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Originally Posted by GeraldSommariva
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Back on the Keyboard I did some more testing some of the Data lines from the keyboard encoder go from +5v to 0v and some go from +5v to -12v as you press the keys. i made a drawing of the keybaord.
sd-keyboard-circuit-diagram
Would it be usual to have pull down resistors from the encoder output to -12v ?
I moved the resistors so they pull down to 0v, now all outputs go from +5v to 0v as I would expect. This is strange as the keyboard was working with the resistors pulling down to -12v. |
What is even stranger is that any of the outputs only went down to 0V, if they all had pull-downs to -12V - I'd expect all to go down to -12V
I presume it was done that way as this keyboard wasn't actually designed for the Triton etc, and so maybe it did actually work OK for a while like this, as resistors would have limited the current.
If all outputs do go down close to 0V OK, with pull-downs to 0V, then maybe this controller IC is fairly-OK after all, which would be rather handy.
Although maybe some of it's output-circuitry is quite fully-working, but it maybe possible to work-around that
- If you feed into a 74HC744 buffer etc. 74HC-series logic-gate, then that will have < 1/3 x 5V = 1.7V Low-threshold and > 2/3 x 5V = 3.3V High-threshold, which will probably be OK, if the keyboard outputs really go all the way to +5V (TTL often only goes upto 3.5V, hence standard 74LS etc. usually has thresholds shifted down a bit and don't have as good noise-immunity of CMOS levels.