Sys 80 Non CPU boot/IRQ issue

[MartyJ]

I asked for some help with this one (few days ago) on RGP.

The following was posted by a fellow RGP'er Kerry:

"U5 Pin 15 is driven by inverter Z26.  Z26 pin 13 should be high (+5V)
normally and go to near 0V when the slam switch is closed.  Z26 pin 12
should be opposite (near 0V normally and go to +5V when slam switch
closed).  Bottom line... make sure U5 PA7 (pin 15) is near 0V (I would
guess it should be about 0.2V). "

My pin 13 is low 0.01v and pin 12 is 5.014v

Slam mod is definately done (removed old one and redid again).

I replaced Z26 this morning, no difference.  Assuming the above is correct, something weird going on (apart from me miscounting pins)

[Marty Machine]

Glad you found the right pins hehehe.

Your measurments on Z26 indicate the slam switch (or mod) must be stuck on.

When Slam-Switch activated :
Z26-13 = Lo.
Z26-12 = Hi, therefore U5-15 also Hi.

and so the opposite is true...
In normal operation (slam switch NOT activated) :
Z26-13 = Hi.
Z26-12 = Lo, therefore U5-15 also Lo.

I'd now be looking at your slam-mod, or the switch itself, shorted diode on it or damaged/bent switch ?? ??

MM.

[MartyJ]

Switch isn't connected.....I'll remove the mod and see difference.

Also, from the schematics can you tell me how do I know if a pin should be high or low?

[Marty Machine]

In this case, Z26-13 is 'held' Hi by the R20 3k resistor upto +5v.
The Capacitor C30 is there for noise filtering, removing some spikes from the switch contacts etc.

So, the slam switch (or output from the mod) on (connector J5-10) will need to go to 0v to change the state of Z26.

Assuming you know that Z26 is in 'invertor' whatever appears on the input (pin13) will be the exact opposite on the output (pin12).


Other than that, you would only know what Hi/Lo signals to expect when you know what the pins do on the chip.
You'd need datasheets on the chips to learn which pins need to be Hi or Lo to do their job etc....

MM.

[MartyJ]

With mod removed:

U5 pin 15=low
Z26 pin 12=low
Z26 pin 13=high

IRQ still locked at high, no pulse.

OK, I understand re the hi/lo signals.  I thought this was something you were deciphering from the schematics...

[Marty Machine]

Ok, so from your recent post, the readings on Z26/U5 are now as expected......DAMMIT