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Author Topic: Help Troubleshooting A500 Multiplexer vs Mice  (Read 596 times)


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Offline WaveyTopic starter

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Help Troubleshooting A500 Multiplexer vs Mice
« on: July 02, 2023, 01:18:37 PM »
Hi All,
I have taken my A500 out of storage and notice the mouse does not behave correctly. It cannot move down or right reliably. Up & Left is ok.
Same behavior for my tank mouse and a PS2 mouse through adaptor.
I have checked the voltage changes at the multiplexer U15. When a Joystick is connected the 4 relevant pins drop from 5v to 0v when the stick is moved in that direction. All good.

When I connected a mouse however it is different.
Moving the mouse up and left causes the voltage to drop relivably from 5v to approx 0v.
But moving down or right only causes voltage to drop to about 2.5v.
Resistor pack RP401 is correct.
Resistance from Multiplexer pins to the actual pins in the joystick port are ok.

Is there another step I can take to check the output of the multiplexer?

Thanks, Wavey
Im not sure now whether to blame mice or multiplexer.
« Last Edit: July 02, 2023, 01:24:35 PM by Wavey »

Offline Castellen

Re: Help Troubleshooting A500 Multiplexer vs Mice
« Reply #1 on: July 03, 2023, 08:42:33 PM »
How are you measuring the mouse input at U15?  The mouse outputs quadrature wavefoms, which is essentially a pair of square waves for each direction, where the frequency of pulses relates to the speed of mouse movement, and which half leads/lags the other relates to the mouse direction.

In other words, if moving the mouse diagonally, there should be square waves on all of the four movement lines.  The duty cycle of the waveform is approximately 50%, so using a DC multimeter, you'll see an average of about 2.5V while it's moving.  Each of the four inputs will randomly stay 'stuck' at 5V or 0V when the movement stops.  You ideally need an oscilloscope to see what the signals are doing.

The four movement inputs for the mouse are the same as for the joystick.  So if the four inputs work correctly with the joystick, then the computer is probably OK.  Use Keir Fraser's excellent Amiga Test Kit to easily see how the computer is reading the joystick/mouse inputs: