making PT reader, was re:weird find: papertape "patch" puncher

From: Andy Holt <andyh_at_andyh-rayleigh.freeserve.co.uk>
Date: Sat Mar 6 01:04:41 2004

Tony wrote:
>
> There have been several designs for homebrew readers, the schematic of
> one is in the back of the S100 handbook. The unidentified chips in that
> schemaitc are obviously 555 timers, used as schmitt triggers (!). The
> only difficult thing to make is the photodiode array for the read head. I
> wonder if you could use plastic fibre-optic cable to lead the light from
> a row of holes to some normal-size photodetectors mounted on a PCB in a
> convenient arrangement. It would be easier than finding some
> photodetectors that you could mount on 0.1" centres.
>
One further catch if you are building a tape reader ... or doing a "radical
rebuild"* on a very old design: make sure that the power to the light source
is reasonably well smoothed ... an AC ripple here can lead to mysterious
inconsistencies when trying to read tapes :-)

* before Tony is inclined to accuse me of vandalism here, I'll mention that
this "rebuild" took place 20 more than years ago when I acquire a couple of
old (then!) ex-mainframe tape readers and converted most of the electronics
from discrete (which I didn't understand) to TTL (which I could get to work)
to interface to a home-brew 6800 system.

Andy
Received on Sat Mar 06 2004 - 01:04:41 GMT