Debugging The UE1 Paper Tape Reader And Amplification Circuit

December 9, 2024 by Maya Posch 2 Comments

The tape reader and amplifiers mounted with the other UE1 modules. (Credit: David Lovett, YouTube)

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’.

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Completing The UE1’s Paper Tape Reader And First Squiggles

November 17, 2024 by Maya Posch 1 Comment

The UE1 tape reader in its nearly finished glory. Note the resistor to regulate the motor speed. (Credit: David Lovett, Usage Electric) — The UE1 tape reader in its nearly finished glory. Note the resistor to regulate the motor speed. (Credit: David Lovett, Usagi Electric)

On today’s installment of UE1 vacuum tube computer construction, we join [David Lovett] once more on the Usagi Electric farm, as he determines just how much work remains before the project can be called done. When we last left off, the paper tape reader had been motorized, with the paper tape being pulled through smoothly in front of the photodiodes. This left [David] with the task to create a PCB to wire up these photodiodes, put an amplification circuit together (with tubes, of course) to amplify the signal from said photodiodes, and add some lighting (two 1-watt incandescents) to shine through the paper tape holes. All of this is now in place, but does it work?

The answer here is a definite kinda, as although there are definitely lovely squiggles on the oscilloscope, bit 0 turns out to be missing in action. This shouldn’t have come as a major surprise, as one of the problems that Bendix engineers dealt with back in the 1950s was effectively the same one: they, too, use the 9th hole on the 8-bit tape as a clock signal, but with this whole being much smaller than the other holes, this means not enough light passes through to activate the photodiode.

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A Teletype By Any Other Name: The Early E-mail And Wordprocessor

November 13, 2024 by Al Williams 40 Comments

Some brand names become the de facto name for the generic product. Xerox, for example. Or Velcro. Teletype was a trademark, but it has come to mean just about any teleprinter communicating with another teleprinter or a computer. The actual trademark belonged to The Teletype Corporation, part of Western Electric, which was, of course, part of AT&T. But there were many other companies that made teleprinters, some of which were very influential.

The teleprinter predates the computer by quite a bit. The original impetus for their development was to reduce the need for skilled telegraph operators. In addition, they found use as crude wordprocessors, although that term wouldn’t be used for quite some time.

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Building A Motor Feed For The UE1 Vacuum Tube Computer’s Paper Tape Reader

November 10, 2024 by Maya Posch 9 Comments

Building a paper tape reader by itself isn’t super complicated: you need a source of light, some photoreceptors behind the tape to register the presence of holes and some way to pull the tape through the reader at a reasonable rate. This latter part can get somewhat tricky, as Usagi Electric‘s [David Lovett] discovered while adding this feature to his vacuum tube-era DIY reader. This follows on what now seems like a fairly simple aspect of the photosensors and building a way to position said photosensors near the paper tape.

As the feed rate of the paper tape is tied to the reading speed, and in the case of [David]’s also contains the clock for the custom tube-based UE1 computer, it determines many of the requirements. With 8 bits per line, the tape forms the ROM for the system, all of which has to be executed and used immediately when read, as there is no RAM to load instructions into. This also necessitates the need to run the tape as an endless loop, to enable ‘jumping’ between parts of this paper-based ROM by simple masking off parts of the code until the desired address is reached.

For the motor a slot car motor plus speed-reduction gear was chosen, with a design to hold these then designed in FreeCAD. Courtesy of his brother’s hobby machine shop and a CAD professional’s help, producing these parts was very easy, followed by final assembly. Guides were added for the tape, not unlike with a cassette player, which allowed the tape to be pulled through smoothly. Next up is wiring up the photodiodes, after which theoretically the UE1 can roar into action directly running programs off paper tape.

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Usagi Electric’s Paper Tape Reader Is Ready To Hop With The Tube Computer

September 16, 2024 by Maya Posch 25 Comments

After previously working out a suitable approach to create a period-correct paper tape reader for his tube-based, MC14500B processor-inspired computer, [David Lovett] over at the Usagi Electric farm is back with a video on how he made a working tape reader.

The assembled paper tape reader as seen from the front with tape inserted. (Credit: David Lovett, Usage Electric, YouTube)

The tape reader’s purpose is to feed data into the tube-based computer, which for this computer system with its lack of storage memory means that the instructions are fed into the system directly, with the tape also providing the clock signal with a constant row of holes in the tape.

Starting the tape reader build, [David] opted to mill the structural part out of aluminum, which is where a lot of machining relearning takes place. Ultimately he got the parts machined to the paper design specs, with v-grooves for the photodiodes to fit into and a piece to clamp them down. On top of this is placed a part with holes that line up with the photodiodes.

Another alignment piece is added to hold the tape down on the reader while letting light through onto the tape via a slot. After a test assembly [David] was dismayed that due to tolerance issues he cracked two photodiodes within the v-groove clamp, which was a hard lesson with these expensive (and rare) photodiodes.

Although tolerances were somewhat off, [David] is confident that this aluminum machined reader will work once he has it mounted up. Feeding the tape is a problem that is still to be solved. [David] is looking for ideas and suggestions for a good approach within the limitations that he’s working with. At the video’s end, he mentions learning FreeCAD and 3D printing parts in the future. That would probably not be period-correct in this situation, but might be something he could get away with for some applications within the retrocomputing space.

We covered the first video and the thought process behind picking small (1.8 mm diameter) photodiodes as a period-correct tape hole sensor for a 1950s-era computing system, like the 1950s Bendix G-15 that [David] is currently restoring.

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Building A Paper Tape Reader To Read Bytes

August 19, 2024 by Maya Posch 19 Comments

Over at the Usagi Electric farm, [David Lovett]’s custom 1-bit, vacuum tube-based computer (UEVTC for short) has been coming along well the past years, matching and exceeding the Motorola MC14500B 1-bit industrial control unit (ICU) that it is heavily inspired by. What is still missing, however, is a faster way to get data into the computer than manually toggling switches. The obvious choice is to make a (punched) paper tape reader, but how does one go about this, and what options exist here? With a few historical examples as reference and the tape reader on the impressive 1950s Bendix G-15 which [David] happens to have lounging around, [David] takes us in a new video through the spiraling complexity of what at first glance seems like a simple engineering challenge.

Photodiodes in the tape reader of the Bendix G-15. (Credit: David Lovett, Usagi Electric)

Punched paper tape saw significant use alongside punched paper cards and magnetic tape, and despite their low bit density, if acid-free paper (or e.g. mylar) is used, rolls of paper tape should remain readable for many decades. So how to read these perforations in the paper? This can be done mechanically, or optically, with in both case the feedrate an important consideration.

Right off the bat the idea of a mechanical reader was tossed out due to tape wear, with [David] digging into his stack of photodetector tubes. After looking at a few rather clunky approaches involving such tubes, the photodiodes in the Bendix G-15’s tape reader were instead used as inspiration for a design. These are 1.8 mm diameter photodiodes, which aren’t super common, but have the nice property that they align exactly with the holes in the paper tape.

This left building a proof-of-concept on a breadboard with some incandescent bulbs and one of the photodiode to demonstrate that a valid logic signal could be produced. This turned out to be the case, clearing the construction of the actual tape reader, which will feature in upcoming videos.

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Homebrew Reader Brings Paper Tape Programs Back To Life

June 15, 2024 by Dan Maloney 11 Comments

We may be a bit biased, but the storage media of yesteryear has so much more personality than that of today. Yes, it’s a blessing to have terabyte SD cards smaller than your pinky nail and be able to access its data with mind-boggling speed. But there’s a certain charm to a mass storage device that can potentially slice off your finger.

We’re overstating the dangers of the venerable paper tape reader, of course, a mass storage device that [David Hansel] recreated a few years back but we only just became aware of. That seems a bit strange since we’ve featured his Arduino-based Altair 8800 simulator, which is what this tape reader is connected to. Mechanically, the reader is pretty simple — just a wooden frame to hold the LEGO Technic wheels used as tape reels, and some rollers to guide the tape through a read head. That bit is custom-made and uses a pair of PCBs, one for LEDs and one for phototransistors. There are nine of each — eight data bits plus the index hole — and the boards are sandwiched together to guide the paper tape.

The main board has an ATmega328 which reads the parallel input from the read head and controls the tape motor. That part is important thanks to Altair Basic’s requirement for a 100- to 200-ms delay at the end of each typed line. The tape reader, which is just being used as sort of a keyboard wedge, can “type” a lot faster than that, so the motor speed is varied using PWM control as line length changes.

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