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From: 4526P@NAVPGS.BITNET (LT Scott A. Norton, USN)
Newsgroups: comp.sys.amiga
Subject: Re: Re: PARITY: odd even mark space none
Message-ID: <0284526P@NAVPGS>
Date: Tue, 28-Jul-87 13:53:00 EDT
Article-I.D.: NAVPGS.0284526P
Posted: Tue Jul 28 13:53:00 1987
Date-Received: Thu, 30-Jul-87 04:00:34 EDT
Organization: Naval Postgraduate School
Lines: 48

In <638@neoucom.UUCP>, wtm@neoucom.UUCP (Bill Mayhew) writes,
> NONE = no parity bits added
> ODD  = data bit appended so tthat check sum of bits = 1
> EVEN = data bit appended so that check sum of bits = 0
> MARK = data bit = 0 is appended
> SPACE = data bit = 1 is appended
> Note that for most practical purposes, 8N parity is the equivalent
> of 7S or 8S, since the the appended bit is at the same logic level
> as the idle state of the output.
Robot sez, "DANGER ROBINSON FAMILY!!"  DBW's VT100 uses the
parity setting to influence whether Kermit transfers will
use 8 bits or just seven.  I found if I set parity to "None",
the IBM mainframe, which sends even parity, wouldn't send
Kermit packets to my Amiga, since it's Ctrl-A begin-packet
characters with parity were hex 81. With "None" parity, VT100
won't strip the parity bit.
>All this parity
>nonsense has often resulted in hours of frustration in getting two
>devices to talk to each other, especially when the manufacturers
>notions of wheter or not the partiy bit is included in the "word
>length" differ!
>--Bill
I think the problem exists because there are two ends to a duplex
serial line.  As a transmitter, you can do five things with the
parity bit: send odd parity, even parity, mark, space, or use the
parity bit as a data bit and send 8 bit characters.  But as a
receiver, you must know what the transmitter is doing, and then
interpret the parity bit.
     
When I see a BBS advertise 8/N/1, I read the N = No parity to mean
that the board will ignore parity bits it receives, and will not be
sending parity information.  It will probably send spaces for
parity, but I will ignore it.
     
For Kermit 8-bit and XMODEM protocols, you don't want the serial
port to mess with the parity, but you want to give it an 8-bit
character and have it sent as 8 bits.
     
So, to avoid ambiguity, the transmitter needs to specify if the
parity bit is being used for error checking ( even or odd ), as an
extra stop bit ( mark or space ), or as data.  Then, the receiver
can take the appropriate action, detecting errors, ignoring marks or
spaces, or using all 8 bits as data.
     
LT Scott A. Norton, USN
     


Naval Postgraduate School
Monterey, CA 93943-5018
4526P@NavPGS.BITNET