Phototransistors as Input Devices, and Paper Tape.

[Martin_H]

Designing my first PCB was an unexpected amount of fun, so I'm designing another one based on the obsolete OP-80a. It was a paper tape reader used by Altair users in the 70's. I don't know if paper tape was used by 6502 or 6800 users, because the Apple I and SWTPC 6800 used ROM monitors and keyboards for input. So, forum users might not be familiar with it.

My first step was reading through the OP-80a's schematics and documentation available online, reproducing the schematic in KiCad, and then doing a board layout. That way I understood how it works. Essentially it uses phototransistors to sense the tape holes, nine 555 timers configured as Schmitt triggers, with pin 5 supplying the reference voltage via an LED and 220-ohm resistor voltage divider. Eight of the phototransistors are for data lines with the ninth using the sprocket hole as the clock pulse.

The schematics state that all the IC's and phototransistors are sensitivity matched, so do not substitute any of them. I've used phototransistors as line sensors and wheel encoders for robots, and they do vary in their sensitivity. In my robots I use a trim pot somewhere in the circuit to compensate, which I think this design should have used. The question is where? Should the pullup network be replaced by nine trim pots, or the reference voltage to the 555 timers?

There's also the nagging question about using a 555 timer as a Schmitt trigger. Shouldn't a real Schmitt trigger IC have been used instead?

I hear you asking where do you get a paper tape in the year 2025? That's super easy, barely an inconvenience.
https://github.com/colemanjw2/ASCII2Pap ... me-ov-file

Also, my work so far is on my Github: https://github.com/Martin-H1/KiCad/tree/main/OP-80A

[barnacle]

Meanwhile: https://unimplementedtrap.com/paper-tape-punch

I might have been tempted to look for opto-sensors with a TTL output, and avoid the whole Schmitt trigger thing; control the trigger voltage with an LED per channel perhaps. But your motivation may differ.

Usagi Electric did some development work on a vintage tape reader not too long ago, for his Boroughs computer. Have a search on Youtube to see his problems.

Neil

[BigDumbDinosaur]

barnacle wrote:

Meanwhile: https://unimplementedtrap.com/paper-tape-punch

When I was in the US Navy, I went to school to learn how to fix Tele-Types and the tape punches attached to them.  The punches weren’t real complicated as mechanisms go—nowhere near as complicated as the innards of the Tele-Type proper.  However, the punches were precision made, with thousandth-of-an-inch fits.  More often than not, the solenoids failed, not the mechanism itself.

BTW, the business ends of the punches were made from some sort of alloy tool steel, through-hardened and the ends hollow-ground to a razor edge.  They were equally adept at taking bites out of one’s fingers as they were in punching very clean holes in paper. 

[Martin_H]

I looked around on the Internet to see other people's ideas for a reader using modern technology. Often, they use a microcontroller and convert the parallel data into async serial via the MCU's UART. Once you get a microcontroller into the mix you can dispense with Schmitt triggers and do everything in software.

Anyway, here's one of those readers in action:
https://youtu.be/uZ2VWKNlPQU?si=ad0XDlE0Whm2S5fh

@BDD, your description makes the laser cutting option sound much safer.

[BigDumbDinosaur]

Martin_H wrote:

@BDD, your description makes the laser cutting option sound much safer.

Depends on how you define “safer.”    Lasers can be plenty dangerous if misused.

Back in the 1960s when I worked on that stuff, the laser was little more than a laboratory curiosity.  Heck, computers were still quite mystifying to the average bloke at the time, and the microprocessor didn’t even exist.  You young whippersnappers have no idea how good you have it! 

[Martin_H]

BigDumbDinosaur wrote:

You young whippersnappers have no idea how good you have it! 

It's been a while since I was called a whippersnapper, but I'll take it.

I've updated the schematic and board layout with the following changes:

Code: Select all

* After reviewing other schematics, the best course of action was adding trim pots to the phototransistors. I calculated the required resistance using the data sheet and came up with 25K. So, the resistor network is 20K and the trim resistors are 10K. That should allow fine tuning each data channel to equal sensitivity.
* Added a power jack to connect an IR illumination assembly above the phototransistors.
* Added four M2 mounting holes to allow connecting a paper tape guide above the phototransistors.

Changes were pushed to GitHub. I have enough parts in inventory to build a prototype, but I have enough on my plate, so I will back burner this for now. However, I like that no microcontroller was used in this design as I feel like that's overkill.

[Martin_H]

I had two crazy thoughts about this project this morning.

If you only used the sprocket and d0 channels, you would produce synchronous serial output. Your tape would be eight times longer, but the device would be much simpler.

However, if you spaced the sprocket and data channel sensors out in the direction of tape motion, the output of each channel is time delayed by the amount of spacing and tape speed. You then OR their output together and produce asynchronous serial. The sprocket channel is the start bit, and each data channel being sampled in sequence. The stop bit is the tricky part because there's no physical analog on the tape. But two sensors on the sprocket channel might work.

One constraint is that all bits must be sampled and transmitted before the next row arrives at the sprocket channel. Placing a narrow slot mask over the phototransistors should increase their selectivity. Given that most tapes have a row spacing of 0.1" this would require a precise construction to say the least.

Basically, you would be constructing a mechanical UART which is a crazy idea. My apologies to Gordon Bell.

[Agumander]

I tried to do something like this once, but it was early into my 3D printing and electronics "career" and I made a lot of mistakes and couldn't get it working.

Putting holes on the tape to control timing makes a lot more sense than what I tried, which was to put a magnet on a gear that would pass by a reed switch.

Maybe I should make a paper tape reader attachment for the GameTank.

[barnacle]

Funny, when I search for 1" paper tape I get surgical tape... think they're trying to tell me something?

Neil

[BigDumbDinosaur]

barnacle wrote:

Funny, when I search for 1" paper tape I get surgical tape... think they're trying to tell me something?

Neil

The operative word being “surgical.”    Not to worry!  The doctors will get you, sooner or later.  I’m living proof of that.

Agumander wrote:

Putting holes on the tape to control timing makes a lot more sense than what I tried, which was to put a magnet on a gear that would pass by a reed switch.

I wouldn’t expect that to be very consistent, since reed switches tend to not be very consistent about the threshold at which they trip.  That’s why the good Lord invented LEDs and phototransistors.