How to use the SERIAL ports

Python3 examples

Send a sequence of bytes

import serial
import time

ser = serial.Serial(
    port='/dev/ttyUSB0',
    baudrate=9600,
    timeout=1,
    parity=serial.PARITY_NONE,
    stopbits=serial.STOPBITS_ONE,
    bytesize=serial.EIGHTBITS
)

packet = [0x54,0x51,0x41,0x12,0x00,0x02,0xFF,0xFF]

# convert list to bytearray
byte_array = bytearray(packet)

while True:
        ser.write(byte_array)
        time.sleep(1)

ser.close()

Read any bytes received on serial line and print out on stdout in hex format:

import sys
import serial

ser = serial.Serial(
    port='/dev/ttyUSB0',
    baudrate=9600,
    timeout=1,
    parity=serial.PARITY_NONE,
    stopbits=serial.STOPBITS_ONE,
    bytesize=serial.EIGHTBITS
)

linecounter=0
bytecounter=0
bytesperline = 8
while True:
    bytesToRead = ser.inWaiting()

    if bytesToRead>0:
        value=ser.read(1)
        if (bytecounter % bytesperline) == 0:
            print("\n[%04X] - " % bytecounter,end='')
            sys.stdout.flush()
            linecounter=linecounter+1

        print("%02X " % ord(value),end='')
        sys.stdout.flush()
        bytecounter=bytecounter+1
        if bytecounter==0:
            print
            bytecounter=0

C example

This is a basic example on how to use a serial port in C.

Refer to these links to understand how it works.

• Termios
• GNU Low-level I/O doc

#include "stdio.h"     
#include "string.h"    
#include "unistd.h"    
#include "fcntl.h"     
#include "errno.h"     
#include "sys/types.h"
#include "sys/stat.h"
#include "stdlib.h"
#include "stdarg.h"
#include "termios.h"

int main(void) {
    char txBuffer[10];
    char rxBuffer[10];
    int fd;
    struct termios tty_attributes;

    if ((fd = open("/dev/ttyS1",O_RDWR|O_NOCTTY|O_NONBLOCK))<0) {
        fprintf (stderr,"Open error on %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    } else {
        tcgetattr(fd,&tty_attributes);

        // c_cflag
        // Enable receiver
        tty_attributes.c_cflag |= CREAD;        

        // 8 data bit
        tty_attributes.c_cflag |= CS8;          

        // c_iflag
        // Ignore framing errors and parity errors. 
        tty_attributes.c_iflag |= IGNPAR;       

        // c_lflag
        // DISABLE canonical mode.
        // Disables the special characters EOF, EOL, EOL2, 
        // ERASE, KILL, LNEXT, REPRINT, STATUS, and WERASE, and buffers by lines.

        // DISABLE this: Echo input characters.
        tty_attributes.c_lflag &= ~(ICANON);     

        tty_attributes.c_lflag &= ~(ECHO);      

        // DISABLE this: If ICANON is also set, the ERASE character erases the preceding input  
        // character, and WERASE erases the preceding word.
        tty_attributes.c_lflag &= ~(ECHOE);     

        // DISABLE this: When any of the characters INTR, QUIT, SUSP, or DSUSP are received, generate the corresponding signal. 
        tty_attributes.c_lflag &= ~(ISIG);      

        // Minimum number of characters for non-canonical read.
        tty_attributes.c_cc[VMIN]=1;            

        // Timeout in deciseconds for non-canonical read.
        tty_attributes.c_cc[VTIME]=0;           

        // Set the baud rate
        cfsetospeed(&tty_attributes,B9600);     
        cfsetispeed(&tty_attributes,B9600);

        tcsetattr(fd, TCSANOW, &tty_attributes);

        txBuffer[0]='A';
        write(fd,txBuffer,1);

        // Read a char
        if (read(fd,&rxBuffer,1)==1) {
            printf("%c",rxBuffer[0]);
            printf("\n");
        }   
    }   

    close(fd);  
    return EXIT_SUCCESS;
}

• For the RoadRunner SOM please read: Using up to 10 serial ports on RoadRunner SOM

Pinout of the serial ports

Configure the device tree

Acqua

usart0: serial@f001c000 {
    status = "okay";
};

usart1: serial@f0020000 {
    status = "okay";
};

usart2: serial@f8020000 {
    status = "okay";
};

usart3: serial@f8024000 {
    status = "okay";
};

uart1: serial@f8028000 {
    status = "okay";
};

uart0: serial@f0024000 {
    status = "okay";
};

Related links

• Atmel usart binding
• DMA bindings

Using the RS485 lines

• Read the original version from Linux documentation

• Kernel >=3.4
• Previous Kernel

usart3: serial@f8028000 {
    compatible = "atmel,at91sam9260-usart";
    reg = <0xf8028000 0x200>;
    interrupts = <8 4 5>;
    pinctrl-names = "default";
    pinctrl-0 = <&pinctrl_usart3 &pinctrl_usart3_rts &pinctrl_usart3_cts >;
    linux,rs485-enabled-at-boot-time;
    rs485-rts-delay = <0 20>;
    status = "okay";
};

import serial,fcntl, struct
 
ser = serial.Serial(
    port='/dev/ttyS2', 
    baudrate=9600, 
    timeout=1,
    parity=serial.PARITY_NONE,
    stopbits=serial.STOPBITS_ONE,
    bytesize=serial.EIGHTBITS
)  
 
fd=ser.fileno()
 
ser.write("0123456789") # Send some char on the RS485 bus
s = ser.read(1)         # Wait for a char
print s
ser.close()

import serial,fcntl, struct
 
ser = serial.Serial(
    port='/dev/ttyS2', 
    baudrate=9600, 
    timeout=1,
    parity=serial.PARITY_NONE,
    stopbits=serial.STOPBITS_ONE,
    bytesize=serial.EIGHTBITS
)  
 
fd=ser.fileno()
serial_rs485 = struct.pack('hhhhhhhh', 1, 0, 0, 0, 0, 0, 0, 0)
fcntl.ioctl(fd,0x542F,serial_rs485)
 
 
ser.write("A")      # Send a "A" char to the serial port
s = ser.read(1)         # Wait for a char
print s
ser.close()

#include "stdio.h"     
#include "string.h"    
#include "unistd.h"    
#include "fcntl.h"     
#include "errno.h"     
#include "sys/types.h"
#include "sys/stat.h"
#include "stdlib.h"
#include "stdarg.h"
#include "termios.h"
#include "linux/serial.h"  
 
#define TIOCSRS485 0x542F
 
int main(void) {
    char txBuffer[10];
    char rxBuffer[10];
    int fd;
    struct termios tty_attributes;
    struct serial_rs485 rs485conf;
 
    if ((fd = open("/dev/ttyS2",O_RDWR|O_NOCTTY|O_NONBLOCK))<0) {
        fprintf (stderr,"Open error on %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    } else {
        tcgetattr(fd,&tty_attributes);
 
        // c_cflag
        // Enable receiver
        tty_attributes.c_cflag |= CREAD;        
 
        // 8 data bit
        tty_attributes.c_cflag |= CS8;          
 
        // c_iflag
        // Ignore framing errors and parity errors. 
        tty_attributes.c_iflag |= IGNPAR;       
 
        // c_lflag
        // DISABLE canonical mode.
        // Disables the special characters EOF, EOL, EOL2, 
        // ERASE, KILL, LNEXT, REPRINT, STATUS, and WERASE, and buffers 
        // by lines.
 
        // DISABLE this: Echo input characters.
        tty_attributes.c_lflag &= ~(ICANON);     
 
        tty_attributes.c_lflag &= ~(ECHO);      
 
        // DISABLE this: If ICANON is also set, the ERASE character 
        // erases the preceding input   
        // character, and WERASE erases the preceding word.
        tty_attributes.c_lflag &= ~(ECHOE);     
 
        // DISABLE this: When any of the characters INTR, QUIT, SUSP, 
        // or DSUSP are received, generate the corresponding signal.    
        tty_attributes.c_lflag &= ~(ISIG);      
 
        // Minimum number of characters for non-canonical read.
        tty_attributes.c_cc[VMIN]=1;            
 
        // Timeout in deciseconds for non-canonical read.
        tty_attributes.c_cc[VTIME]=0;           
 
        // Set the baud rate
        cfsetospeed(&tty_attributes,B9600);     
        cfsetispeed(&tty_attributes,B9600);
 
        tcsetattr(fd, TCSANOW, &tty_attributes);
 
        // Set RS485 mode: 
        rs485conf.flags |= SER_RS485_ENABLED;
        if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {
            printf("ioctl error\n");
        }
 
        txBuffer[0]='A';
        write(fd,txBuffer,1);
 
        // Read a char
        if (read(fd,&rxBuffer,1)==1) {
            printf("%c",rxBuffer[0]);
            printf("\n");
        }   
    }   
 
    close(fd);  
    return EXIT_SUCCESS;
}

Related links

• Linux Kernel RS485 documentation
• Device Tree Binding for RS485

Sergio Tanzilli
Systems designer, webmaster of www.acmesystems.it and founder of Acme Systems srl

Personal email: tanzilli@acmesystems.it
Web pages: https://www.acmesystems.it --- https://www.acmestudio.it
Github repositories: https://github.com/tanzilli --- https://github.com/acmesystems
Telegram group dedicated to the Acme Systems boards: https://t.me/acmesystemssrl