By request one of my long lost blogs was a MODbus RTU application for a PIC 18F27J53. You could easily plop this onto many of the 18F series micros. It doesn’t do much but it’s a great stepping stone for throwing in your own IO. I’ll probably follow up with an Arduino version in a while.. I’ll put it on my “todo” list.
The code…
/* File: main.c Author: Charles M Ihler Contact at: http://iradan.com * Created on November 8, 2014, 2:37 PM * Target Device: 18F27J53 Development Board by AtomSoft * Project: MODbus/RTU slave * Notes We will ignore both query high bytes for now TODO: Pull all hard coded IO out eventually and set it up via USB? Maybe a terminal application for enabling points? * * Version: 0.02 Controlled sends caned reply on request to address on RS-232 0.03 CRC works on RX 0.04 No actual IO yet.. Function 0x02 (Read Input Status) Started 0.05 Ported to 18F17J53 for expanded IO/USB options. 12MHz XTAL with CPUDIV set to 16MHz * / //#ifndef _XTAL_FREQ //#define _XTAL_FREQ 16000000 // //#define __delay_us(x) _delay((unsigned long)((x)(_XTAL_FREQ/4000000.0))) //#define __delay_ms(x) _delay((unsigned long)((x)*(_XTAL_FREQ/4000.0))) //#endif // PIC18F27J53 Configuration Bit Settings // 'C' source line config statements include // #pragma config statements should precede project file includes. // Use project enums instead of #define for ON and OFF. // CONFIG1L pragma config WDTEN = OFF // Watchdog Timer (Disabled - Controlled by SWDTEN bit) pragma config PLLDIV = 3 // PLL Prescaler Selection (Divide by 3 (12 MHz oscillator input)) pragma config CFGPLLEN = ON // PLL Enable Configuration Bit (PLL Enabled) pragma config STVREN = ON // Stack Overflow/Underflow Reset (Enabled) pragma config XINST = OFF // Extended Instruction Set (Enabled) // CONFIG1H pragma config CPUDIV = OSC3_PLL3// CPU System Clock Postscaler (CPU system clock divide by 3 from 48MHz) pragma config CP0 = OFF // Code Protect (Program memory is not code-protected) // CONFIG2L pragma config OSC = HSPLL // Oscillator (HS+PLL, USB-HS+PLL) pragma config SOSCSEL = HIGH // T1OSC/SOSC Power Selection Bits (High Power T1OSC/SOSC circuit selected) pragma config CLKOEC = OFF // EC Clock Out Enable Bit (CLKO output enabled on the RA6 pin) pragma config FCMEN = OFF // Fail-Safe Clock Monitor (Disabled) pragma config IESO = OFF // Internal External Oscillator Switch Over Mode (Disabled) define _XTAL_FREQ 16000000 //defined for delaychar ctxt[10], buf1, testcrc[4]; //buff serial string volatile unsigned int ping, isrcall, index, reading, new_rx; int address, crc_high, crc_low, i, querylength; unsigned int result; //these variables are for fuctions to be enabled/disabled and pushed into application code via later int frame, z, bufferbits, query_reg_address, y; char data_out[64]; char reg_address[255];/* * Interrupt Service * UART RX and Timer 1 * */ void interrupt ISR() { if (PIR1bits.RCIF) // see if interrupt caused by incoming data { isrcall = 0x01; char temp; temp = RCREG; // read the incoming data buf1 = temp; if(temp==address && reading==0) //if my address.. { index = 0; //reset index reading = 1; //from now on go to else if LATCbits.LATC1 = 1; new_rx = 0; ping = 1; } else if(reading == 1) //in middle of GPS sentence { //TODO reset timeout timer ctxt[index] = temp; //load it up index++; //increment index ping = 1; //this is for debugging if(index > 6) //1+7 = master frame. { index = 0; //reset index reading = 0; //no longer storing the string new_rx = 1; //"ding" T1CONbits.TMR1ON = 0; } } } //RCSTA2bits.FERR = 0; //Clear errors //RCSTA2bits.OERR = 0; //time out timer, if tripped new_rx=0; if (PIR1bits.TMR1IF) { new_rx=0; ping = 0; T1CONbits.TMR1ON = 0; PIR1bits.TMR1IF = 0; if (reading) { reading = 0; LATCbits.LATC0 = 1; } } } void init_io(void) {INTCONbits.GIE = 0; //no interruptions please ADCON0 = 0b00000000; //don't need any ADC ADCON1 = 0b00000000; //speed Vref=AVdd, VssRef=AVss LATA = 0x00; LATB = 0x00; LATC = 0x00; TRISAbits.TRISA0 = 1; // address 1 TRISAbits.TRISA1 = 1; // address 2 TRISAbits.TRISA2 = 1; // address 3 TRISAbits.TRISA3 = 0; // output TRISAbits.TRISA5 = 0; // output TRISBbits.TRISB0 = 0; // TRISBbits.TRISB1 = 1; // TRISBbits.TRISB2 = 0; // TRISBbits.TRISB3 = 0; // TRISBbits.TRISB4 = 0; // TRISBbits.TRISB5 = 1; // TRISBbits.TRISB6 = 0; // TRISBbits.TRISB7 = 0; // TRISCbits.TRISC0 = 0; // timer LED output TRISCbits.TRISC1 = 0; // output TRISCbits.TRISC2 = 0; // LED test output TRISCbits.TRISC3 = 0; // output //TRISCbits.TRISC4 = 1; // USB //TRISCbits.TRISC5 = 1; // USB TRISCbits.TRISC6 = 0; // output TX1 TRISCbits.TRISC7 = 1; // input RX1} void __delay_10ms(unsigned char n) //__delay functions built-in can't be used for much at this speed… so! { while (n-- != 0) { __delay_ms(10); } } void uart_xmit(unsigned int mydata_byte) {while(!TXSTAbits.TRMT); // make sure buffer full bit is high before transmitting TXREG = mydata_byte; // transmit data} void write_uart(const char txt) { //this send a string to the TX buffer //one character at a time while(txt) uart_xmit(*txt++); } void serial_init(void) { //9600 8N1TXSTA1bits.SYNC = 0; TXSTA1bits.BRGH=1; // select low speed Baud Rate (see baud rate calcs below) TXSTA1bits.TX9=0; // select 8 data bits TXSTA1bits.TXEN = 1; // enable transmit RCSTA1bits.SPEN=1; // serial port is enabled RCSTA1bits.RX9=0; // select 8 data bits RCSTA1bits.CREN=1; // receive enabled SPBRG1=102; SPBRGH1=0; PIR1bits.RCIF=0; // make sure receive interrupt flag is clear PIE1bits.RCIE=1; // enable UART Receive interrupt INTCONbits.PEIE = 1; // Enable peripheral interrupt INTCONbits.GIE = 1; // enable global interrupt __delay_10ms(5); // give time for voltage levels on board to settle} void run_timer (void) { T1CONbits.TMR1ON = 0; TMR1=0; TMR1L=0x00; TMR1H=0xAA; T1CONbits.TMR1CS = 0x01; T1CONbits.T1CKPS = 0x01; PIE1bits.TMR1IE = 1; T1CONbits.TMR1ON = 1; // PIR1bits.TMR1IF=0; } void check_my_address(void) { /* * Determine what address the unit is based on DIP switches * PORTAbits.RA0 Switch 1 * PORTAbits.RA1 Switch 2 */if (PORTAbits.RA0 & PORTAbits.RA1) address = 0x04; if (!PORTAbits.RA0 & PORTAbits.RA1) address = 0x03; if (PORTAbits.RA0 & !PORTAbits.RA1) address = 0x02; if (!PORTAbits.RA0 & !PORTAbits.RA1) address = 0x01;} /* TODO Poll the inputs * I will probably use a 74138 to address banks with 74HC245 for byte/banks of inputs I will likely use the same set up for "Coils" For input registers (analog inputs) I will consider perhaps some kind of A/D input with CD4066+74HC138 * */ void poll_my_inputs(void) {//just stuff it with something for now for (z = 0; z < 64; ++z) { reg_address[z] = z; }} /* did the chicken come before the egg? this MODbus CRC bit was gently borrowed from the internet.. I can't determine who wrote it.. it shows up in an Ardiuno Sketch for MODbus Modbus over serial line - RTU Slave Arduino Sketch with referenced of Juan Pablo Zometa, Samuel Marco, Andras Tucsni, Philip Costigan It also shows up on a number of websites and forums uncredited… many PIC or C based .. so who knows, but I didn't write it The MODbus CRC-16 function is outlined on the Modicon site for reference */ unsigned int modbus_crc(unsigned char buf[], int start, int cnt) { int i,j; unsigned temp, temp2, flag; temp=0xFFFF; for (i=start; i<cnt; i++){ temp=temp ^ buf[i]; for (j=1; j<=8; j++){ flag=temp & 0x0001; temp=temp >> 1; if (flag) temp=temp ^ 0xA001; } } /*** Reverse byte order. ***/ temp2=temp >> 8; temp= (temp << 8) | temp2; return(temp); } int check_master_crc (void) {int diff, diff1, diff2; //what we send back char data[6]; result = 0x0000; crc_high = 0x00; crc_low = 0x00; data[0] = address; //this is ugly but all master queries are 6bytes+CRC so it'll do data[1] = ctxt[0]; //function data[2] = ctxt[1]; //start_addressH data[3] = ctxt[2]; //start_addressL data[4] = ctxt[3]; //# of regH data[5] = ctxt[4]; //# of regL result = modbus_crc(data,0,6); crc_high = result >> 8; crc_low = result & 0x00FF; diff1 = ctxt[5] ^ crc_high; diff2 = ctxt[6] ^ crc_low; diff = diff1 + diff2; //this spits back an XORed value between calculated and received CRC return(diff);} void respond_input_status(void) { //ctxt[1] start_addressH //ctxt[2] start_addressL //ctxt[3] # of regH //ctxt[4] # of regL querylength = ctxt[4]; //ignoring upper byte for now (TODO) query_reg_address = ctxt[2]; //ignoring upper address byte (TODO)frame = 0; //data frame counter do { data_out[frame] = reg_address[query_reg_address]; //TODO make this do somethng querylength--; frame++; } while (querylength>0); querylength = ctxt[4]; //reload so we can count time transmitting data result = modbus_crc(data_out,0,2); crc_high = result >> 8; crc_low = result & 0x00FF; //uart_xmit response address, function, data_out[frames], CRC uart_xmit(address); uart_xmit(ctxt[0]); frame = 0; do { uart_xmit(data_out[frame]); //TODO make this do somethng querylength--; frame++; } while (querylength>0); uart_xmit(crc_high); uart_xmit(crc_low);} int main(void) { OSCCONbits.SCS = 0x00;new_rx=0; init_io(); serial_init(); run_timer(); check_my_address(); y += address; write_uart("MODBus "); uart_xmit(y); LATCbits.LATC0 = 1; LATCbits.LATC1 = 1; __delay_10ms(50); LATCbits.LATC0 = 0; LATCbits.LATC1 = 0; if (address == 0x01){ LATCbits.LATC0 = 1; } if (address == 0x02) { LATCbits.LATC1 = 1; } __delay_10ms(50); LATCbits.LATC0 = 0; LATCbits.LATC1 = 0; INTCONbits.GIE = 1; INTCONbits.PEIE = 1; while (1) { //If read data, not me and then if no 232 traffic for >800us then any xmit over //104 us (9600) x 8 quiet periods minus some for wiggle. if (ping) { run_timer(); } if (new_rx) { //this device was polled //testing response i = check_master_crc(); //check master query crc if (i==0x00) { //CRC Ok? if (ctxt[0] == 2) { //coil status query respond_input_status(); } //TODO other types of functions go here } //all done, get ready for new query new_rx = 0; } //testcrc[0] = 0xFF; //testcrc[1] = 0x02; //right now this is only thing processed //result = modbus_crc(testcrc,0,2); //crc_high = result >> 8; //crc_low = result & 0x00FF; __delay_10ms(10); LATCbits.LATC2 = 1; //LED blinky test __delay_10ms(10); LATCbits.LATC0 = 0; LATCbits.LATC1 = 0; LATCbits.LATC2 = 0; }}