Tim's own version of the Catweasel/Compaticard/whatever

From: Richard Erlacher <richard_at_idcomm.com>
Date: Wed Jul 5 17:55:20 2000

I agree that the device in question doesn't offer OC/OD outputs, a tristate
driver would work as well, and the CPLD drives a bit MORE (8 mA) than the
typical HC/HCT device (6 mA). I think the CPLD would provide an easy-to
build alternative that would require some assembly, but which could be
modified without wiring or soldering/.

If you look at FDC's used in many systems, including some of the ones used
in PC-clones, didn't bother with OC drivers. It's not critical unless you
have more than one controller on the cable. That happens rather seldom.
HCT's by the way, don't have open collectors. In fact, they don't have
collectors at all. It doesn't matter, though. If you drive a line with a
tristate buffer with the appropriate polarity at its input, it will behave
pretty much as an OC device does, only better and faster. If you don't
believe that, try plugging a 74126 with an input grounded into a circuit
prevously occupied by a 7403. of course it must be wired such that the
logic functions the same, but it's a valid comparison. The more common 7438
is not valid for comparison because they didn't get around to building a
tristate buffer in the '126 pinout. When you hit the enable with a high,
the output will go low, lickety-split, just like the OC. You'll need a
pullup in either case.

Dick
----- Original Message -----
From: Chuck McManis <cmcmanis_at_mcmanis.com>
To: <classiccmp_at_classiccmp.org>
Sent: Wednesday, July 05, 2000 12:48 PM
Subject: Re: Tim's own version of the Catweasel/Compaticard/whatever


> Actually my point on using the PLD was that it would simplify
> layout/building/retrofitting. I've checked and the CPLDs do not support
> "true" open collector outputs so you are stuck with the 74HCT drivers in
> any case.
>
> --Chuck
>
>
Received on Wed Jul 05 2000 - 17:55:20 BST

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