RS-485 Library

RS-485 is a multipoint communication which allows multiple devices to be connected to a single bus. mikroPascal PRO for ARM 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:

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).

Important :

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 multiple UART modules it is possible to initialize them and then switch by using the UART_Set_Active routine.

Library Dependency Tree

External dependencies of RS-485 Library

Stellaris

The following variable must be defined in all projects using RS-485 Library:	Description :	Example :
`var RS485_rxtx_pin : sbit; sfr; external;`	Control RS-485 Transmit/Receive operation mode	`var RS485_rxtx_pin : sbit at GPIO_PORTA_DATA2_bit;`
`var RS485_rxtx_pin_direction : sbit; sfr; external;`	Direction of the RS-485 Transmit/Receive pin	`var RS485_rxtx_pin_direction : sbit at GPIO_PORTA_DIR2_bit;`

STM32

The following variable must be defined in all projects using RS-485 Library:	Description :	Example :
`var RS485_rxtx_pin : sbit; sfr; external;`	Control RS-485 Transmit/Receive operation mode	`var RS485_rxtx_pin : sbit at GPIOA_ODR.B6;`

RS485Master_Init

Prototype	procedure RS485Master_Init();
Description	Initializes MCU as a Master for RS-485 communication.
Parameters	None.
Returns	Nothing.
Requires	External dependencies of the library from the top of the page must be defined before using this function. UART HW module needs to be initialized. See UARTx_Init.
Example	Stellaris // RS485 module pinout var rs485_rxtx_pin : sbit at GPIO_PORTA_DATA2_bit; // set transcieve pin rs485_rxtx_pin_direction : sbit at GPIO_PORTA_DIR2_bit; // set transcieve pin direction // End of RS485 module pinout ... UART1_Init(9600); // initialize UART1 module RS485Master_Init(); // intialize MCU as a Master for RS-485 communication STM32 // RS485 module pinout var rs485_rxtx_pin : sbit at GPIOA_ODR.B6; // set transcieve pin // End of RS485 module pinout ... UART1_Init(19200); // initialize UART1 module RS485Master_Init(); // intialize MCU as a Master for RS-485 communication
Notes	None.

RS485Master_Receive

Prototype	procedure RS485Master_Receive(var data : array[10] of byte);
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. Data will be stored in the following manner: `data_buffer[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 routine automatically adjusts `data[4]` and `data[5]` upon every received message. These flags need to be cleared by software.
Returns	Nothing.
Requires	MCU must be initialized as a Master for RS-485 communication. See RS485Master_Init.
Example	var msg : array[8] of byte; ... RS485Master_Receive(msg);
Notes	None.

RS485Master_Send

Prototype	procedure RS485Master_Send(var buffer : array[20] of byte; datalen : byte; slave_address : byte);
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
Returns	Nothing.
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	var msg : array[8] of byte; ... // send 3 bytes of data to Slave with address 0x12 RS485Master_Send(msg, 3, 0x12);
Notes	None.

RS485Slave_Init

Prototype	procedure RS485Slave_Init(slave_address : byte);
Description	Initializes MCU as a Slave for RS-485 communication.
Parameters	`Slave_address:` Slave address
Returns	Nothing.
Requires	External dependencies of the library from the top of the page 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: Stellaris // RS485 module pinout var rs485_rxtx_pin : sbit at GPIO_PORTA_DATA2_bit; // set transcieve pin rs485_rxtx_pin_direction : sbit at GPIO_PORTA_DIR2_bit; // set transcieve pin direction // End of RS485 module pinout ... UART1_Init(9600); // initialize UART1 module RS485Slave_Init(160); // intialize MCU as a Slave for RS-485 communication with address 160 STM32 // RS485 module pinout var rs485_rxtx_pin : sbit at GPIOA_ODR.B6; // set transcieve pin // End of RS485 module pinout ... UART1_Init(19200); // initialize UART1 module RS485Slave_Init(160); // intialize MCU as a Slave for RS-485 communication with address 160
Notes	None.

RS485Slave_Receive

Prototype	procedure RS485Slave_Receive(var data_buffer : array[20] of byte);
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_buffer[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 The routine automatically adjusts `data[4]` and `data[5]` upon every received message. These flags need to be cleared by software.
Returns	Nothing.
Requires	MCU must be initialized as a Slave for RS-485 communication. See RS485Slave_Init.
Example	var msg : array[8] of byte; ... RS485Slave_Read(msg);
Notes	None.

RS485Slave_Send

Prototype	procedure RS485Slave_Send(var data : array[20] of byte; datalen : byte);
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.
Returns	Nothing.
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	var msg : array[8] of byte; ... // send 2 bytes of data to the Master RS485Slave_Send(msg, 2);
Notes	None.

Library Example

The example demonstrates working with the ARM as a Master node in RS-485 communication. Master sends message to Slave with address 160 and waits for a response. After the response is received, the first byte of received data is incremented and sent back to the Slave. The received data is displayed on GPIO_PORTE while error on receiving (0xAA) and number of consecutive unsuccessful retries are displayed on GPIO_PORTD.

Stellaris

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program RS485_Master_Example;

var dat : array[10] of byte;   // buffer for receving/sending messages
    i, j : byte;
    cnt : longint;

var rs485_rxtx_pin           : sbit at GPIO_PORTA_DATA2_bit;  // set transcieve pin
    rs485_rxtx_pin_direction : sbit at GPIO_PORTA_DIR2_bit;   // set transcieve pin direction

// Interrupt routine
procedure interrupt(); iv IVT_INT_UART0;
begin
  RS485Master_Receive(dat);
  UART_ICR_RXIC_bit := 1;           // ensure interrupt not pending
end;

  begin
    cnt := 0;

    GPIO_Digital_Output(@GPIO_PORTD, _GPIO_PINMASK_ALL);
    GPIO_PORTD_DATA := 0;
    
    GPIO_Digital_Output(@GPIO_PORTE, _GPIO_PINMASK_ALL);
    GPIO_PORTE_DATA := 0;

    UART0_Init(19200);         // initialize UART0 module
    Delay_ms(100);

    RS485Master_Init();        // initialize MCU as Master

    UART_IM_RXIM_bit := 1;         // enable uart rx interrupt
    NVIC_IntEnable(IVT_INT_UART0); // enable interrupt vector

    EnableInterrupts();            // enable core interrupts

    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);

    while (TRUE) do
      begin                              // upon completed valid message receiving
                                         //   data[4] is set to 255
        Inc(cnt);
        if (dat[5] <> 0) then            // if an error detected, signal it
          GPIO_PORTD_DATA := 0xAA;       //   by setting GPIO_PORTD to 0xAA
        if (dat[4] <> 0) then            // if message received successfully
          begin
            cnt := 0;
            dat[4] := 0;                 // clear message received flag
            j := dat[3];
            for i := 1 to dat[3] do      // show data on GPIO_PORTE
              GPIO_PORTE_DATA := dat[i-1];
            dat[0] := dat[0]+1;          // send back to master
            Delay_ms(1);
            RS485Master_Send(dat,1,160);
          end;

        if (cnt > 100000) then                // if in 100000 poll-cycles the answer
          begin
            Inc(GPIO_PORTD_DATA);             //   was not detected, signal
            cnt := 0;                         //   failure of send-message
            RS485Master_Send(dat,1,160);
            if (GPIO_PORTD_DATA > 10) then    // if sending failed 10 times
              begin
                RS485Master_Send(dat,1,50);   //   send message on broadcast address
              end;
          end;
      end;
  end.

STM32

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program RS485_Master_Example;

var dat : array[20] of byte;   // buffer for receving/sending messages
    i, j : byte;
    cnt : longint;

var rs485_rxtx_pin           : sbit at GPIOA_ODR.B6;    // set transcieve pin

// Interrupt routine
procedure interrupt(); iv IVT_INT_USART1;
begin
  RS485Master_Receive(dat);
end;

  begin
    cnt := 0;

    GPIO_Digital_Output(@GPIOD_BASE, _GPIO_PINMASK_ALL);
    GPIOD_ODR := 0;
    
    GPIO_Digital_Output(@GPIOC_BASE, _GPIO_PINMASK_ALL);
    GPIOC_ODR := 0;

    UART1_Init(19200);              // initialize UART1 module
    Delay_ms(100);

    RS485Master_Init();             // initialize MCU as Master

    USART1_CR1.B5 := 1;             // enable uart rx interrupt
    NVIC_IntEnable(IVT_INT_USART1); // enable interrupt vector

    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);

    while (TRUE) do
      begin                              // upon completed valid message receiving
                                         //   data[4] is set to 255
        Inc(cnt);
        if (dat[5] <> 0) then            // if an error detected, signal it
          GPIOC_ODR := 0xAA;             //   by setting portd to 0xAA
        if (dat[4] <> 0) then            // if message received successfully
          begin
            cnt := 0;
            dat[4] := 0;                 // clear message received flag
            j := dat[3];
            for i := 1 to dat[3] do      // show data on PORTD
              GPIOD_ODR := dat[i-1];
            dat[0] := dat[0]+1;          // send back to master
            Delay_ms(1);
            RS485Master_Send(dat,1,160);
          end;

        if (cnt > 100000) then                // if in 100000 poll-cycles the answer
          begin
            Inc(GPIOC_ODR);                   //   was not detected, signal
            cnt := 0;                         //   failure of send-message
            RS485Master_Send(dat,1,160);
            if (GPIOC_ODR > 10) then          // if sending failed 10 times
              begin
                RS485Master_Send(dat,1,50);   //   send message on broadcast address
              end;
          end;
      end;
  end.

Stellaris

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program RS485_Slave_Example;

var dat : array[20] of byte;                           // buffer for receving/sending messages
    i, j : byte;

var rs485_rxtx_pin           : sbit at GPIO_PORTA_DATA2_bit;    // set transcieve pin
    rs485_rxtx_pin_direction : sbit at GPIO_PORTA_DIR2_bit;     // set transcieve pin direction

// Interrupt routine
procedure interrupt(); iv IVT_INT_UART0;
begin
  RS485Slave_Receive(dat);
  UART_ICR_RXIC_bit := 1;           // ensure interrupt not pending
end;

begin
  GPIO_Digital_Output(@GPIO_PORTD, _GPIO_PINMASK_ALL);
  GPIO_PORTD_DATA := 0;

  GPIO_Digital_Output(@GPIO_PORTE, _GPIO_PINMASK_ALL);
  GPIO_PORTE_DATA := 0;

  UART0_Init(19200);         // initialize UART0 module
  Delay_ms(100);

  UART_IM_RXIM_bit := 1;         // enable uart rx interrupt
  NVIC_IntEnable(IVT_INT_UART0); // enable interrupt vector

  EnableInterrupts();            // enable core interrupts

  RS485Slave_Init(160);      // Intialize MCU as slave, address 160

  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;

  while (TRUE) do
    begin
      if (dat[5] <> 0) then           // if an error detected, signal it by
        begin
          GPIO_PORTD_DATA := 0xAA;    //   setting GPIO_PORTD to 0xAA
          dat[5] := 0;
        end;
      if (dat[4] <> 0) then           // upon completed valid message receive
        begin
          dat[4] := 0;                //   data[4] is set to 0xFF
          j := dat[3];

          for i := 1 to dat[3] do     // show data on GPIO_PORTE
            GPIO_PORTE_DATA := 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
        end;
    end;
end.

STM32

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program RS485_Slave_Example;

var dat : array[20] of byte;                           // buffer for receving/sending messages
    i, j : byte;

var rs485_rxtx_pin           : sbit at GPIOA_ODR.B6;    // set transcieve pin

// Interrupt routine
procedure interrupt(); iv IVT_INT_USART1;
begin
  RS485Slave_Receive(dat);
end;

begin
  GPIO_Digital_Output(@GPIOD_BASE, _GPIO_PINMASK_ALL);
  GPIOD_ODR := 0;

  GPIO_Digital_Output(@GPIOC_BASE, _GPIO_PINMASK_ALL);
  GPIOC_ODR := 0;

  UART1_Init(19200);              // initialize UART1 module
  Delay_ms(100);

  USART1_CR1.B5 := 1;             // enable uart rx interrupt
  NVIC_IntEnable(IVT_INT_USART1); // enable interrupt vector

  RS485Slave_Init(160);           // Intialize MCU as slave, address 160
  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;

  while (TRUE) do
    begin
      if (dat[5] <> 0) then           // if an error detected, signal it by
        begin
          GPIOC_ODR := 0xAA;          //   setting portd to 0xAA
          dat[5] := 0;
        end;
      if (dat[4] <> 0) then           // upon completed valid message receive
        begin
          dat[4] := 0;                //   data[4] is set to 0xFF
          j := dat[3];

          for i := 1 to dat[3] do     // show data on PORTD
            GPIOD_ODR := 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
        end;
    end;
end.

Message format and CRC calculations

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

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const START_BYTE : byte = 0x96;  // 10010110
const STOP_BYTE  : 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  xor address;
crc_send := crc_send xor data[0];    // if exists
crc_send := crc_send xor data[1];    // if exists
crc_send := crc_send xor data[2];    // if exists
crc_send := crc_send not crc_send;
if ((crc_send = START_BYTE) or (crc_send = STOP_BYTE)) then
   crc_send := crc_send + 1;
NOTE: DATALEN<4..0> can not take the START_BYTE<4..0> or STOP_BYTE<4..0> values.

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