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Posting-Version: version B 2.10.2 9/18/84; site peora.UUCP
Path: utzoo!watmath!clyde!cbosgd!ihnp4!houxm!hjuxa!petsd!peora!jer
From: jer@peora.UUCP (J. Eric Roskos)
Newsgroups: net.arch
Subject: Re: All the Chips that (Don't) Fit
Message-ID: <2034@peora.UUCP>
Date: Tue, 18-Mar-86 09:21:16 EST
Article-I.D.: peora.2034
Posted: Tue Mar 18 09:21:16 1986
Date-Received: Wed, 19-Mar-86 05:24:24 EST
References: <5100022@ccvaxa> <615@hoptoad.uucp>
Organization: Concurrent Computer Corporation, Orlando, Fl
Lines: 36

> >  I find it hard to understand why they would consider the UART setting of
> >  8 data bits _and_ 1 parity bit useless (in the Z8530). Not everybody uses
> >  7 bit ASCII, I don't care what's in the eigth bit so I'm gonna mask it
> >  off.  Not even all UNIX systems. (What I would give for a true meta key
> >  in my EMACS).
>
> The problem was that the chip is configurable to: 1 start bit, 8 data
> bits, 1 parity bit (whose value is ignored) and 1 stop bit.  Nobody, I
> mean nobody, sends data that way; they send 1 start, 8 data, 1 stop.
> (One of the data bits might be a parity bit, if desired.)

I think what the original poster (the one with the "> >") meant was that
there *are* cases in which you'd want to send 1 start bit, 8 data bits,
1 parity bit, and 1 stop bit -- except that if the parity bit's value
is ignored this makes it useless.

Nowadays people don't use parity bits very often for communication with
terminals, because terminals tend to give a sort of visual data integrity
check because they're usually run in full-duplex mode, echoing back what
the user typed, so the user can see whether what was typed was what got
there. (In fact, with Unix you often can't even use parity checking on a
terminal if you want to, due to applications that set 8-bit mode in order
to get the wakeup-on-every-character feature, and then send their data out
without parity bits.  Of course with System V you don't have that problem
since all those parameters are separate.)

So the parity bits are more likely to be useful for data transfers over
serial lines.  But data transfers over serial lines are usually done in
8-bit mode.  So 8 data bits + 1 parity bit would be useful, as long as
the parity bit was checked.

Of course, that sort of data transfer is usually checked via a block check
character (checksum, CRC, etc).  But if you're going to have a parity bit
at all, you might as well provide it for 8 data bits too.
-- 
E. Roskos