RS-485 Library
RS-485 is a multipoint communication which allows multiple devices to be connected to a single bus. The mikroC 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).
- START byte value =
150 - STOP byte value =
169 - Address
50is the broadcast address for all Slaves (packets containing address50will be received by all Slaves except the Slaves with addresses150and169).
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 : |
|---|---|---|
extern sfr sbit RS485_rxtx_pin; |
Control RS-485 Transmit/Receive operation mode | sbit RS485_rxtx_pin at GPIO_PORTA_DATA2_bit; |
extern sfr sbit RS485_rxtx_pin_direction; |
Direction of the RS-485 Transmit/Receive pin | sbit RS485_rxtx_pin_direction at GPIO_PORTA_DIR2_bit; |
STM32
| 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 GPIOA_ODR.B6; |
Library Routines
- RS485Master_Init
- RS485Master_Receive
- RS485Master_Send
- RS485Slave_Init
- RS485Slave_Receive
- RS485Slave_Send
RS485Master_Init
| Prototype |
void 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.
|
| Example |
Stellaris// RS485 module pinout sbit RS485_rxtx_pin at GPIO_PORTA_DATA2_bit; // set transcieve pin sbit RS485_rxtx_pin_direction at GPIO_PORTA_DIR2_bit; // set transcieve pin direction // end RS485 module pinout ... UART1_Init(9600); // initialize UART1 module RS485Master_Init(); // intialize MCU as a Master for RS-485 communication STM32// RS485 module pinout sbit RS485_rxtx_pin at GPIOA_ODR.B6; // set transcieve pin // end 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 |
void RS485Master_Receive(char *data_buffer); |
|---|---|
| Description |
Receives messages from Slaves. Messages are multi-byte, so this routine must be called for each byte received. |
| Parameters |
The routine automatically adjusts |
| Returns |
Nothing. |
| Requires |
MCU must be initialized as a Master for RS-485 communication. See RS485Master_Init. |
| Example |
char msg[8]; ... RS485Master_Receive(msg); |
| Notes |
None. |
RS485Master_Send
| Prototype |
void RS485Master_Send(char *data_buffer, char datalen, char slave_address); |
|---|---|
| Description |
Sends message to Slave(s). Message format can be found at the bottom of this page. |
| Parameters |
|
| 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 |
char msg[8]; ... // send 3 bytes of data to Slave with address 0x12 RS485Master_Send(msg, 3, 0x12); |
| Notes |
None. |
RS485Slave_Init
| Prototype |
void RS485Slave_Init(char Slave_address); |
|---|---|
| Description |
Initializes MCU as a Slave for RS-485 communication. |
| Parameters |
|
| Returns |
Nothing. |
| Requires |
External dependencies of the library from the top of the page must be defined before using this function.
|
| Example |
Stellaris// RS485 module pinout sbit RS485_rxtx_pin at GPIO_PORTA_DATA2_bit; // set transcieve pin sbit RS485_rxtx_pin_direction at GPIO_PORTA_DIR2_bit; // set transcieve pin direction // end 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 sbit RS485_rxtx_pin at GPIOA_ODR.B6; // set transcieve pin // end 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 |
void RS485Slave_Receive(char *data_buffer); |
|---|---|
| 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 |
The routine automatically adjusts |
| Returns |
Nothing. |
| Requires |
MCU must be initialized as a Slave for RS-485 communication. See RS485Slave_Init. |
| Example |
char msg[8]; ... RS485Slave_Read(msg); |
| Notes |
None. |
RS485Slave_Send
| Prototype |
void RS485Slave_Send(char *data_buffer, char datalen); |
|---|---|
| Description |
Sends message to Master. Message format can be found at the bottom of this page. |
| Parameters |
|
| 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 |
char msg[8]; ... // send 2 bytes of data to the Master RS485Slave_Send(msg, 2); |
| Notes |
None. |
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 GPIO_PORTE, while error on receive (0xAA) and number of consecutive unsuccessful retries are displayed on GPIO_PORTD. Slave displays received data on GPIO_PORTE, while error on receive (0xAA) is displayed on GPIO_PORTD.
RS485 Master code:
Stellaris
char dat[10]; // buffer for receving/sending messages
char i,j;
sbit RS485_rxtx_pin at GPIO_PORTA_DATA2_bit; // set transcieve pin
sbit RS485_rxtx_pin_direction at GPIO_PORTA_DIR2_bit; // set transcieve pin direction
// Interrupt routine
void interrupt() iv IVT_INT_UART0 {
RS485Master_Receive(dat);
UART_ICR_RXIC_bit = 1; // ensure interrupt not pending
}
void main(){
long 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 (1){
// upon completed valid message receiving
// data[4] is set to 255
cnt++;
if (dat[5]) { // if an error detected, signal it
GPIO_PORTD_DATA = 0xAA; // by setting GPIO_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 GPIO_PORTE
GPIO_PORTE_DATA = 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) {
GPIO_PORTD_DATA ++;
cnt = 0;
RS485Master_Send(dat,1,160);
if (GPIO_PORTD_DATA > 10) // if sending failed 10 times
RS485Master_Send(dat,1,50); // send message on broadcast address
}
}
}
STM32
char dat[10]; // buffer for receving/sending messages
char i,j;
sbit RS485_rxtx_pin at GPIOA_ODR.B6; // set transcieve pin
// Interrupt routine
void interrupt() iv IVT_INT_USART1 ics ICS_AUTO {
RS485Master_Receive(dat);
}
void main(){
long cnt = 0;
GPIO_Digital_Output(&GPIOC_BASE, _GPIO_PINMASK_ALL);
GPIOC_ODR = 0;
GPIO_Digital_Output(&GPIOD_BASE, _GPIO_PINMASK_ALL);
GPIOD_ODR = 0;
UART1_Init(19200); // initialize UART1 module
Delay_ms(100);
RS485Master_Init(); // initialize MCU as Master
USART1_CR1bits.RXNEIE = 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 (1) {
// upon completed valid message receiving
// data[4] is set to 255
cnt++;
if (dat[5]) { // if an error detected, signal it
GPIOC_ODR = 0xAA; // by setting PORTC 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 PORTD
GPIOD_ODR = 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) {
GPIOC_ODR ++;
cnt = 0;
RS485Master_Send(dat,1,160);
if (GPIOC_ODR > 10) // if sending failed 10 times
RS485Master_Send(dat,1,50); // send message on broadcast address
}
}
}
RS485 Slave code:
Stellaris
char dat[9]; // buffer for receving/sending messages
char i;
sbit RS485_rxtx_pin at GPIO_PORTA_DATA2_bit; // set transcieve pin
sbit RS485_rxtx_pin_direction at GPIO_PORTA_DIR2_bit; // set transcieve pin direction
// Interrupt routine
void interrupt() iv IVT_INT_UART0 {
RS485Slave_Receive(dat);
UART_ICR_RXIC_bit = 1; // ensure interrupt not pending
}
void main() {
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 (1) {
if (dat[5]) { // if an error detected, signal it by
GPIO_PORTD_DATA = 0xAA; // setting GPIO_PORTD to 0xAA
dat[5] = 0;
}
if (dat[4]) { // upon completed valid message receive
dat[4] = 0; // data[4] is set to 0xFF
for (i = 1; i <= dat[3];i++){
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
}
}
}
STM32
char dat[9]; // buffer for receving/sending messages
char i;
sbit RS485_rxtx_pin at GPIOA_ODR.B6; // set transcieve pin
// Interrupt routine
void interrupt() iv IVT_INT_USART1 ics ICS_AUTO {
RS485Slave_Receive(dat);
}
void main() {
GPIO_Digital_Output(&GPIOC_BASE, _GPIO_PINMASK_ALL);
GPIOC_ODR = 0;
GPIO_Digital_Output(&GPIOD_BASE, _GPIO_PINMASK_ALL);
GPIOD_ODR = 0;
UART1_Init(19200); // initialize UART1 module
Delay_ms(100);
USART1_CR1bits.RXNEIE = 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 (1) {
if (dat[5]) { // if an error detected, signal it by
GPIOC_ODR = 0xAA; // setting PORTC to 0xAA
dat[5] = 0;
}
if (dat[4]) { // upon completed valid message receive
dat[4] = 0; // data[4] is set to 0xFF
for (i = 1; i <= dat[3];i++){
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
}
}
}
Message format and CRC calculations
Q: How is CRC checksum calculated on RS485 master side?
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.
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