On Tue, Feb 28, 2017 at 5:12 PM, Tapley, Mark via cctalk <cctalk@classiccmp.org> wrote: > On Feb 28, 2017, at 9:55 AM, Jim Brain via cctalk <cctalk@classiccmp.org> > wrote: > >> Analog, which is my nemesis, curses me again.
I fail to see how anyone can be a good digital designer and not understand analogue electronics. >> >> I have a cute idea for a cassette port project for the Tandy line of >> computers >> (the ones with the cassette port). I have a Coco 3 on the bench, so I scoped >> the output line while doing 'csave "jim"'. The signal looks to be just >> under 1V >> PtP (0-1V on the scope), and rests at about .3V when not sending data. That sounds about right. IIRC the CoCo 3 uses a custom chip that contains a 6 bit DAC for this. Older machines just use a couple of TTL output port lines and a few resistors. The CoCo3 service manual suggests the output is 1V peak-to-peak >> >> I have tried 6 different ways to boost the signal to 5V digital, to no >> avail, and so >> I ask humbly if someone with analog knowledge might be able to assist. > > Warning, I am not an electrical engineer at all, only opinionated. > > What about: > > (analog out) -> A/D conversion -> tight software loop -> D/A conversion -> > (analog in) ? Oh for $deity's sake. Why does everyone now insist on using a million components when a dozen will do? I stick to classic computers to get away from this. In any case it won't help that much. You should have some analgue 'signal conditioning' circuitry before the ADC (to get the input signal into the right voltage range for the ADC), ditto after the DAC. Otherwise you are wasting the performance of said converters. To get back to the problem. What do you _actually_ want the output to become? An analogue signal or a digital one? An op-amp will boost the signal. Look at the LM324 (a very cheap and common part, I have chosen it because the input voltage range goes down to 0 so you can run it off a single 5V supply). You could use it as a *5 amplifier by adding 2 or 3 resistors. If you want a digital signal then look at a comparator chip like the LM339. This compares the voltages on the 2 inputs, the output changes state as they pass each other (if you see what I mean). So you cassout signal to one input, a stable voltage of, say 0.5V on the other. There are couple of gotchas with this chip. The first is that it has open collector outputs, so you need a pullup resistor. The second is that it tends to oscillate at switchover. You can add hysteresis with a couple of resistors to prevent this. Where do you get the 0.5V reference from? Well, if your 5V supply is stable you can use that, divided down with a couple of resistors. Say 9.1k and 1k in series across the supply, comparator input to the junction. Or perhaps 0.7V from a forward biased silicon diode (diode cathode to ground, anode to +5V via a 4k7 or so resistor, take the 0.7V from the diode anode). Do you have 'The Art of Electronics'? If not buy it. Yes, it's expensive But it explains all this and more. -tony
