27 Jun
2025
27 Jun
'25
10:27 a.m.
On 27/06/2025 12:00, Adrian Godwin via cctalk wrote:
Interesting that some readers didn't require the
sprocket hole. Whilst
I can appreciate that they didn't mechanically drive on it, I assumed
they'd use it as a clock signal to sample the data levels. Even that
could be avoided, but then a nul would be indistinguishable from the
space between punch positions.
Did they make nul an illegal character, or determine it using a
flywheel sync ? I appreciate there were out-of-band ASCII characters
such as EOT but weren't there binary format tapes too ?
I'm not sure they didn't - it was something I should have found out
fifty years ago. And the same readers coped with 5 and 8 hole tapes,
where the sprocket is in a different position. Where there were no holes
other than the sprocket they flew through. I don't think it was
necessary to read a NUL, but it might have been. And there may have been
different handling for the eight and five bit. I shall have to ask Mr Onion!
If anyone wants to theorise, the roller was controlled by a solenoid and
could move the tape forward one character at a time, or apparently
free-run. In other words it didn't seem to unload and load on each
character. At the time I just used it - I didn't think that much about it.