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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 5th Apr 2019, 10:28 am   #21
SiriusHardware
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Default Re: Mk14 programming

Quote:
Originally Posted by SiriusHardware View Post
I've never really looked into how much reverse voltage a LED can withstand.
Not much, as it turns out. The first 3mm red LED I Googled has a maximum reverse voltage of 4V. So Tony's suggestion is wise, as indeed his suggestions invariably are.
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Old 5th Apr 2019, 6:07 pm   #22
SiriusHardware
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Default Re: Mk14 programming

Herewith, the interface revised to incorporate Tony's modification.
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Old 6th Apr 2019, 1:24 pm   #23
julie_m
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Default Re: Mk14 programming

LEDs usually have a PIV between 4 and 6V. Then the reverse-biased diode begins to conduct as much current as is necessary to bring the voltage down to this level. If there is no series element limiting the current, it will go boom. But as long as the LED's ballast resistor is chosen large enough to limit the power dissipation in this state to a safe level (bearing in mind that the reverse voltage is between 2-3 times the forward voltage), it will not be harmed by being reverse-biased; and will light up again when forward-biased.
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Old 6th Apr 2019, 1:46 pm   #24
SiriusHardware
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Default Re: Mk14 programming

For many years at work we had a product which was programmed via a phototransistor which could see through a hole in the product casing - the actual data was just standard asynchronous serial data so to program it we used a 'light pen' which consisted of nothing more than an LED and a resistor installed in the body of a BIC ballpen and connected to the RS232 TX and ground of the programming PC.

That lasted for over ten years, and was still working when taken out of service.

There aren't many applications (when you think about it) where voltage of incorrect polarity is intentionally applied to LEDs - usually the power is either absent or of the correct polarity, so it had never occurred to me to wonder whether LEDs might be less tolerant of reverse voltage than, or handle it differently to, a standard diode.

So, I have learned something new over the past couple of days.
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Old 6th Apr 2019, 6:45 pm   #25
JohnBHanson
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Default Re: Mk14 programming

Just a quick note. The RS232 TX line can be used as a source of -12 Volts to feed into
the RX circuit if it is required that the RS232 RX has a full voltage swing.

Worth noting for the *very* purists.

Or could use the other control line if held inactive.

Probably not worth it if your interface accepts 0/5V levels but could be a get out of jail otherwise.
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Old 6th Apr 2019, 9:11 pm   #26
SiriusHardware
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Default Re: Mk14 programming

In the original (unpublished) version of the MK14-to-PC interface I did actually use, or tried to use, RTS set to -9V and the rail now shown going to COM port GND was taken to RTS instead.

But, whenever I forced DTR high and RTS low, I then found that the port did not seem to want to receive data even though I was getting a nice +/- swing on the RX input as expected.

Rather than bother finding out why, I left DTR / RTS at their default states and just moved that rail to the GND terminal.

It worked just as well with no -ve pulldown applied to the RS232 input, just as you had suggested.
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