26 Aug
2006
26 Aug
'06
3:40 p.m.
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
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.
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?
-tony