RS-485 is a multipoint communication which allows multiple devices to be connected to a single bus. The mikroC PRO for 8051 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).

Note: The library uses the UART module for communication. The user must initialize the appropriate UART module before using the RS-485 Library. For MCUs with two UART modules it is possible to initialize both of them and then switch by using the UART_Set_Active function. See the UART Library functions.

• START byte value = 150
• STOP byte value = 169
• Address 50 is the broadcast address for all Slaves (packets containing address 50 will be received by all Slaves except the Slaves with addresses 150 and 169).

External dependencies of RS-485 Library

Library Routines

• RS485Master_Init
• RS485Master_Receive
• RS485Master_Send
• RS485Slave_Init
• RS485Slave_Receive
• RS485Slave_Send

RS485Master_Init

// rs485 module pinout
sbit rs485_transceive at P3_2_bit;      // transmit/receive control set to port3.bit2
...
UART1_Init(9600);                           // initialize UART1 module
RS485Master_Init();                         // intialize MCU as a Master for RS-485 communication

RS485Master_Receive

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

RS485Master_Send

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

RS485Slave_Init

// rs485 module pinout
sbit rs485_transceive at P3.B2;      // transmit/receive control set to port3.bit2
...
UART1_Init(9600);                   // initialize UART1 module
RS485Slave_Init(160);                // intialize MCU as a Slave for RS-485 communication with address 160

RS485Slave_Receive

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

RS485Slave_Send

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 P0, while error on receive (0xAA) and number of consecutive unsuccessful retries are displayed on P1. Slave displays received data on P0, while error on receive (0xAA) is displayed on P1. Hardware configurations in this example are made for the Easy8051B board and AT89S8253.

RS485 Master code:

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

sbit rs485_transceive at P3_2_bit;       // set transcieve pin

// Interrupt routine
void interrupt() org IVT_ADDR_ES {
  EA_bit = 0;                          // Clear global interrupt enable flag
  if(RI_bit) {                         // Test UART receive interrupt flag
    RS485Master_Receive(dat);          // UART receive interrupt detected,
                                       //   receive data using RS485 communication
    RI_bit = 0;                        // Clear UART interrupt flag
    }
  EA_bit = 1;                          // Set global interrupt enable flag
}

void main(){
  long cnt = 0;

  P0 = 0;
  P1 = 0;

  UART1_Init(4800);                    // 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);

  ES_bit = 1;                          // Enable UART interrupt
  RI_bit = 0;                          // Clear UART RX interrupt flag
  EA_bit = 1;                          // Enable interrupts

  while (1){
                                       // upon completed valid message receiving
                                       //   data[4] is set to 255
    cnt++;
    if (dat[5])  {                     // if an error detected, signal it
      P1 = 0xAA;                       //   by setting PORT1 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 PORT0
        P0 = 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) {
      P2++;
      cnt = 0;
      RS485Master_Send(dat,1,160);
      if (P2 > 10)                     // if sending failed 10 times
        RS485Master_Send(dat,1,50);    //   send message on broadcast address
    }
  }
}

RS485 Slave code:

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

sbit rs485_transceive at P3_2_bit;     // set transcieve pin

// Interrupt routine
void interrupt() org IVT_ADDR_ES {
  EA_bit = 0;                          // Clear global interrupt enable flag
  if(RI_bit) {                         // Test UART receive interrupt flag
    RS485Slave_Receive(dat);           // UART receive interrupt detected,
                                       //   receive data using RS485 communication
    RI_bit = 0;                        // Clear UART interrupt flag
    }
  EA_bit = 1;                          // Set global interrupt enable flag
}

void main(){

  P0 = 0;
  P1 = 0;

  UART1_Init(4800);                    // 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

  ES_bit = 1;                          // Enable UART interrupt
  RI_bit = 0;                          // Clear UART RX interrupt flag
  EA_bit = 1;                          // Enable interrupts

  while (1) {
    if (dat[5])  {                     // if an error detected, signal it by
      P1 = 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++){
        P0 = 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 PC to 8051 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?

_RS485_START_BYTE = 0x96; // 10010110
_RS485_STOP_BYTE  = 0xA9; // 10101001


PACKAGE:
-------- 
_RS485_START_BYTE 0x96
ADDRESS
DATALEN
[DATA1]            // if exists
[DATA2]            // if exists
[DATA3]            // if exists
CRC
_RS485_STOP_BYTE  0xA9


DATALEN bits
------------
bit7 = 1  MASTER SENDS
       0  SLAVE  SENDS
bit6 = 1  ADDRESS WAS XORed with 1, IT WAS EQUAL TO _RS485_START_BYTE or _RS485_STOP_BYTE
       0  ADDRESS UNCHANGED
bit5 = 0  FIXED
bit4 = 1  DATA3 (if exists) WAS XORed with 1, IT WAS EQUAL TO _RS485_START_BYTE or _RS485_STOP_BYTE
       0  DATA3 (if exists) UNCHANGED
bit3 = 1  DATA2 (if exists) WAS XORed with 1, IT WAS EQUAL TO _RS485_START_BYTE or _RS485_STOP_BYTE
       0  DATA2 (if exists) UNCHANGED
bit2 = 1  DATA1 (if exists) WAS XORed with 1, IT WAS EQUAL TO _RS485_START_BYTE or _RS485_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 == _RS485_START_BYTE) || (crc_send == _RS485_STOP_BYTE))
   crc_send++;
  
NOTE: DATALEN<4..0> can not take the _RS485_START_BYTE<4..0> or _RS485_STOP_BYTE<4..0> values.

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