Designing a PS_ON and PS_OFF circuit for Power Supply

[Cyo]

Hey guys,

I'm in the process of designing a new power supply for an old Japanese computer. I have a pin out of the voltages for all pins and I can successfully turn the computer on by wiring a pico PSU to a breadboard with the 44pin high density backplane connector that connects to the computer. I press the power switch on my pico breakout board and it shorts the PS_ON line to ground on the pico turning the computer on. However when I do this I cannot press the front power button on the computer to turn it off or on.

I'm in a dilemma. Currently the design can turn the computer on but I require a second switch in the rear specifically making the front power button obsolete. There is also a software shutdown feature in the computer software that also shuts the PSU off and I'm not sure how to implement this in a circuit.

The pin that performs a software shutdown is an active high and goes low when the PSU cuts off basically when you select computer shutdown in the operating system. This I do know.  I'm welcome to any suggestions to try out by others more experienced here

[m k]

What kind of a switch the original one is?

Active high means that level is low when shutdown is not shutting the power.
To use something like that the PSU must support it.

[Jwillis]

What about a soft latch circuit and relay.  A CD4011 Nand gate comes to mind.

[m k]

NAND is good, even better that 4011 is a quartet.
Shorting NAND inputs is usually simple, that way you get an inverter.
By inverting NAND inputs you get OR.
You still have a spare gate so you can get NOR.

https://en.wikipedia.org/wiki/NAND_logic

[SmokedComponent]

Using CD4093 (Schmitt-trigger NAND) is better in this case, when interfacing with buttons and slow(ish) events and not other CMOS/TTL logic.

[m k]

One is still missing, open collector.
(cd74hct03)
https://en.wikipedia.org/wiki/Open_collector

For the device in hand,
PWR_ON ->
PWR_GOOD ->
PWR_OFF/ON <-

Many times the first one is the only one that is presented.
In that case the current power on method must be replaced.