Hi
If you programatically ‘set’ an output bit, the corresponding on-board LED goes on and the output voltage goes to zero! (not +5v as I expected). When you clear an output bit, the output voltage goes high. So the on-board leds always show the inverse of the output voltage. Is there something wrong with my board?
Thanks
Richard Horsman
The “outputs” are not really outputs in the classic sense, in that they do not provide and output signal or voltage. They open-collector transistors with a base-emitter bias applied when the 8055 output bit is set. This causes the output terminal to be connected to ground through the saturated output transistor, pulling it low.
What you are reading when the output bit is not set is just a floating voltage because output transistor is no longer conducting.
I.e. it is a “switch” to ground connected to nothing at the 8055 “output” terminal.
Here’s [color=#0000BF]a good PDF explaining more about how they work and how to use them[/color].
goodmorning , i use to have the same there .
i put +5 volt on one pin of a relay and the other pin i put it on the board connector (exp) Q1 when you active the output to high(0 volt)the output goes 0 the relay is activate
[quote=“eftychios”]goodmorning , i use to have the same there .
i put +5 volt on one pin of a relay and the other pin i put it on the board connector (exp) Q1 when you active the output to high(0 volt)the output goes 0 the relay is activate[/quote]
Yes, that is the way open collector outputs work…