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[General] Logic HIGH on digital input?

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Junior Member level 2
Apr 10, 2015
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Hi all.

At the moment I'm working on a simple LED indicator (up/down) for a motor controller. It has to indicate if the motor is going in one direction or the other. On the controller there is a control port (D-SUB9) and a switch is connected through a three wire cable to this control port to connect one common wire C to either wire A (up) or B (down). Unfortunately i don't have any more data about the controller.

I suspected that the common wire would be a logic high signal that gets connected through the switch to either INPUT A (wire A) or B (Wire B). I disconnected the cable and switch and measured directly at the control (D-SUB9) connector on the motor controller. The pin connected to wire C measured high (4.7 V). However the other pins connected to wire A and B also measured high (4.7 V). How is this possible when there should be in- and outputs on this control port? I was suspecting the input pins to measure low. My idea was to detect when input A or B is high and then indicate this with two LEDs. If all the voltage are logic high then this isn't possible.

Maybe I overlooked something. Any help is appreciated.


I was suspecting the input pins to measure low.
It is very usual, that inactive inputs are high. But it is a weak high. A switch can pull it down to indicate it is activated. This is called low-active signalling.

If all the voltage are logic high then this isn't possible.
Why not? A simple bjt or FET invert signal logic...

The description is not clear. A drawing could help.


I made a drawing in Paint. You are right that the input can be a weak high, due to a pull-up resistor. But if it is low-active signalling then i would suspect that if input A and B measures high (4.7 V) pin C should measure 0V (GND). So when a connection is made through the switch between either C-A or C-B then PIN C should be low to activate the input ports right? Otherwise the input levels on input A or B would stay high.

I'm not entirely sure which pins are input/outputs, but for low-active signalling at least one should measure low. Any help appreciated.



you say you measure 4.7V.
4.7V against what? I suspect against GND. So you should have GND... to pull down the inputs.

..if all three signals continously have the same logic (high) level then the whole circuit makes no sense....

I don´t know what A, B or C is. Maybe they are actively driven and C is only low in times when input state is polled. I don´t know. Maybe a scope will tell you.

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