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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old Yesterday, 4:18 pm   #201
ajgriff
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Default Re: Commodore PET 2001

Some very useful thoughts and ideas. Further investigation is ongoing but I can confirm that Vcc is spot on at pin 8 of both op-amps.

Having had a closer look at the down at heel C64 type unit it's PCB layout doesn't correspond exactly to any of the versions in Commodore's 1984 service manual (see post#120) but the circuit does match the schematic for the #NP-090 board aside from the use of a different manufacturer's quad op-amp. This in turn is virtually identical to the PET datasette circuit except that the two dual op-amps have been replaced by the single quad op-amp. Images of the two schematics are attached for the purposes of comparison. Helpfully the first schematic shows expected waveforms at different points in the circuit.

All this does open up some possibilities for comparative testing including the option of board swapping. Unfortunately they are not directly compatible largely because of the different positioning of the read/write switch. The battle between me and the PET continues. I apologise on behalf of us both that we are such slow workers!

Alan
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Old Yesterday, 5:46 pm   #202
Mark1960
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Default Re: Commodore PET 2001

The data format is designed to remove the dependence on frequency response required to reproduce a square wave output.

The write circuit uses a push/pull driver to drive the head with plus or minus approx 0.5ma, recording two opposite flux polarities to the tape.

In read mode the transition between flux polarities will generate a positive or negative pulse from the head. If the transitions are close enough together it will look like a sine wave. The first op amp in the read circuit is a differential amplifier. This is ac coupled to the next stage to remove any dc bias due to input offset voltages in the first op amp and the second op amp includes a low pass filter in the negative feedback. The third op amp is additional filtering. The final op amp is configured as a comparator with positive feedback to generate hysteresis, so a positive pulse will switch the output high and a negative pulse will switch the output low. This recovers data similar to the data recorded though though it might have some slight lengthening or shortening of the mark/space ratio.

The data is coded in the delay between high to low transitions. The longer delay is logic 1 and a short delay is logic 0. Low to high transitions are ignored but obviously required to set up for the next high to low transition. This method of coding is tolerant of tape speed variation between record and playback as it only measures the delay since the last transition, unlike the more traditional cassette interface using a simple serial protocol which is dependent on timing since the last start bit.

One disadvantage is the length of a block of data on the tape will change depending on the ratio of 1s to 0s in the data, so its not a good scheme for recording blocks of data, and then going back to rewrite only selected blocks. Though this isn’t a problem for the intended purpose of recording programs.

I’m not sure why they didn’t include low to high transitions in the coding to double the data rate. Maybe this would have made the software more complicated.

Last edited by Mark1960; Yesterday at 5:48 pm. Reason: Added purpose of third op amp
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Old Yesterday, 9:15 pm   #203
ajgriff
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Default Re: Commodore PET 2001

Interesting background information and will help me to check the waveform shapes. That's if I ever manage to find any sensible looking signals of course.

Although I think the PET datasette's tape head is ok I thought I'd compare the out of circuit head resistances of the heads in the two machines. The PET's head reads more or less exactly 310Ω whereas the C64's is 195Ω. I appreciate that the operational impedance of the head coils would be a more relevant measurement but it does seem odd that the resistances are so far apart bearing in mind that they both feed identical input circuitry. Is this another red herring?

Alan
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