Triggering an SR Latch with multiple reset lines

Question

Ok, so I'm relatively new to electrical engineering having worked on the software side of life in the past. I'm trying to work a simple SR Latch where only one output is enabled at once. To do this, I've tried resetting the whole latch with each button press (connected to the screw terminals in my diagram).

Unfortunately I've come across the issue of triggering the set lines at the same time. I've fixed this with diodes but now have the problem where I'm triggering both the reset line and the set line at the same time on whichever button I press. This obviously causes invalid states and completely unpredictable behaviour.

How can I reset the whole SR Latch whilst still being able to set one output high after? I thought about using capacitors on each set line but I'm thinking I'm probably missing something obvious with logic gates here.

Answer 1

What you want is called a "radio button circuit". Like the pushbutton tuning on old car radios, when you press any button, that button's signal is selected and all other signals are blocked. In your case each of the four buttons sets its own flipflop, while resetting all other flipflops.

If you agree with all of that, you can search for radio button circuits and schematics. You will see that a 4-button circuit cannot be done with only one logic package (not counting a microcontroller).

The basic approach is for each ff to have one set input from its button, and three reset inputs from the other three buttons through a 3-input OR gate. There are four, 3-input OR gates, one for each ff. Each flipflop's OR gate has inputs from the other three buttons

Three-input OR gates come three to a package, so that's two more packages. There are diode-OR tricks you can use to reduce the package count.

Note: I keep saying OR gate, but you might have to use NOR gates, depending on the logic polarity of your flipflops.