I'm currently having a strange issue with what I think is a 'floating' signal.
The setup: I have a bank of inputs (which are connected to a resistor and LED acting as a pull-down) connected to inputs and outputs of a D-type flip flop IC (SN74ALS374AN). The fact I'm using all 8 shouldn't matter as the issue occurs on any one in isolation. They are connected to input and output as I will only have enable or clock on at any given time, and never both (for a bidirectional bus). The datasheet of the chip says it is suitable for this.
The problem: See the image attached. When I disconnect wire A from the flip flop IC (connection B) and read the voltage between A and ground, I get 0v or 5v depending on whether the switch is on. However when I connect A to B (the goal being to feed this logic 1 or 0 signal to the IC), suddenly the voltage reads as 1.5v (which I believe to be a floating value).
Can anyone explain this behavior? Am I doing something wrong?
A few things to note: - It doesn't matter if I disconnect Q and D on the IC - If I connect to Q only, then the logic low is retained, so the problem is really just the D (or input) of the flip flop - The datasheet for the IC doesn't seem to shed any light on this
Any suggestions would be very much appreciated. If I haven't explained anything properly or you need me to test anything else then please let me know.
Many thanks, Matt