On Tue, Sep 16, 2014 at 01:59:36PM -0500, drlegendre . wrote:
[...]
Now the schematics we have do show the fuse, I believe
it's indicated as F1.
But there's no rating on the diagram. Can anyone confirm that this is
actually a 4A fuse? Why on earth would they have such a high-rated fuse
protecting those lines?
I went and looked at the schematics of a few other Amiga models to see how the
mouse/joystick ports are implemented. For all but one of the models I checked,
pin 7 of both joystick/mouse ports are commoned and are connected to +5v via an
unmarked fuse. So that's not much help.
We hit the jackpot with the A500 though: each pin 7 is connected separately to
+5v via a 4R7 current-limiting resistor. Ohm's Law tells us that it limits the
short-circuit current to 1A, but since this would mean dumping 5W into what is
probably a 0.5W resistor, what you effectively have there is a fuse that makes
a nasty stench when it blows.
What sort of controller-port peripheral device could
ever need that much
current - and is a common DB-9 connector (or for that matter, the PC board
traces) even capable of that kind of current?
400mA maybe?
The 4R7 effectively limits it to 100mA per port, so I'd be tempted to go even
lower than that to ~200mA. Further, a resettable fuse is cheap[0] and will save
you having to wave the soldering iron the next time the port is shorted out.
[0] Farnell are listing suitable fuses for under 50p.