Linux Serial Programming for POSIX Operating Systems

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LinuxSerial Programmingfor POSIX Operating
Systems
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What Are Serial Communications ?

• Speed in bits-per-second (bps) . Represents
  the number of ones and zeroes that can be sent in
  one second.
• The modulation rate can be lower than the
  bits-per-second (bps) rate when we encode more
  than one bits in a single change of symbol. The
  number of changes per second in the signal is
  termed baud rate.
• Half Duplex It is necessary to control RTS,
  CTS, and possibly DCD Signals
• Flow Control
• None
• Hardware RS232 RTS CTS signals
• Software Special Characters start XON (or DC1)
  or stop (XOFF or DC3)
• Full Duplex No need to Control RTS and CTS
  Signals
• Asynchronous of Synchronous
• Communication via RS-232
• Communication via RS-485 (Differential Mode)

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About Serial Ports

• On Windows
• Com1, Com2, Com3
• On Linux
• /dev/ttyS0, /dev/ttyS1, /dev/ttyS2
• On Solaris/SunOS
• /dev/ttya, /dev/ttyb, /dev/ttyc
• On POSIX systems, a serial port is a file
  (/dev/ttyS0). Thus, the Open( ) and Close ( )
  functions have to be used to work with the serial
  port.

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Input Concepts for Serial Devices - 1

• The appropriate concept has to be chosen for the
  intended application. Whenever possible, do not
  loop reading single characters to get a complete
  string because there may get characters lost.
• Canonical Input Processing
• This is the normal processing mode for terminals.
  A read( ) will only return a full line of input.
  It means that the process (or thread) will not
  proceed until a full line is received !!!. A
  line is by default terminated by a NL (ASCII LF),
  and end of file, or and end of line character. A
  CR (DOS/Windows default end-of-line) will not
  terminate a line with the default settings.

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Input Concepts for Serial Devices - 2

• Non-Canonical Input Processing
• Non-Canonical Input Processing will handle a
  fixed amount of characters per read, and allows
  for a character timer (to specify RX timeout).
• When the serial port is programmed to read a
  number N of chars, it will not return N chars.
  It will return the number of chars already
  available in the input buffer !!!!!!!!!!. The
  user has to control it.
• Two parameters control the behavior of this mode
  
• c_cc VTIME sets the character timer
• c_cc VMIN sets the minimum number of characters
  to receive before satisfying the read.
• If MIN gt 0 and TIME0, MIN sets the number of
  chars to receive before the read is satisfied. As
  TIME is zero, the timer is not used.
• If MIN0 and TIME gt 0, TIME serves as a timeout
  value. The read will be satisfied if a single
  char is read, or TIME is exceeded.

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Input Concepts for Serial Devices - 3

• The two modes above can be used in synchronous
  and asynchronous modes.
• In synchronous mode the read () operation will
  block until the read is satisfied.
• In asynchronous mode the read () statement will
  return immediately and send a signal to the
  calling program upon completion. This signal can
  be received by a signal handler.
• There is also Raw input Raw input is
  unprocessed, that is, the receiver does not
  interpretate any character. To accomplish this,
  deselect the ICANON, ECHO, ECHOE and ISIG options
  when using raw input
• Options.c_flag (ICANON ECHO ECHOE ISIG)

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Configuring a Serial Port

• indclude global.h
• include ltsys/ioctl.hgt
• include ltsys/io.hgt
• include ltasm/system.hgt
• include ltsys/termios.hgt
• include ltsys/fcntl.hgt
• define BAUDRATE B9600
• define MODEMDEVICE /dev/ttyS2
• define
• FILE input
• FILE output
• Int fd, tty,
• struct termios oldtio, newtio
• Struct ..
• void configure_serial ()
• fd open (MODEMDEVICE, O_RDWR O_NOCTTY
  O_NONBLOCK)
• if (fd lt 0) perror (MODEMDEVICE) exit (-1)
• tcgetattr (fd, oldtio) // save current
  port settings //

O_RDWR Open for Read and Write Mode O_NOCTTY
tell Unix this prog is no controlling terminal
for that port O_NONBLOCK Non Blocking
Mode c_flag Control Options (control speed, no
data bits, parity, stop bits, etc) i_flag Input
Options (enable/disable parity check, ignore
break, etc) o_flag Output options ( Map lower
to upper case, map CR to NL, etc) l_flag line
options ( TCSANOW specifies all changes to
occur NOW without waiting for output data to
finish sending or input data finish
receiving TCIFLUSH Flush Input and Output
buffers and make the change c_cc char array
control char definitions as well as timeout
parameters
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RECEIVING A CHARACTER

• Void recebe_caracter (char ch, char tout)
• char carac
• int
• for (I0 IltN, I) bufI \0 // Limpa
  buffer de recepçao
• timeout FALSE // Timeout de caractere
• do
• res read (fd, buf, 1)
• if (res 0) timeout TRUE
• carac buf 0
• printf (carac lido 02X\r\n, carac)
• while (( buf0 ! EOT) (timeout FALSE)
• if (timeout TRUE)
• - - - - - - - - -
• - - - - - - - - -
• - - - - - - - - -
• - - - - - - - - -

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Sending a Message Function

• Void envia_mensagem (int tamanho) //
  FUNÇÃO DE ENVIAR
• int flags, nbytes, para1
• char ch
• long int I
• ioctl (fd, TIOCMGET, flags) // Get the
  current MODEM status bits
• for (I 0 I lt 1000000 I) // espera 10 ms
  antes de acionar RTS
• flags TIOCM_RTS // prepara RTS (Request to
  Send)
• if (( ioctl (fd, TIOCMSET, flags) lt 0)) perror
  ( IOCTL) // Seta RTS na serial
• while ((( inb (0x3ED) 0x60) ! 0x60)) //
  3EDttyS2 3FD ttyS0 2FD ttyS1
• for (I 0 I lt 50000 I) // espera 10ms
  antes de enviar dados
• nbytes write (fd, buf_tx, tamanho) //
  transmite msg
• while ((( inb(0x3ED) 0x40) ! 0x40)) //
  Espera transmitir - valores para COM2
• for (I 0 Ilt50 I) // espera 10ms antes de
  retirar RTS
• flags TIOCM_RTS // prepara variavel de
  RTS, resetando
• if ((ioctl (fd, TIOCMSET, flags) lt 0)) perror (
  IOCTL) // reseta RTS, na serial

Draw Picture with Half Duplex RS 485 or RS 422
Scheme
NOTE Under Linux the serial port is configured
using the ioctl( ) system call
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CLOSING THE SERIAL COMMUNICATION

• Void fecha_e_restabelece () // FUNÇÃO DE
  FECHAMENTO
• tcsetattr (fd, TCSANOW, oldtio) // restabelece
  configuração anterior
• close (fd) // fecha dispositivo da serial
• fclose (input)
• fclose (output)