Embedded Lab

RS485 MODBUS

RS485 is basically a long-range reliable industrial version of serial communication. It is a hardware layer that can support anything from simple serial communication to complex software-based communication layers. MODBUS is such a software layer or protocol that can be used to reliably exchange data between devices. MODBUS (particularly MODBUS RTU) is very popular among industrial devices like energy meters, PLCs and sensor devices. MODBUS has some similarity with I2C in some regards.

Another similar communication medium is the Controller Area Network (CAN). CAN is not supported internally by STC microcontrollers unlike some advanced microcontrollers like the STM32s and other ARMs but with external hardware like MCP2551 and MCP2515 CAN communication can also be achieved. CAN communication is beyond the scope of this tutorial.

To know more about MODBUS, visit these links:

• https://en.wikipedia.org/wiki/Modbus
• https://modbus.org/docs/PI_MBUS_300.pdf
• https://modbus.org/docs/Modbus_Application_Protocol_V1_1b.pdf

Code

 #include "STC8xxx.h"
 #include "BSP.h"
 #include "LCD.c"
 #include "lcd_print.c"
  
 #define DIR_RX                                  P12_low
 #define DIR_TX                                  P12_high
  
  
  
  
 #define TX_buffer_length                        8
 #define RX_buffer_length                        10
  
 unsigned char cnt = 0x00;
 unsigned char RX_buffer[RX_buffer_length];
  
 static const unsigned char TX_buffer[TX_buffer_length] = {0x01, 0x03, 0x00, 0x00, 0x00, 0x02, 0xC4, 0x0B};
  
 void setup(void);
 void flush_RX_buffer(void);
 void send_read_command(void);
 unsigned int make_word(unsigned char HB, unsigned char LB);
 void get_HB_LB(unsigned int value, unsigned char *HB, unsigned char *LB);
 unsigned int MODBUS_RTU_CRC16(unsigned char *data_input, unsigned char data_length);
  
 void UART_2_ISR(void)       
 interrupt 8
 {
   if(check_UART_2_RX_flag)
   {
     RX_buffer[cnt++] = UART2_read_buffer();
   }
 }
  
 void main(void)
 {
   unsigned int value = 0x0000;
   unsigned int CRC_check_1 = 0x0000;
   unsigned int CRC_check_2 = 0x0000;
  
   setup();
  
   LCD_goto(0, 0);
   LCD_putstr("R.H / %:");
   LCD_goto(0, 1);
   LCD_putstr("Temp/ C:");
   print_symbol(5, 1, 0);
  
   while(1)
   {
       send_read_command();
  
       CRC_check_1 = MODBUS_RTU_CRC16(RX_buffer, 7);
       CRC_check_2 = make_word(RX_buffer[8], RX_buffer[7]);
  
       if(CRC_check_1 == CRC_check_2)
       {
          value = make_word(RX_buffer[5], RX_buffer[6]);
          print_F(11, 0, (value / 10.0), 1);
  
          value = make_word(RX_buffer[3], RX_buffer[4]);
          print_F(11, 1, (value / 10.0), 1);
       }
  
       else
       {
         LCD_goto(12, 0);
         LCD_putstr("--.-");
         LCD_goto(12, 1);
         LCD_putstr("--.-");
       }
  
       P55_toggle;
       delay_ms(1000);
   };
 }
  
 void setup(void)
 {
   CLK_set_sys_clk(IRC_24M, 2, MCLK_SYSCLK_no_output, MCLK_out_P54);
   
   P12_push_pull_mode;
   P55_open_drain_mode;
   
   LCD_init();
   LCD_clear_home();
   load_custom_symbol();
   
   UART2_pin_option(0x00);
   
   UART2_init(9600, \
              UART2_timer_1T, \
              12000000);
   
   _enable_UART_2_interrupt;
   _enable_global_interrupt;
 }
  
 void flush_RX_buffer(void)
 {
   signed char i = (RX_buffer_length - 1);
  
   while(i > -1)
   {
     RX_buffer[i] = 0x00;
     i--;
   };
 }
  
 void send_read_command(void)
 {
   unsigned char i = 0x00;
  
   flush_RX_buffer();
  
   DIR_TX;
  
   for(i = 0; i < TX_buffer_length; i++)
   {
     UART2_write_buffer(TX_buffer[i]);
   }
  
   cnt = 0;
   DIR_RX;
  
   delay_ms(600);
 }
  
 unsigned int make_word(unsigned char HB, unsigned char LB)
 {
   unsigned int tmp = 0;
  
   tmp = HB;
   tmp <<= 8;
   tmp |= LB;
  
   return tmp;
 }
  
 void get_HB_LB(unsigned int value, unsigned char *HB, unsigned char *LB)
 {
   *LB = (value & 0x00FF);
   *HB = ((value & 0xFF00) >> 8);
 }
  
 unsigned int MODBUS_RTU_CRC16(unsigned char *data_input, unsigned char data_length)
 {
   static const unsigned int CRC_table[256] =
   {
      0x0000, 0xC0C1, 0xC181, 0x0140, 0xC301, 0x03C0, 0x0280, 0xC241,
      0xC601, 0x06C0, 0x0780, 0xC741, 0x0500, 0xC5C1, 0xC481, 0x0440,
      0xCC01, 0x0CC0, 0x0D80, 0xCD41, 0x0F00, 0xCFC1, 0xCE81, 0x0E40,
      0x0A00, 0xCAC1, 0xCB81, 0x0B40, 0xC901, 0x09C0, 0x0880, 0xC841,
      0xD801, 0x18C0, 0x1980, 0xD941, 0x1B00, 0xDBC1, 0xDA81, 0x1A40,
      0x1E00, 0xDEC1, 0xDF81, 0x1F40, 0xDD01, 0x1DC0, 0x1C80, 0xDC41,
      0x1400, 0xD4C1, 0xD581, 0x1540, 0xD701, 0x17C0, 0x1680, 0xD641,
      0xD201, 0x12C0, 0x1380, 0xD341, 0x1100, 0xD1C1, 0xD081, 0x1040,
      0xF001, 0x30C0, 0x3180, 0xF141, 0x3300, 0xF3C1, 0xF281, 0x3240,
      0x3600, 0xF6C1, 0xF781, 0x3740, 0xF501, 0x35C0, 0x3480, 0xF441,
      0x3C00, 0xFCC1, 0xFD81, 0x3D40, 0xFF01, 0x3FC0, 0x3E80, 0xFE41,
      0xFA01, 0x3AC0, 0x3B80, 0xFB41, 0x3900, 0xF9C1, 0xF881, 0x3840,
      0x2800, 0xE8C1, 0xE981, 0x2940, 0xEB01, 0x2BC0, 0x2A80, 0xEA41,
      0xEE01, 0x2EC0, 0x2F80, 0xEF41, 0x2D00, 0xEDC1, 0xEC81, 0x2C40,
      0xE401, 0x24C0, 0x2580, 0xE541, 0x2700, 0xE7C1, 0xE681, 0x2640,
      0x2200, 0xE2C1, 0xE381, 0x2340, 0xE101, 0x21C0, 0x2080, 0xE041,
      0xA001, 0x60C0, 0x6180, 0xA141, 0x6300, 0xA3C1, 0xA281, 0x6240,
      0x6600, 0xA6C1, 0xA781, 0x6740, 0xA501, 0x65C0, 0x6480, 0xA441,
      0x6C00, 0xACC1, 0xAD81, 0x6D40, 0xAF01, 0x6FC0, 0x6E80, 0xAE41,
      0xAA01, 0x6AC0, 0x6B80, 0xAB41, 0x6900, 0xA9C1, 0xA881, 0x6840,
      0x7800, 0xB8C1, 0xB981, 0x7940, 0xBB01, 0x7BC0, 0x7A80, 0xBA41,
      0xBE01, 0x7EC0, 0x7F80, 0xBF41, 0x7D00, 0xBDC1, 0xBC81, 0x7C40,
      0xB401, 0x74C0, 0x7580, 0xB541, 0x7700, 0xB7C1, 0xB681, 0x7640,
      0x7200, 0xB2C1, 0xB381, 0x7340, 0xB101, 0x71C0, 0x7080, 0xB041,
      0x5000, 0x90C1, 0x9181, 0x5140, 0x9301, 0x53C0, 0x5280, 0x9241,
      0x9601, 0x56C0, 0x5780, 0x9741, 0x5500, 0x95C1, 0x9481, 0x5440,
      0x9C01, 0x5CC0, 0x5D80, 0x9D41, 0x5F00, 0x9FC1, 0x9E81, 0x5E40,
      0x5A00, 0x9AC1, 0x9B81, 0x5B40, 0x9901, 0x59C0, 0x5880, 0x9841,
      0x8801, 0x48C0, 0x4980, 0x8941, 0x4B00, 0x8BC1, 0x8A81, 0x4A40,
      0x4E00, 0x8EC1, 0x8F81, 0x4F40, 0x8D01, 0x4DC0, 0x4C80, 0x8C41,
      0x4400, 0x84C1, 0x8581, 0x4540, 0x8701, 0x47C0, 0x4680, 0x8641,
      0x8201, 0x42C0, 0x4380, 0x8341, 0x4100, 0x81C1, 0x8081, 0x4040
    };
  
   unsigned char temp = 0;
   unsigned int CRC_word = 0xFFFF;
  
   while(data_length--)
   {
       temp = *data_input++ ^ CRC_word;
       CRC_word >>= 8;
       CRC_word ^= CRC_table[temp];
   }
  
   return CRC_word;
 } 

Schematic

Explanation

In this example, a MODBUS-based SHT20 relative humidity and temperature sensor is read with a STC micro. The sensor accepts MODBUS RTU data frames.

The setup is similar to the one we have already seen in the UART example and it should easy by now. The only exceptions are the UART module and the use of its interrupt.

 CLK_set_sys_clk(IRC_24M, 2, MCLK_SYSCLK_no_output, MCLK_out_P54);
   
 P12_push_pull_mode;
 P55_open_drain_mode;
   
 UART2_pin_option(0x00);
   
 UART2_init(9600, \
            UART2_timer_1T, \
            12000000);
  
 _enable_UART_2_interrupt;
 _enable_global_interrupt; 

The UART’s interrupt is used for receiving data only.

 void UART_2_ISR(void)       
 interrupt 8
 {
   if(check_UART_2_RX_flag)
   {
     RX_buffer[cnt++] = UART2_read_buffer();
   }
 } 

For sending data, the following function is used. In this function, previously received data are cleared first in order to make the micro ready to receive new batch of data and to make sure that past data do not make any conflict with the new ones. The onboard MAX485’s data direction is set to transmission mode and the transmission (TX) buffer data are sent via UART. MAX485’s mode operation is reset back to reception mode in order to receive new data.     

static const unsigned char TX_buffer[TX_buffer_length] = {0x01, 0x03, 0x00, 0x00, 0x00, 0x02, 0xC4, 0x0B};
  
 ....
  
 void send_read_command(void)
 {
   unsigned char i = 0x00;
  
   flush_RX_buffer();
  
   DIR_TX;
  
   for(i = 0; i < TX_buffer_length; i++)  
   {
     UART2_write_buffer(TX_buffer[i]);
   }
  
   cnt = 0;
   DIR_RX;
  
   delay_ms(600);
 } 

The TX buffer contains the following information in the following order. This is the standard frame that should be sent by a host device to read holding registers. MODBUS RTU strictly prohibits other data frame formats and doing so will only lead to errors.

When the sensor receives these bytes in this order, it responds back with a reception or RX frame which is something like the frame shown below. This frame is similar to the TX frame but the important stuffs are the relative humidity and temperature data. These are the stuff that we mainly need. The values in frame shown below is just for giving an example.    

As per MODBUS RTU frame shown above, the slave ID is 0x01, the function code is 0x03 and 4 data bytes have been sent by the sensor along with the CRC fields. Function code 0x03 stands for reading holding registers. The temperature and the relative humidity are computed according to the calculation shown below:

The table above shows that the high and low bytes are combined to make word and the word is divided by 10 to get desired outputs.

For more information on MODBUS, please refer to the docs mentioned in the beginning of this section. MODBUS RTU, itself, is a big topic and it is not possible to fully cover it in this tutorial. The code runs by querying the SHT20 sensor. After receiving all data from sensor, two Cyclic Redundancy Checks (CRC) are performed in order to ensure that valid and error-free data have been received. One CRC is the embedded with the RX frame and other is calculated using a CRC lookup table. Details of CRC lookup table and formula are covered in MODBUS RTU documentations. If CRCs match then the RX frame is considered to contain valid data and with the RX data, we can compute relative humidity and temperature values.   

 send_read_command();
  
 CRC_check_1 = MODBUS_RTU_CRC16(RX_buffer, 7);
 CRC_check_2 = make_word(RX_buffer[8], RX_buffer[7]);
  
 if(CRC_check_1 == CRC_check_2)
 {
      value = make_word(RX_buffer[5], RX_buffer[6]);
      print_F(11, 0, (value / 10.0), 1);
  
      value = make_word(RX_buffer[3], RX_buffer[4]);
      print_F(11, 1, (value / 10.0), 1);
 }
  
 else
 {
      LCD_goto(12, 0);
      LCD_putstr("--.-");
      LCD_goto(12, 1);
      LCD_putstr("--.-");
 }
  
 P55_toggle;
 delay_ms(1000); 

Demo

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