Paper tape and 8th bit?

Dave Dunfield dave06a at dunfield.com
Sat Aug 26 17:00:54 CDT 2006


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
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
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).

I checked the tape and it's not a read-error the 9th bit is punched
on certain characters..

Anyone shed some light?

Dave

--
dave06a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Collector of vintage computing equipment:
                http://www.classiccmp.org/dunfield/index.html



More information about the cctalk mailing list