat[i-1];
      }                                // increment received dat[0]
      dat[0] = dat[0]+1;               // send back to master
      Delay_ms(1);
      RS485Master_Send(dat,1,160);

    }
   if (cnt > 100000) {
      PORTD ++;
      cnt = 0;
      RS485Master_Send(dat,1,160);
      if (PORTD > 10)                  // if sending failed 10 times
        RS485Master_Send(dat,1,50);    //   send message on broadcast address
     }
  }

}

RS485 Slave code:

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char dat[9];             // buffer for receving/sending messages
char i,j;

sbit  rs485_rxtx_pin at RC2_bit;             // set transcieve pin
sbit  rs485_rxtx_pin_direction at TRISC2_bit;   // set transcieve pin direction

// Interrupt routine
void interrupt() {
 RS485Slave_Receive(dat);
}

void main() {
  ANSEL  = 0;                        // Configure AN pins as digital I/O
  ANSELH = 0;
  C1ON_bit = 0;                      // Disable comparators
  C2ON_bit = 0;
  
  PORTB = 0;
  PORTD = 0;
  TRISB = 0;
  TRISD = 0;


  UART1_Init(9600);                  // initialize UART1 module
  Delay_ms(100);
  RS485Slave_Init(160);              // Intialize MCU as slave, address 160

  dat[4] = 0;                        // ensure that message received flag is 0
  dat[5] = 0;                        // ensure that message received flag is 0
  dat[6] = 0;                        // ensure that error flag is 0

  RCIE_bit = 1;                      // enable interrupt on UART1 receive
  TXIE_bit = 0;                      // disable interrupt on UART1 transmit
  PEIE_bit = 1;                      // enable peripheral interrupts
  GIE_bit = 1;                       // enable all interrupts

  while (1) {
    if (dat[5])  {                   // if an error detected, signal it by
      PORTD = 0xAA;                  //   setting portd to 0xAA
      dat[5] = 0;
    }
    if (dat[4]) {                    // upon completed valid message receive
      dat[4] = 0;                    //   data[4] is set to 0xFF
      j = dat[3];
      for (i = 1; i <= dat[3];i++){
        PORTB = dat[i-1];
      }
      dat[0] = dat[0]+1;             // increment received dat[0]
      Delay_ms(1);
      RS485Slave_Send(dat,1);        //   and send it back to master
    }
  }
}

HW Connection

Example of interfacing PIC16F887 MCU to PIC16F887 MCU MCU via RS485 bus with LTC485 as RS-485 transceiver

Example of interfacing PIC16F887 MCU to PIC16F887 MCU MCU via RS485 bus with LTC485 as RS-485 transceiver

Message format and CRC calculations

Q: How is CRC checksum calculated on RS485 Master side?

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START_BYTE = 0x96; // 10010110
STOP_BYTE  = 0xA9; // 10101001


PACKAGE:
-------- 
START_BYTE 0x96
ADDRESS
DATALEN
[DATA1]            // if exists
[DATA2]            // if exists
[DATA3]            // if exists
CRC
STOP_BYTE  0xA9


DATALEN bits
------------
bit7 = 1  MASTER SENDS
       0  SLAVE  SENDS
bit6 = 1  ADDRESS WAS XORed with 1, IT WAS EQUAL TO START_BYTE or STOP_BYTE
       0  ADDRESS UNCHANGED
bit5 = 0  FIXED
bit4 = 1  DATA3 (if exists) WAS XORed with 1, IT WAS EQUAL TO START_BYTE or STOP_BYTE
       0  DATA3 (if exists) UNCHANGED
bit3 = 1  DATA2 (if exists) WAS XORed with 1, IT WAS EQUAL TO START_BYTE or STOP_BYTE
       0  DATA2 (if exists) UNCHANGED
bit2 = 1  DATA1 (if exists) WAS XORed with 1, IT WAS EQUAL TO START_BYTE or STOP_BYTE
       0  DATA1 (if exists) UNCHANGED
bit1bit0 = 0 to 3 NUMBER OF DATA BYTES SEND


CRC generation :
----------------
crc_send  = datalen ^ address;
crc_send ^= data[0];    // if exists
crc_send ^= data[1];    // if exists
crc_send ^= data[2];    // if exists
crc_send = ~crc_send;
if ((crc_send == START_BYTE) || (crc_send == STOP_BYTE))
   crc_send++;
  
NOTE: DATALEN<4..0> can not take the START_BYTE<4..0> or STOP_BYTE<4..0> values.
Copyright (c) 2002-2012 mikroElektronika. All rights reserved.
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