Re: Where to start on Missile Command PCB repair

On Fri, 1 Nov 2013 08:08:42 -0500, you wrote:

>On Nov 1, 2013 7:52 AM, "Grant Thienemann" <grant.thienemann@gmail.com>
>wrote:

>> Ok not to keep getting off topic, but as this is related to the Missile
>> Command repair, I'm going to ask. When I look for 74191 I can't find
>> anything on Digikey, etc, but when I search for 74HC191 it comes up, is
>> that the same thing?

>Close. HCT is closer. Either should work.

Not always, though. 74HC is "high-speed CMOS"; its switching thresholds
for what it considers "low" and "high" logic levels are the usual <33% of
Vcc = Low, >66% of Vcc = High. In a 5Vdc circuit, this works out to <1.65V
= Low, >3.3V = High.

However -- TTL gate inputs consider anything <0.8V to be "low", and >2V to
be "high". On the output side, 0 - 0.5V is considered an acceptable "low"
output signal level, while 2.7V - 5V is an acceptable "high."

So -- mixing 74HC with TTL *may* work. It will *probably* work. It would
take a very unlucky coincidence of worst-case scenarios for the input and
output drive voltages of the two different chips to fall within the
"mismatch zone"... but it *can* happen.

74HCT, on the other hand, is "high-speed CMOS, TTL-compatible"; it's
specifically designed so that its input and output signal levels fall
within the same ranges as "normal" TTL.
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A: Because it reverses the natural flow of conversation.

Q: Why is top-posting bad?
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Received on Fri Nov 1 19:56:23 2013