I'm not sure what caused the failure, but I'd suggest putting 1k resistors between them instead of direct connections. CMOS data goes through a 1k resistor just fine, and there's a lot less of risk of damage from accidental connections or static.
If you pick an arbitrary tri-state latch, you might end up with one that is high-Z when there is no Vcc. Or you might end up with one that shorts to ground. With most CMOS parts, any voltage you put on a powered-down chip finds its way to Vcc and attempts to power the whole circuit. It's a handy trick for when you need it, but it's probably what's doing you in right now.
If you use a part specifically designed to be high-impedance when powered down then it's not a big deal... but I don't know of any octal/quad tri-state latches that fall in this category. Usually these are things like RS-232 or RS-423 transceivers (which are not all that high impedance, but are usually better than a few K ohms.)
I'm working on a sync serial interface between 2 processors using 2 data and
1 clock. I had to replace 1 board because I was sending data while it was off. By sourcing or sinking one of the IO pins, I fried the board. I'm trying to think of a good way to avoid this. What I'm thinking of is using a octal (or quad) latch which buffers the IO and is controlled by the 'master' processor via the OC or enable pin. As long as the output of the latch is hi-Z then I shouldn't have a problem. Right?
Just put 220R or so resistors in series with the port pins, that will limit the current, and you can then scope either end to see if there are any conflicts
martin
Serious error. All shortcuts have disappeared. Screen. Mind. Both are blank.
Why not use a real RS232 transceiver. Maxim makes at least 1 with 3 drivers/3 receivers in one pack - single supply with built in charge pump. I think these are available in SM only. Motorola made an (MC145407?) that had several drivers/receivers in one 20 pin DIP package, 5V only, but I don't see it at the ON web site. I don't know what bit rate you are running, but these devices are good up to about 250 Kbs.
I can't remember if it is part of the RS232 spec, but the original EIA specification actually had pins assigned to the clock leads. If you go back to before UARTs were invented in the 1970s, all modems that I had seen that ran faster than 300 Baud were synchronous. Just to show how things have changed, we once had a Western Electric 103A 300 Baud modem. It was the size of a large DVD player, with about 10 plug in boards.
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