The goal was simple: blink a LED, connected to PPI 82C55.

The program is straightforward: set all ports to output, change PC7 to 0, wait a moment, change PC7 to 1, wait a moment. Repeat.

But...

At first everything went OK: voltmeter shows me 0 V and 5 V at 1 sec frequency. Then I have connect a LED.

And nothing happened.

PC7 stays at 5V.

Okay, let change PC7 to whole PC port. Blink, blink, voltmeter OK, connect LED... and died. The whole PC port was now dead. Some of pins were 0, some 1.

No short connections. No "Vcc on GND". No such errors. But PPI was dead.

Another chip, everything OK on the voltmeter, so connect wires (not any LED, just wires!), let's measure voltage on the other ends of wires... nothing... dead. The whole PPI seems to be dead!

OMG OMG OMG!

I throwed the NEC 82C55AP to the recyc... okay, not recycle bin, maybe the problem is outside the PPI. So I pulled out older Intersil 82C55, connect it...

... and everything is blinking, flashing and working!

(Programmed in BASIC this time... Isn't it sweet?!)

Two days ago: I pulled out the Intersil one, connected the NEC one, everything is working now... Hooray!