Is the R6520 a drop in replacement for the MC6820?

_________________
Bill

Thanks!

_________________
Bill

Wondering if that W65C21S maybe a little too aggressive WRT switching speeds for an older board?

I've had a 74ACT04 render an older board inoperative due to switching spikes on the power rails messing with other chips.

_________________
Bill

Given that the rise and fall times are rather abrupt on AC and ACT outputs, my first thought would be ringing on the traces driven by the 'ACT04 outputs. But spikes on the power rails are also a possibility. If you're curious enough to do an experiment, you could try soldering a supply bypass capacitor directly on top of the 'ACT04. (The original bypass caps on the old board were perhaps not well chosen and perhaps not well situated and optimally connected.)

-- Jeff

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

That ship sailed Jeff. No longer have the board. I guess I could ask the owner to loan it back to me..

There were major spikes and HF ringing on the on the traces, but the culprit in preventing operation was an unconnected refrigerable one-shot that just kept retriggering and gumming up the works.

If I triggered my scope off any of the 74ACT04 outputs I would see spikes on the power rails. One in particular created a spike (actually HF ring) that registered as over 700mV on my meager 100MHz scope. In reality it was probably a lot worse. Certainly enough to re-trigger the one-shot. The one-shot (74LS123) was adjacent to the 74ACT04 so got the noise first hand.

The board was badly laid out too with "once in a while" bypass caps between the power rails rather than between the chips power pins. Think of a row of ICs with +/- power rails both above and below the row of chips. The bypass caps went between the adjacent rails so basically there was no proper decoupling of any IC. I'd say (without having in front of me) that it had maybe one cap for every 4 ICs.

_________________
Bill

Well, I guess the pertinent question now is how a W65C21S will behave in this other old board you mentioned.

It'd be fairly easy to upgrade the supply bypassing if that seems appropriate. But if ringy outputs seem to be an issue, then as you know the easiest remedy (if you can get away with it) is series termination -- just a 30 or 50 ohm (approx) resistor located close to the IC pin and connected between it and whatever it connects to.

Will the W65C21S have a lot of its peripheral pins operating as outputs? In that case you'll need a lot of resistors, and might wanna consider using a resistor array (possibly SMT) rather than discrete resistors.

(And of course the data bus pins sometimes act as outputs, too... )

-- Jeff

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

The 6821/W65C21S is not going in the previously discussed board but into the one below to replace that beautiful but maybe faulty purple ceramic one. This board is not great WRT to bypassing, but is better than the previously discussed one by a measurable margin.


As you can see from the reverse, the process you mention is underway - not completed yet, but this project has been and on again, off again one since 2015.

It's now on again and I'll get those caps completed before the W65C21S arrives. Most of the I/O off that chip already has termination resistors.


As always, thanks Jeff!

_________________
Bill