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Old 17 February 2021, 18:54   #1

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Thread to mod any Amiga for USB/PS2 mice?

I have recently prepared two Rev. 6A A500s as reference machines to test hardware and software on. They're unmodded and sporting matching-yellowed tank mice, however I'd like all the Amigas I fix up to support USB/PS2 mice (via adapters I have), so time to mod.

Zetr0 measured PC current limit to PS2 @ 275mA, so a mouse (or perhaps a PC joystick via adapter) would need that much plus what the serial converter draws (let's say 50mA)? So 325mA (2W resistor to still act as a cheap engineer's fuse(TM)).

However, Rev6 schematics only show a 4.7Ohm resistor to pin 7. This (alone) would limit power draw to 1060mA, not 50mA as legend has it.

Also, if power can be overdrawn there must also be a diode in series (small as possible voltage drop desired).

Presumably, a 1010mA current limiter resistor in parallel with the 4.7Ohm one, and a diode in series is provided in the power supply circuit. Where is it shown, so that the correct resistor value can be calculated?

This is all simple observations, so there must be more to consider. It doesn't stand to reason that anyone would use a 2W resistor as a fuse, yet going by just the schematic up to 2W must be provided to the PC mouse. Could the experienced modders here shed more light?

And to simplify the move to modern mice, would it be a good idea with a list of mobo revisions and resistor numbers to identify and replace with what value?

Last edited by Photon; 18 February 2021 at 00:36.
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Old 21 February 2021, 04:38   #2
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At a fulll load, you would have 0 volts and thus no power, only corrent.

The maximun power draw would be at 2,5V and then you'd need a step-up regulator to get the 5V that a P/S2 or USB mouse would require.

You do need to account for the power capacity of the resistor which would likely be less than that.

On the other hand, although especially old PS/2 mice and keyboards might draw a lot of current, the semi modern that is more or less the newest that has a ball rather than being optical will likely draw very little current. Not sure what the modern optical will draw. At least the LEDs that produce the red light has improved significally since LEDs were first used in consumer electronics in the 1970's. There is an image sensor and a processor of sorts that interprets what it sees and that probably consume some current though.

If it's only for your own personal usage then it seems like a better idea to take the 5V from the floppy- or video port. Both those are fed without a series resistor (but there still is a limit, especially with the higher amp PSUs you at least in theory risk damaging the PCB if you overload the output).
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Old 22 February 2021, 19:29   #3

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I must have been unclear. All I asked really is how to calculate the replacement resistor value when there is no current limiting circuit shown. And then I looked for support that maybe we could create a list of revisions and resistor numbers to replace.
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Old 22 February 2021, 21:45   #4
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Replace resistor with some PTC fuse - at least it will be sane solution immune to fail.
Calculation is simple - U (in Volts)/I (Current in Amps) - lets assume that mouse should work with 4.5V supply still OK and it need 200mA then 0.5V (5V-4.5V)/0.2=2.5Ohm

PTC reset-able fuses for example https://www.digikey.com/en/products/...protection/150
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