26 Aug
2006
26 Aug
'06
4:56 p.m.
Subject: Re: Paper tape and 8th bit?
From: ard at p850ug1.demon.co.uk (Tony Duell)
Date: Sat, 26 Aug 2006 23:40:58 +0100 (BST)
To: cctalk at classiccmp.org
The punch and reader "dumb" devices though some did parity local that
was more exception than general practice.
Hi Guys,
I'm attempting to recover some software from paper-tape.
Never actually had a PT reader in my "altair days", I fooled around
with magnetic tape systems (audio and 9-track), and later went to
the NorthStar disk system... Did a bit of paper-tape stuff with the
university systems, but I don't recall any real details.
Anyway - I'm using an OP-80A which is a very simple manual feed
reader - you position a light over the unit and pull the tape through
wire guides over an optical sensor and it provides parallel data.
This all appears to work OK.
The tape I've been testing with is a Processor Technology "BASIC
VDM DRIVER" - it contains driver software for the PT VDM-1 video
board.
My question - Does anyone know what they are using the 8th bit
for? - I get nicely readable ASCII BASIC source out of it, except
Well, AFAIK few punches [1] or readers ever did anything with the 8th bit
other than just transfer it to/from the system. So what it means depends
on the device that punched that tape.
[1] I say 'few', not none because the serial adapter board for the 4070
can be set up to make this a locally-generated parity bit. In other words
you can send it 7 bit characters (or 8 bit with the top bit ignored) and
it will punch them with the 8th level an even parity bit. All
link-selectable.
> that the 8th bit seems to be somewhat randomly set on certain
> characters. If I strip the 8th bit I get what appears to be legit BASIC
> code.
>
> I thought it might be parity, however this does not appear to be
In PT systems that may have been used as a "page" (VT or FF) marker.
I have the VDM-1 and the basic driver is ascii and the resulting program
load the binary driver.
That would have been my first guess too. Normally even
parity, since then
totally blank and all holes are valid characters (the latter can be used
to overpunch any other character, and was origianlly simply ignored by
the system).
Of course on a binary tape (true binary, not Intel-Hex or something),
it's just anothe bit.
It's an Ascii punch of the PT basic "LIST: command. The 8th hole may
well be random trash or a particular character in memory that had the
high bit set.
the case - the
codes 0A (00001010) and 0D (00001101) both
appear with the 8th bit clear - If the 8th bit were parity, one or
the other should have it set. Other characters always have it set,
for example 'T' (54) seems to always appears as (D4).
Doe any character exist in both forms (with the top bit both clear and
set)? If so, could it be something like a 'start of statement' marker or
'start of keyword' or soemthing like that?
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