The application is an old keyboard from mid-1970's.
The setup is such that from the PSU, it is providing 9-pins of 4.8V to the keyboard, and 1-pin with 8.33V (that's underload, nominal is 5V/8.5V).
I have a working keyboard and used an oscope to monitor behavior. So I'm trying to understand how it is working...
For simplicity, say it just has 4-lines @ 5V and a P (parity) line also at 5V. Then has normal 8.5V, 5V and a ground pin.
When a key is pressed, suppose pins 2, 4, and the P-line (parity) get "push down" to 0.2V (essentially 0, but while under load 0.2V is what I measured).
Also, there is a KBD STROBE line with a similar behavior - only difference is, the "pushed down" voltage stays as long as the key is pressed (e.g. key A), but the probe pulses for about 60ms only once, and won't strobe again until the key pressed is released.
So, how does that work? The pressing of the key is setup to induce that much resistance to those lines? Doesn't that mean a lot of lines and a lot of resistors? Is there an easier way? And what modern device could replicate this, i.e. to dynamically apply resistance to a set of parallel voltage inputs?
Thanks!
The setup is such that from the PSU, it is providing 9-pins of 4.8V to the keyboard, and 1-pin with 8.33V (that's underload, nominal is 5V/8.5V).
I have a working keyboard and used an oscope to monitor behavior. So I'm trying to understand how it is working...
For simplicity, say it just has 4-lines @ 5V and a P (parity) line also at 5V. Then has normal 8.5V, 5V and a ground pin.
When a key is pressed, suppose pins 2, 4, and the P-line (parity) get "push down" to 0.2V (essentially 0, but while under load 0.2V is what I measured).
Also, there is a KBD STROBE line with a similar behavior - only difference is, the "pushed down" voltage stays as long as the key is pressed (e.g. key A), but the probe pulses for about 60ms only once, and won't strobe again until the key pressed is released.
So, how does that work? The pressing of the key is setup to induce that much resistance to those lines? Doesn't that mean a lot of lines and a lot of resistors? Is there an easier way? And what modern device could replicate this, i.e. to dynamically apply resistance to a set of parallel voltage inputs?
Thanks!