Paper tape and 8th bit?

[Allison]

>
>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
>
>> 
>> 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.

The punch and reader "dumb" devices though some did parity local that
was more exception than general practice.

>
>> 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?

Easier, what is the punch pattern on the tape that corrosponds to the 8th 
bit?

My tape is deeply burried since I stopped using back in '76.  I know the 
box it's in far enough down the stack to not warrent looking.


Allison