Blinking light minutia

So, did we always know it would come down to blinking lights?
Not sure I did, because there is a big reason to avoid them. That reason is that it takes a lot more chips, just to run the lights. It takes a lot of mux's (at least that is how AK did it.) It also takes those light driver chips. hmm gotta look that up, back in a minute....

I'm back, but no more knowledgeable than I was when I left. It seems that the standard KI-10 Customer print set (which was by and large all us field service grunts ever saw) does not contain the actual schematics for the freaking blinking lights! It seems that this information was only in the manufacturing print set. So we know a wire goes off to somewhere there is a light (like AR BIT 0) , but that circuit is a mystery to us.

I sorta get this. Whether or not the lights worked was superfluous to the thing actually working. It coulda worked just fine with no lights at all, so they were there just for us to admire, and on the rare occasion use them for troubleshooting.

So, the customer would never need these and various power supply details unless they were building their own KI-10 which DEC would not have been in favor of. Hey, what about me? ha

BUT I started this post because blinking lights are COOL! And, whether or not the finished product has them, I'm gonna need them while prototyping, so I can "SEE" what's happening.

And now finally for the electrical and logic minutia (more teaching). I remembered that in the 74XX logic chips there were some that were built to be interface chips. To drive things OTHER than just another 74XX chip.

So, for example, there is the 7404 chip. This is one of the 3 most basic 74XX chips.
It is simply 6 NOT's. Whatever goes in comes out the opposite.
If the input is high, the output is low and visa versa.

OK goddamit here are the other 2.
These are the 7400, which is 4 NAND gates (that's an AND gate with a NOT output.
And the 7402 which is 4 NOR gates (yes, you get it, an OR with a NOT output.

I mention these here, because every other digital circuit can be made from these basic building blocks.
These form adders, registers, counters, etc. So really (and frankly you could do it all with just the 7400's) if you understand these you have the basis for any digital circuit.

Can we get back to the subject please? I said this was minutia and I need to get to it.

So, the 7404 is 6 inverters. But those guys also had the 7405 and the 7407 chips.
Now if you look below, you will see that the logic is identical. It looks like 6 inverters. Just like the 7404.

Well, as I said (it seems like forever ago) this chip was built to interface to other things. The way they did that was by making the output "open collector".
OK really in the weeds now, See the arrow? It it pointing to a part of a transistor which is NOT connected. It's OPEN. The circuit is completed OUTSIDE of the chip, which makes it usable to connect to other things.


And here we show why that is important. A blinking light can be driven by one of these.

Finally, notice that I mentioned there was a 7405 and a 7407. What's the difference? I started to ask myself that question as well, because I forgot.  I could buy a 7405 for a quarter (literally), but the 7407 was more like 60 cents. So el cheapo me went and bought (10) 7405's without thinking about it.

These are still on the parts I'm waiting for, but I did start to wonder. So, I looked at the spec's and it turns out that the 7405 isn't really powerful enough. It can only handle an output of 8 ma. My led's generally need about 30 ma, so if I connected one to a 7405 there would be smoke!

The 7407 chip can handle 40 ma (that's milliamp) outputs, or happily BLINKING LIGHTS
I just ordered some of those so I can start to work on the light show.