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From: plocher@uwspan.UUCP (John Plocher)
Newsgroups: comp.dcom.modems
Subject: Re: 8-bit byte can be sent w/o xform ?
Message-ID: <123@uwspan.UUCP>
Date: Tue, 13-Oct-87 16:25:09 EDT
Article-I.D.: uwspan.123
Posted: Tue Oct 13 16:25:09 1987
Date-Received: Thu, 15-Oct-87 06:31:32 EDT
References: <167100015@uiucdcsb> <8215@sri-unix.ARPA>
Reply-To: root@uwspan.UUCP (Admin)
Organization: UW-Madison Spanish Dept
Lines: 61
Summary: The secret is to beat the rocks together, guys!

+---- In <8215@sri-unix.ARPA> Alan Larson writes:
| +---- In <167100015@uiucdcsb> kkim@uiucdcsb.cs.uiuc.edu writes:
| |   as i understand, most 1200-bps modems send data in the unit of
| |seven data bits preceded and followed by some start and stop bits.
| |there is usually one parity bit which may or may not be used.
| +----
|You understand incorrectly.
+----

Talk about a helpful followup :-(

ALL modems send whatever bit pattern is sent to them by the computer.  If
the computer sends 6 bit data to the modem, the modem will send 6 bit data.
similarly, if the computer sends 8 bits, the modem does too.

(With pictures:
The usual way a computer talks to a modem is through a RS-232C serial port;
The bit stream looks like this:
		... <start bit> <data bits> <parity bit> <stop bits> ...
  Number:	... <   1     > < 6,7,or 8> <  0 or 1  > <1,1.5,or2> ...
  Value:        ... [   1     ] [variable ] [0,1, none ] [ 1's     ]

		   Usually the total number of bits is 10, so you get:

			Start	1	1	 1	 1
			Data	7	8	 7	 6
			Stop	1	1	 2	 1.5
			Parity	1	none	 none 	 1
			----------     ---	---	----
			       10      10	10	9.5 (almost 10 :-)

		71E = 1 0101010 1 1	(1 start, 7 data, even parity,& 1 stop)
		81N = 1 10101010  1	(1 start, 8 data, no parity,  & 1 stop)
		72N = 1 0101010   11    (1 start, 7 data, no parity,  & 2 stop)

 end of pictures)

The reason you see a 7 bit data path when using modems (which DO support
an 8 bit data path) is that the interface between the computer and the
modem is set to use a 7 bit data path.  In addition to that, some programs
which manipulate data assume that only 7 bits of the 8 are valid.  Programs
like /bin/mail (and others) don't really like it when they have 8 bit
characters shoved at them :-)  On the other hand, uucico, zm, kermit, umodem,
and a host of other file transfer methods are perfectly happy with all 8 bits!

If the serial port which is connected to the modem is set to 81N (8 data bits,
1 stop bit, and no parity) with a command like

    stty 1200 -parenb cs8 -cstopb < /dev/ttyd1

then the modem will be able to transmit 8 bit data.  (Be sure that the computer
at the other end of the line also is set up for 8 bit data, too! :-)

If you are using a PC with Procomm or Qmodem or... there is a menu (with a
name like "dial" or "configuration"...) which lets you set these things for
each system you call.  Note that you MUST have an 8 bit data path if you
are using XMODEM, YMODEM, ZMODEM, SEALINK, TELINK or other similar file
transfer methods.  Only Kermit (and Blast) will work with a 7 bit data path.

Hope this helps...

-John