Embedded System Laboratory Serial Port

Embedded System LaboratorySerial Port

Suthit [email protected]

Contents

Basic of Serial Communications1

Configuring the Serial Port2

Advanced Serial Programming3

Appendix A, RS-232 Pinouts4

2 Embedded System Laboratory

Appendix B, ASCII Codes5

What are Serial Communications?

Computers transfer information (data) one or more bits at a time. Serial refers to the transfer of data one bit at a time. Serial communications include most network devices, keyboards, mice, MODEMs, and terminals.

When doing serial communications each word (i.e. byte or character) of data you send or receive is sent one bit at a time. Each bit is either on or off. The terms you'll hear sometimes are mark for the on state and space for the off state.

The speed of the serial data is most often expressed as bits-per-second ("bps") or baudot rate ("baud"). This just represents the number of ones and zeroes that can be sent in one second.


What is RS-232?

RS-232 is a standard electrical interface for serial communications defined by the Electronic Industries Association ("EIA"). RS-232 actually comes in 3 different flavors (A, B, and C) with each one defining a different voltage range for the on and off levels. The most commonly used variety is RS-232C, which defines a mark (on) bit as a voltage between -3V and -12V and a space (off) bit as a voltage between +3V and +12V. The RS-232C specification says these signals can go about 25 feet (8m) before they become unusable. You can usually send signals a bit farther than this as long as the baud is low enough.


http://www.eia.org/

What is RS-232? (Cont.)


Pin Description Pin Description Pin Description Pin Description Pin Description

1 Earth Ground 6DSR - Data Set Ready

11 Unassigned 16Secondary RXD

21Signal Quality Detect

2TXD - Transmitted Data

7GND - Logic Ground

12Secondary DCD

17 Receiver Clock 22 Ring Detect

3RXD - Received Data

8DCD - Data Carrier Detect

13Secondary CTS

18 Unassigned 23Data Rate Select

4RTS - Request To Send

9 Reserved 14Secondary TXD

19Secondary RTS

24 Transmit Clock

5CTS - Clear To Send

10 Reserved 15 Transmit Clock 20DTR - Data Terminal Ready

25 Unassigned

Signal Definitions


GND The common signal ground connection. All the other signals refer to this

reference. This must be connected at both connectors.

TXD The data signal from the PC.

RXD The data signal to the PC.

DCD A control signal to the PC. The modem is receiving a valid carrier signal from

the remote modem.

Signal Definitions (Cont.)


CTS A control signal to the PC. The remote host is ready to receive your data.

RTS A control signal from to the PC. The PC is ready to receive data.

DSR A control signal to the PC. The modem is ready to answer or originate a call –

as opposed to being in a test mode.

DTR A control signal from the PC. The PC uses this to enable the modem. The

modem is allowed to go on-line or answer a call.

Asynchronous Communications


Accessing Serial Ports


System Port 1 Port 2

IRIX® /dev/ttyf1 /dev/ttyf2

HP-UX /dev/tty1p0 /dev/tty2p0

Solaris®/SunOS® /dev/ttya /dev/ttyb

Linux® /dev/ttyS0 /dev/ttyS1

Digital UNIX® /dev/tty01 /dev/tty02 Routines:

Usage: The stty utility shall set or report on terminal I/O characteristics for the device

that is its standard input.

stty [ -a| -g]

http://www.opengroup.org/onlinepubs/009695399/utilities/stty.html

http://www.opengroup.org/onlinepubs/009695399/utilities/stty.html

Example Access Serial Port

Download: Virtual Serial Ports Emulator << Click Download: Serial Terminal << Click Install Virtual Serial Ports Emulator Install Serial Terminal


http://www.eterlogic.com/downloads/SetupVSPE.zip

http://sourceforge.net/projects/realterm/files/

Example Access Serial Port (Cont.)

Click Create new device… Select Device type: pair Click Next Select your virtual serial port 1 and 2 Click Finish



Open Vmware Click Edit virtual machine setting Add Serial Port Connection:

Use physical serial port

Select your virtual serial port



Practice: A list of Serial Port name

Practice: Change baud rate to 38400

Practice: Enable parity bit

Practice: Print only one line containing string $


root@slax:# dmesg | grep tty

root@slax:# stty –F /dev/ttyS0 38400

root@slax:# stty –F /dev/ttyS0 parenb

root@slax:# cat /dev/ttyS0 | grep $ -m 1

Contents







Opening and Closing terminal

Routines:

Usage: The open() function establishes the connection between a file and a file

descriptor. It creates an open file description that refers to a file and a file descriptor that refers to that open file description. The file descriptor is used by other I/O functions to refer to that file. The path argument points to a pathname naming the file.

The close() function will deallocate the file descriptor indicated by fildes. To deallocate means to make the file descriptor available for return by subsequent calls to open() or other functions that allocate file descriptors. All outstanding record locks owned by the process on the file associated with the file descriptor will be removed (that is, unlocked).


open (*path, oflag, ... )close (fildes)

http://www.opengroup.org/onlinepubs/7990989775/xsh/open.html

http://www.opengroup.org/onlinepubs/7990989775/xsh/open.html

http://www.opengroup.org/onlinepubs/7990989775/xsh/close.html

Getting and Setting terminal

Routines:

Usage: Most systems support the POSIX terminal (serial) interface for changing

parameters such as baud rate, character size, and so on. The first thing you need to do is include the file <termios.h>; this defines the terminal control structure as well as the POSIX control functions.

These get and set terminal attributes, respectively; you provide a pointer to a termios structure that contains all of the serial options available:


tcgetattr (fildes, *termios_p) tcsetattr (fildes, optional_actions, *termios_p); cfgetispeed (*termios_p) cfsetispeed (*termios_p, speed_t) cfgetospeed (*termios_p) cfsetospeed (*termios_p, speed_t)

http://www.opengroup.org/onlinepubs/7990989775/xsh/tcgetattr.html

http://www.opengroup.org/onlinepubs/7990989775/xsh/tcsetattr.html

http://www.opengroup.org/onlinepubs/7990989775/xsh/cfgetispeed.html

http://www.opengroup.org/onlinepubs/7990989775/xsh/cfsetispeed.html

http://www.opengroup.org/onlinepubs/7990989775/xsh/cfgetospeed.html

http://www.opengroup.org/onlinepubs/7990989775/xsh/cfsetospeed.html

Reading and Writing terminal

Routines:

Usage: The read() function attempts to read nbyte bytes from the file associated with

the open file descriptor, fildes, into the buffer pointed to by buf. If nbyte is 0, read() will return 0 and have no other results. The write() function attempts to write nbyte bytes from the buffer pointed to

by buf to the file associated with the open file descriptor, fildes. If nbyte is 0, write() will return 0 and have no other results if the file is a

regular file; otherwise, the results are unspecified.


read (fildes, *buf, nbyte) write (fildes, *buf, nbyte)

http://www.opengroup.org/onlinepubs/7990989775/xsh/read.html

http://www.opengroup.org/onlinepubs/7990989775/xsh/write.html

Example Configuring the Serial Port

Practice: vi serial.c


#include <unistd.h>#include <sys/stat.h>#include <fcntl.h>#include <stdlib.h>#include <string.h>#include <stdio.h>#include <errno.h> #include <termios.h>

int main() { int port; int count = 0; char buf[] = "\r\nPlease press any key... in 10 sec\r\n"; struct termios tty_attributes;


Practice: (Cont.)


port = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NONBLOCK); tcgetattr(port, &tty_attributes); tty_attributes.c_cflag = CS8 | CLOCAL | CREAD; tty_attributes.c_iflag = IGNPAR | IGNBRK; tty_attributes.c_oflag = 0;

cfsetospeed(&tty_attributes,B9600); // Set the baud rate cfsetispeed(&tty_attributes,B9600); tcflush(port, TCIOFLUSH); tcsetattr(port, TCSANOW, &tty_attributes); sleep(1);

write(port, buf, sizeof(buf)); sleep(10);


Practice: (Cont.)


while (read(port, &buf[count], 1) > 0) { count++; } buf[count++] = 0; printf("Your pressed key: %s\n", buf); write(port, buf, sizeof(char) * count);

close(port); exit(0);}

Contents







Advanced Serial Programming

Routines:

Usage: The ioctl() function performs a variety of control functions on STREAMS

devices. For non-STREAMS devices, the functions performed by this call are unspecified. The request argument and an optional third argument (with varying type) are passed to and interpreted by the appropriate part of the STREAM associated with fildes.

The fcntl() function provides for control over open files. The fildes argument is a file descriptor.


Ioctl (fildes, request, ...) fcntl (fildes, cmd, ...)

http://www.opengroup.org/onlinepubs/7990989775/xsh/ioctl.html

http://www.opengroup.org/onlinepubs/7990989775/xsh/fcntl.html

Advanced Serial Programming


Request Description POSIX Function

TCGETS Gets the current serial port settings. tcgetattr

TCSETS Sets the serial port settings immediately.

tcsetattr(fd, TCSANOW, &options)

TCSETSF Sets the serial port settings after flushing the input and output buffers.

tcsetattr(fd, TCSAFLUSH, &options)

TCSETSWSets the serial port settings after allowing the input and output buffers to drain/empty.

tcsetattr(fd, TCSADRAIN, &options)

TCSBRK Sends a break for the given time. tcsendbreak, tcdrain

TCXONC Controls software flow control. tcflow

TCFLSH Flushes the input and/or output queue. tcflush

TIOCMGET Returns the state of the "MODEM" bits. None

TIOCMSET Sets the state of the "MODEM" bits. None

FIONREAD Returns the number of bytes in the input buffer. None

Example Advance Serial Programming

Practice: vi interface.c


#include <termios.h>#include <stdio.h>#include <unistd.h>#include <fcntl.h>#include <sys/signal.h>#include <sys/types.h>

#define BAUDRATE B38400#define FALSE 0#define TRUE 1

volatile int STOP=FALSE;

void signal_handler_IO (int status); int wait_flag=TRUE; FILE *input;FILE *output;int status;


Practice: (Cont.)


main(){ char Param_strings[7][80]; char message[90];

int fd, tty, c, res, i, error; char In1, Key; struct termios oldtio, newtio; struct termios oldkey, newkey; struct sigaction saio; char buf[255];

input = fopen("/dev/tty", "r"); output = fopen("/dev/tty", "w");


Practice: (Cont.)


tty = open("/dev/tty", O_RDWR | O_NOCTTY | O_NONBLOCK); tcgetattr(tty,&oldkey); newkey.c_cflag = BAUDRATE | CRTSCTS | CS8 | CLOCAL | CREAD; newkey.c_iflag = IGNPAR; newkey.c_oflag = 0; newkey.c_lflag = 0; //ICANON; newkey.c_cc[VMIN] = 1; newkey.c_cc[VTIME] = 0; tcflush(tty, TCIFLUSH); tcsetattr(tty,TCSANOW,&newkey); fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NONBLOCK);


Practice: (Cont.)


//install the serial handler before making the device asynchronous saio.sa_handler = signal_handler_IO; sigemptyset(&saio.sa_mask); //saio.sa_mask = 0; saio.sa_flags = 0; saio.sa_restorer = NULL; sigaction(SIGIO,&saio,NULL);

// allow the process to receive SIGIO fcntl(fd, F_SETOWN, getpid()); // Make the file descriptor asynchronous (the manual page says only // O_APPEND and O_NONBLOCK, will work with F_SETFL...) fcntl(fd, F_SETFL, FASYNC);

tcgetattr(fd, &oldtio); // save current port settings // set new port settings for canonical input processing


Practice: (Cont.)


newtio.c_cflag = B9600 | CRTSCTS | CS8 | 0 | 0 | 0 | CLOCAL | CREAD; newtio.c_iflag = IGNPAR; newtio.c_oflag = 0; newtio.c_lflag = 0; //ICANON; newtio.c_cc[VMIN]=1; newtio.c_cc[VTIME]=0; tcflush(fd, TCIFLUSH); tcsetattr(fd,TCSANOW,&newtio);

while (STOP==FALSE) { status = fread(&Key,1,1,input); if (status==1) //if a key was hit {


Practice: (Cont.)


switch (Key) {

case 0x1b: /* Esc */ STOP=TRUE;break;default: fputc((int) Key,output); write(fd,&Key,1);break;

} } if (wait_flag==FALSE) { res = read(fd,buf,255);


Practice: (Cont.)


if (res>0) { printf("%s\n", buf); } wait_flag = TRUE; }

}

tcsetattr(fd,TCSANOW,&oldtio); tcsetattr(tty,TCSANOW,&oldkey); close(tty); close(fd);


Practice: (Cont.)


fclose(input); fclose(output);}

void signal_handler_IO (int status){ printf("received SIGIO signal.\r\n"); wait_flag = FALSE;}

Contents







RS-232 Pinouts RS-232 uses a 9-pin D-Sub connector

Appendix A, RS-232 Pinouts


Pin Description Pin Description

1 DCD - Data Carrier Detect 6 Data Set Ready

2 RXD - Received Data 7 RTS - Request To Send

3 TXD - Transmitted Data 8 CTS - Clear To Send

4 DTR - Data Terminal Ready 9 Ring Detect

5 GND - Logic Ground

RS-232 Pinouts RS-232 comes in three flavors (A, B, C) and uses a 25-pin D-Sub connector:




1 Earth Ground 14 Secondary TXD

2 TXD - Transmitted Data 15 Transmit Clock

3 RXD - Received Data 16 Secondary RXD

4 RTS - Request To Send 17 Receiver Clock

5 CTS - Clear To Send 18 Unassigned

6 DSR - Data Set Ready 19 Secondary RTS

7 GND - Logic Ground 20 DTR - Data Terminal Ready

8 DCD - Data Carrier Detect 21 Signal Quality Detect

9 Reserved 22 Ring Detect

10 Reserved 23 Data Rate Select

11 Unassigned 24 Transmit Clock

12 Secondary DCD 25 Unassigned

13 Secondary CTS

RS-422 Pinouts RS-422 also uses a 25-pin D-Sub connector, but with differential signals:




1 Earth Ground 14 TXD+

2 TXD- - Transmitted Data 15 Transmit Clock-

3 RXD- - Received Data 16 RXD+

4 RTS- - Request To Send 17 Receiver Clock-

5 CTS- - Clear To Send 18 Unassigned

6 DSR - Data Set Ready 19 RTS+

7 GND - Logic Ground 20 DTR- - Data Terminal Ready

8 DCD- - Data Carrier Detect 21 Signal Quality Detect

9 Reserved 22 Unassigned

10 Reserved 23 DTR+

11 Unassigned 24 Transmit Clock+

12 DCD+ 25 Receiver Clock+

13 CTS+

Contents







Appendix B, ASCII Codes


Most Significant Character

LeastSignificantCharacter

Hex 0 1 2 3 4 5 6 7

0 NUL DLE Space 0 @ P ` p

1 SOH DC1 ! 1 A Q a q

2 STX DC2 " 2 B R b r

3 ETX DC3 # 3 C S c s

4 EOT DC4 $ 4 D T d t

5 ENQ NAK % 5 E U e u

6 ACK SYN & 6 F V f v

7 Bell ETB ' 7 G W g w

8 BS CAN ( 8 H X h x

9 HT EM ) 9 I Y i y

A LF SUB * : J Z j z

B VT ESC + ; K [ k {

C FF FS , < L \ l |

D CR GS - = M ] m }

E SO RS . > N ^ n ~

F SI US / ? O _ o DEL

Appendix B, ASCII Codes (Cont.)


Embedded System Laboratory Serial Port

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