RS-485 Library

RS-485 is a multipoint communication which allows multiple devices to be connected to a single bus. mikroBasic PRO for PIC32 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

The following variable must be defined in all projects using RS-485 Library:	Description :	Example :
`dim RS485_rxtx_pin as sbit sfr atomic external`	Control RS-485 Transmit/Receive operation mode	`dim RS485_rxtx_pin as sbit at RF2_bit`
`dim RS485_rxtx_pin_direction as sbit sfr atomic external`	Direction of the RS-485 Transmit/Receive pin	`dim RS485_rxtx_pin_direction as sbit at TRISF2_bit`

RS485Master_Init

Prototype	sub procedure RS485Master_Init()
Description	Initializes MCU as a Master for RS-485 communication.
Parameters	None.
Returns	Nothing.
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 routine. UART HW module needs to be initialized. See UARTx_Init.
Example	' RS485 module pinout dim RS485_rxtx_pin as sbit at RF2_bit dim RS485_rxtx_pin_direction as sbit at TRISF2_bit ' End of RS485 module pinout ... UART1_Init(9600) ' initialize UART1 module RS485Master_Init() ' intialize MCU as a Master for RS-485 communication
Notes	None.

RS485Master_Receive

Prototype	sub procedure RS485Master_Receive(dim byref data as byte[10])
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	dim msg as byte[8] ... RS485Master_Receive(msg)
Notes	None.

RS485Master_Send

Prototype	sub procedure RS485Master_Send(dim byref buffer as byte[20], dim datalen as byte, dim address as 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	dim msg as byte[8] ... ' send 3 bytes of data to slave with address 0x12 RS485Master_Send(msg, 3, 0x12)
Notes	None.

RS485Slave_Init

Prototype	sub procedure RS485Slave_Init(dim slave_address as byte)
Description	Initializes MCU as a Slave for RS-485 communication.
Parameters	`Slave_address:` Slave address
Returns	Nothing.
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 routine. UART HW module needs to be initialized. See UARTx_Init.
Example	Initialize MCU as a Slave with address 160: ' RS485 module pinout dim RS485_rxtx_pin as sbit at RF2_bit dim RS485_rxtx_pin_direction as sbit at TRISF2_bit ' 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
Notes	None.

RS485Slave_Receive

Prototype	sub procedure RS485Slave_Receive(dim byref data as byte[20])
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	dim msg as byte[8] ... RS485Slave_Read(msg)
Notes	None.

RS485Slave_Send

Prototype	sub procedure RS485Slave_Send(dim byref data as byte[20], dim datalen as 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	dim msg as byte[8] ... ' send 2 bytes of data to the Master RS485Slave_Send(msg, 2)
Notes	None.

Library Example

The example demonstrates 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 PORTB while error on receiving (0xAA) and number of consecutive unsuccessful retries are displayed on PORTD. Hardware configurations in this example are made for the LV-32MX v6 board and PIC32MX460F512L.

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program RS485_Master_Example

dim dat as byte[10]              ' buffer for receving/sending messages
    i, j as byte
    cnt as longint

dim rs485_rxtx_pin  as sbit at RF2_bit                ' set transcieve pin
    rs485_rxtx_pin_direction as sbit at TRISF2_bit    ' set transcieve pin direction

' Interrupt routine
sub procedure interrupt() iv IVT_UART_2 ilevel 7 ics ICS_SRS
  RS485Master_Receive(dat)
  U2RXIF_bit = 0                 ' ensure interrupt not pending
end sub

main:
  cnt = 0

  CHECON  = 0x32
  AD1PCFG = 0xFFFF

  PORTB = 0
  PORTD = 0
  TRISB = 0
  TRISD = 0

  UART2_Init(19200)        ' initialize UART2 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

  U2IP0_bit = 1            ' set interrupt
  U2IP1_bit = 1            ' priority
  U2IP2_bit = 1            ' to 7

  URXISEL1_U2STA_bit = 0   ' 0x = Interrupt flag bit is set when a character is received
  U2RXIF_bit = 0           ' ensure interrupt not pending

  MVEC_bit = 1             ' Interrupt controller configured for multi vectored mode
  asm
    ei R0                  ' Enable all interrupts
  end asm

  U2RXIE_bit = 1                     ' enable intterupt

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

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

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program RS485_Slave_Example

dim dat as byte[20]                           ' buffer for receving/sending messages
    i, j as byte

dim rs485_rxtx_pin as sbit at RF2_bit                   ' set transcieve pin
    rs485_rxtx_pin_direction as sbit at TRISF2_bit      ' set transcieve pin direction

' Interrupt routine
sub procedure interrupt() iv IVT_UART_2 ilevel 7 ics ICS_SRS
  RS485Slave_Receive(dat)
  U2RXIF_bit = 0                      ' ensure interrupt not pending
end sub

main:
  CHECON  = 0x32
  AD1PCFG = 0xFFFF

  TRISB = 0
  TRISD = 0
  PORTB = 0
  PORTD = 0

  UART2_Init(19200)         ' initialize UART2 module
  Delay_ms(100)

  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

  U2IP0_bit = 1            ' set interrupt
  U2IP1_bit = 1            ' priority
  U2IP2_bit = 1            ' to 7

  URXISEL1_U2STA_bit = 0   ' 0x = Interrupt flag bit is set when a character is received
  U2RXIF_bit = 0           ' ensure interrupt not pending

  MVEC_bit = 1             ' Interrupt controller configured for multi vectored mode
  asm
    ei R0                  ' Enable all interrupts
  end asm

  U2RXIE_bit = 1                      ' enable intterupt

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

      for i = 1 to dat[3]         ' show data on PORTB
        PORTB = dat[i-1]
      next i

      dat[0] = dat[0]+1           ' increment received dat[0]
      Delay_ms(1)
      RS485Slave_Send(dat,1)      '   and send it back to master
    end if
  wend
end.

HW Connection

Example of interfacing PC to PIC32 MCU via RS485 bus with LTC485 as RS-485 transceiver

Example of interfacing PC to PIOC32 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  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
end if
NOTE: DATALEN<4..0> can not take the START_BYTE<4..0> or STOP_BYTE<4..0> values.

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