415 lines
9.1 KiB
HTML
415 lines
9.1 KiB
HTML
<HTML
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><HEAD
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><TITLE
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>Getting started</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.63
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"><LINK
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REL="HOME"
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TITLE="Serial Programming HOWTO"
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HREF="index.html"><LINK
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REL="PREVIOUS"
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TITLE="Introduction"
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HREF="intro.html"><LINK
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REL="NEXT"
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TITLE="Program Examples"
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HREF="x115.html"></HEAD
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><DIV
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>Serial Programming HOWTO</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="intro.html"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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></TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="x115.html"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="AEN56"
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>2. Getting started</A
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></H1
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="AEN58"
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>2.1. Debugging</A
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></H2
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><P
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> The best way to debug your code is to set up another Linux box, and
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connect the two computers via a null-modem cable. Use miniterm
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(available from the LDP programmers guide
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(<TT
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CLASS="LITERAL"
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>ftp://sunsite.unc.edu/pub/Linux/docs/LDP/programmers-guide/lpg-0.4.tar.gz</TT
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>
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in the examples directory) to transmit characters to your Linux
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box. Miniterm can be compiled very easily and will transmit all
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keyboard input raw over the serial port. Only the define statement
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<TT
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CLASS="LITERAL"
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>#define MODEMDEVICE "/dev/ttyS0"</TT
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> has to be checked. Set it to
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<TT
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CLASS="LITERAL"
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>ttyS0</TT
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> for COM1, <TT
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CLASS="LITERAL"
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>ttyS1</TT
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> for COM2, etc.. It is
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essential for testing, that <EM
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>all</EM
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> characters are transmitted raw
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(without output processing) over the line. To test your connection,
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start miniterm on both computers and just type away. The characters
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input on one computer should appear on the other computer and vice
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versa. The input will not be echoed to the attached screen.
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</P
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><P
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> To make a null-modem cable you have to cross the TxD (transmit) and
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RxD (receive) lines. For a description of a cable see sect. 7 of the
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Serial-HOWTO.
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</P
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><P
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> It is also possible to perform this testing with only one computer, if
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you have two unused serial ports. You can then run two miniterms off
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two virtual consoles. If you free a serial port by disconnecting the
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mouse, remember to redirect <TT
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CLASS="LITERAL"
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>/dev/mouse</TT
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> if it exists. If you
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use a multiport serial card, be sure to configure it correctly. I had
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mine configured wrong and everything worked fine as long as I was
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testing only on my computer. When I connected to another computer, the port
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started loosing characters. Executing two programs on one computer
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just isn't fully asynchronous.
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</P
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="AEN69"
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>2.2. Port Settings</A
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></H2
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><P
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> The devices <TT
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CLASS="LITERAL"
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>/dev/ttyS*</TT
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> are intended to hook up terminals to
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your Linux box, and are configured for this use after startup. This
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has to be kept in mind when programming communication with a raw
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device. E.g. the ports are configured to echo characters sent from the
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device back to it, which normally has to be changed for data
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transmission.
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</P
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><P
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> All parameters can be easily configured from within a program. The
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configuration is stored in a structure <TT
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CLASS="LITERAL"
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>struct termios</TT
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>, which
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is defined in <TT
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CLASS="LITERAL"
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><asm/termbits.h></TT
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>:
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<TABLE
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BORDER="1"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="SCREEN"
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> #define NCCS 19
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struct termios {
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tcflag_t c_iflag; /* input mode flags */
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tcflag_t c_oflag; /* output mode flags */
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tcflag_t c_cflag; /* control mode flags */
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tcflag_t c_lflag; /* local mode flags */
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cc_t c_line; /* line discipline */
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cc_t c_cc[NCCS]; /* control characters */
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};
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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>
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</P
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><P
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> This file also includes all flag definitions. The input mode flags in
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<TT
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CLASS="LITERAL"
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>c_iflag</TT
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> handle all input processing, which means that the
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characters sent from the device can be processed before they are read
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with <TT
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CLASS="LITERAL"
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>read</TT
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>. Similarly <TT
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CLASS="LITERAL"
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>c_oflag</TT
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> handles the output
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processing. <TT
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CLASS="LITERAL"
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>c_cflag</TT
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> contains the settings for the port, as
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the baudrate, bits per character, stop bits, etc.. The local mode
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flags stored in <TT
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CLASS="LITERAL"
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>c_lflag</TT
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> determine if characters are echoed,
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signals are sent to your program, etc.. Finally the array
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<TT
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CLASS="LITERAL"
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>c_cc</TT
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> defines the control characters for end of file, stop,
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etc.. Default values for the control characters are defined in
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<TT
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CLASS="LITERAL"
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><asm/termios.h></TT
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>. The flags are described in the manual
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page <TT
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CLASS="LITERAL"
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>termios(3)</TT
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>. The structure <TT
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CLASS="LITERAL"
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>termios</TT
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> contains the
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<TT
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CLASS="LITERAL"
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>c_line</TT
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> (line discipline) element, which is not used in POSIX compliant systems.
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</P
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="AEN88"
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>2.3. Input Concepts for Serial Devices</A
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></H2
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><P
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> Here three different input concepts will be presented. The appropriate
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concept has to be chosen for the intended application. Whenever
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possible, do not loop reading single characters to get a complete
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string. When I did this, I lost characters, whereas a <TT
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CLASS="LITERAL"
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>read</TT
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> for
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the whole string did not show any errors.
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</P
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><DIV
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CLASS="SECT3"
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><H3
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CLASS="SECT3"
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><A
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NAME="AEN92"
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>2.3.1. Canonical Input Processing</A
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></H3
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><P
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> This is the normal processing mode for terminals, but can also be
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useful for communicating with other dl input is processed in
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units of lines, which means that a <TT
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CLASS="LITERAL"
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>read</TT
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> will only return a
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full line of input. A line is by default terminated by a <TT
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CLASS="LITERAL"
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>NL</TT
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>
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(ASCII <TT
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CLASS="LITERAL"
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>LF</TT
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>), an end of file, or an end of line character. A
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<TT
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CLASS="LITERAL"
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>CR</TT
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> (the DOS/Windows default end-of-line) will not terminate a
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line with the default settings.
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</P
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><P
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> Canonical input processing can also handle the erase, delete word, and
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reprint characters, translate <TT
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CLASS="LITERAL"
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>CR</TT
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> to <TT
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CLASS="LITERAL"
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>NL</TT
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>, etc..
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</P
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></DIV
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><DIV
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CLASS="SECT3"
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><H3
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CLASS="SECT3"
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><A
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NAME="AEN102"
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>2.3.2. Non-Canonical Input Processing</A
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></H3
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><P
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> Non-Canonical Input Processing will handle a fixed amount of
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characters per read, and allows for a character timer. This mode
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should be used if your application will always read a fixed number of
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characters, or if the connected device sends bursts of characters.
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</P
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></DIV
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><DIV
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CLASS="SECT3"
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><H3
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CLASS="SECT3"
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><A
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NAME="AEN105"
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>2.3.3. Asynchronous Input</A
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></H3
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><P
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> The two modes described above can be used in synchronous and asynchronous
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mode. Synchronous is the default, where a <TT
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CLASS="LITERAL"
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>read</TT
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> statement will
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block, until the read is satisfied. In asynchronous mode the
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<TT
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CLASS="LITERAL"
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>read</TT
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> statement will return immediatly and send a signal to the
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calling program upon completion. This signal can be received by a
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signal handler.
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</P
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></DIV
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><DIV
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CLASS="SECT3"
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><H3
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CLASS="SECT3"
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><A
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NAME="AEN110"
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>2.3.4. Waiting for Input from Multiple Sources</A
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></H3
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><P
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> This is not a different input mode, but might be useful, if you are
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handling multiple devices. In my application I was handling input over
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a TCP/IP socket and input over a serial connection from another computer
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quasi-simultaneously. The program example given below will wait for
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input from two different input sources. If input from one source
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becomes available, it will be processed, and the program will then
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wait for new input.
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</P
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><P
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> The approach presented below seems rather complex, but it is important
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to keep in mind that Linux is a multi-processing operating system. The
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<TT
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CLASS="LITERAL"
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>select</TT
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> system call will not load the CPU while waiting for
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input, whereas looping until input becomes available would slow down
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other processes executing at the same time.
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</P
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></DIV
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></DIV
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
|
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HREF="intro.html"
|
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>Prev</A
|
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></TD
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><TD
|
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
|
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><A
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HREF="index.html"
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>Home</A
|
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
|
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><A
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HREF="x115.html"
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>Next</A
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></TD
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></TR
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
|
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>Introduction</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
|
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VALIGN="top"
|
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> </TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
|
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>Program Examples</TD
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></TR
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> |