[Simh] PDP paper tape reader end of file conditions

[Jörg Hoppe]

[...]
 > DEC didn't make much investment in error detection for its paper tape
 > equipment. In the 18b family, the PDP-9 was the first 18-bit system to
 > feature a "reader empty" flag. The PDP-8 never had one. (The PDP-11 > 
had one from the get go.)
 >
 > That begs the question of what these early PDP's did when they ran out
 > of tape. The reader logic was very simple; when the reader saw a 0->1
 > (dark to light) transition on the feed hole, it strobed a character. 
 > So there might be a last garbage character when the tape ran out, but 
 > after that, the feed hole always saw light, and there would be no
 > further transitions.
Additional (and neither relevant, nor asked-for) detail from repair 
work: On the PC05 reader the phototransitor array contained 10 channels: 
8x data, 1x feed, 1x "tape present". But only 9 of these were amplified 
and evaluated. On some early PC11/15s data was not strobed by feed hole, 
but by stepper motor pulse, and on these the physical "tape present" 
signal was connected instead of "feed". This should give a better "end 
of tape" detection, but at the cost of data reliability.
Why did DEC never used all 10 optical channels?

http://www.retrocmp.com/stories/dec-pc05-papertape/240-dec-pc05-model-evolution

Joerg