After recently putting together the paper tape reader for his custom tube-based UE1 computer, [David Lovett] did get squiggles on the outputs, but not quite the right ones. In the most recent video, these issues are addressed one by one, so that this part of the UE1 1-bit computer can be called ‘done’. Starting off the list of issues were the odd readings from the photodiodes, which turned out to be due to the diodes being misaligned and a dodgy solder joint. This allowed [David] to move on to building the (obviously 6AU6 tube-based) amplifier for the photodiode output signals.
Much like the Bendix G-15’s tape reader which served as inspiration, this also meant adding potentiometers to adjust the gain. For the clock signal on the tape, a clock recovery PCB was needed, which should provide the UE1 computer system with both the clocks and the input data.
Using the potentiometers on the amplification board, the output signals can be adjusted at will to give the cleanest possible signal to the rest of the system, which theoretically means that as soon as [David] adds the permanent wiring and a few utility boards to allow the code to manipulate the tape reader (e.g. halt) as well as manual inputs. The UE1 computer system is thus being pretty close to running off tape by itself for the first time and with it being ‘complete’.
I love how the clock signal handled in paper tape. instead of having some sort of delay circuit to make sure the data lines have settled, the paper tape simply has smaller holes for the clock signal. this guarantees that clock is always triggered after data, and is always low before data starts to change.
I only realized this after seeing the oscilloscope waveform. I thought the smaller holes for clock were just for… I don’t know, looking cool or something.
It’s a left over from the mechanical age.
The small holes were grabbed by a pin wheel which moved and positioned the tape. For the data bits, there were pins with about the same thickness as the small holes, and those pins fell into the holes by a mechanical mechanism, so they needed some clearance to allow for tolerances.