Paper tape and 8th bit?

[Dave Dunfield]

> It's an example of an early microsoft bug ;-)
> 
> In ALTAIR basic (and most of the MS basics) LIST converts the tokenized 
> line back to ascii,using the token as an index into the keyword table. The 
> keywords are stored with the MSB of the last character set. When they list 
> the keyword, they fail to strip off the 8th bit.  The end of every keyword 
> lists with the MSB set.

Ah... - thanks. This makes perfect sense. The tape does not contain an
input stream such as you would "type in" via an ASR-33, nor does it contain
an internal representation of the BASIC program that you could load directly
into memory - what it contains is a listing of the program as produced by
the LIST command. That explains the high bit on the end of tokenized
words, and also explains the null characters following newlines - these are
TTY pads. Thinking about it, this makes further sense, as a likely way that
these tapes were created is to LIST the program to an ASR-33 punch.


I've gotten the OP-80A to work quite reliably - I have it mounted in a little
bracket that hangs it off the edge of my desk so that I can unfold/refold
the paper-tape in piles on the floor as I draw it through.

The OP-80A instructions tell you to use a 40w or 60w bulb as the light
source and to lower it until you get the right light level to read the holes
in the tape.

I'm using a "luxo lamp", and find that even a 40w bulb is too bright - I can't
move the lamp high enough to keep it from seeing through the tape, so I
am using a variac which gives me very good control over the intensity.
With this setup I can adjust it to read very reliably and consistantly.

I've written utilities to read tapes into binary files via the OP-80A
connected to the parallel port, as well as to compare two files (ignoring
the leader/trailer lenghts) and report on the exact differences (bytes
missing) between them - I almost always get identical reads, and the
data I am getting looks right.


Regards,
Dave

--
dave06a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Collector of vintage computing equipment:
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