Xref: utzoo comp.os.msdos.programmer:471 alt.msdos.programmer:2029
Newsgroups: comp.os.msdos.programmer,alt.msdos.programmer
Path: utzoo!telly!robohack!druid!darcy
From: darcy@druid.uucp (D'Arcy J.M. Cain)
Subject: Re: Microsoft C & serial comms
Message-ID: <1990Aug12.235217.25638@druid.uucp>
Organization: D'Arcy Cain Consulting, West Hill, Ontario
References: <990@dcl-vitus.comp.lancs.ac.uk> <32673@cup.portal.com>
Date: Sun, 12 Aug 90 23:52:17 GMT

In article <32673@cup.portal.com> PLFaith@cup.portal.com (Patrick L Faith) writes:
>well "break" is used a couple of different ways, but they all come
>down to this, if the serial port does not transfer a character at
>the specified baud rate for the 8/7 bits of time it takes, then
>there is a break set in the serial driver(atleast if you are using u8250).
>So breaks are all to easy to send, just dont put anything out through
>the port for a given amount of time - 500 mSec is fine - so is 2 seconds.
>I like the book "C programmers guide to serial communications" of its 600+
>pages it devotes about 2 pgs to break, which isn't indexed either.
>
Bzzzzzzzzzzzzzzzzzzzzt!!!!

In what universe?  Do you mean that a caller to a BBS puts out a break between
each keystroke while connected?  Pay attention class.  There will be a pop
quiz at the end.

on a serial connection the normal state of the 8250 SOUT pin is a logic 1. 
When a character is output a frame is assembled consisting of  1 start bit,
7 or 8 data bits, an optional parity bit and 1 or 2 stop bits.  The start
bit is a 0, the stop bits are 1 and the data and parity follows the data
and 8250 parity generation setup.  So say you transmit 'A' at 7 bit odd
parity 1 stop then the frame looks like this:
             0 1 0 0 0 0 0 1 1
When we add the idle bits before and after the character then we have:
     1 1 1 1 1 1 1 1 1 1 1 0 1 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1
As you can see the first 0 bit (the start bit) signals the receiver that
a character is starting.  It then assembles the next 7 bits into a character
and one more bit for the parity.  If the next bit is not a 1 then a framing
error occurs.

To send a break, you set a logic 0 long enough to have all zeros for
the start, data, parity and stop bits.  This special case is recognized
by the receiver as a break condition.  It looks like this:
      1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1
As long as the number of zeros is more than the total of a normal frame.

To generate this on a PC you set bit 6 in the line control register to 1
for the desired length of time.  This register is at port 0x3fb for COM1
and 2fb for COM2.  Don't forget to read the current contents first.  Here
is a snippet:

    int lcr;
    ...
    lcr = inportb(0x3fb);   /* or 0x2fb for COM2 */
    outportb(0x3fb, lcr | 0x40);
    delay(); /* for sufficient time */
    outportb(3fb, lcr);   /* restore it */

-- 
D'Arcy J.M. Cain (darcy@druid)     |
D'Arcy Cain Consulting             |   MS-DOS:  The Andrew Dice Clay
West Hill, Ontario, Canada         |   of operating systems.
+ 416 281 6094                     |