Marty Watt wrote:
> Can U4 be a 78HCT240, instead of a 78HC240? I'd think not, based on
> information in the Data Book by NA5N. But RS had the "T" version,
> not the regular version ...
>
> What would be the result of using a "T" version, which has a higher
> response time, but is a bit tighter on voltages? Seems we're
> overdriving the chip anyway, since the expectation is for 7.8v on
> several pins of U4. (The Data Book shows the Vcc of the normal
> 78HC240 as 2.0-6.0v, the "T" version is 4.5-5.5v). The Vhi is a bit
> lower (1.1 V). By the way, just what is Vcc? I'm assuming it's
> some sort of operating voltage range, but I'm an idiot, and not sure
> of such things ...
The 74HCT logic is 74HC logic that has been modified to have input
voltage thresholds compatible with TTL logic. The Vcc (supply voltage)
range has been specified as that of TTL, too. HCT is normally used when
connecting TTL logic to HC logic, since a TTL gate has a more limited
output voltage swing and may not adequately drive the HC.
I'd definitely prefer the HC part (not the HCT) in the 38S application,
but I suspect the HCT part would work OK.
Vcc actually means something like 'collector supply' - which is a reference
to TTL. You'll sometimes see 'Vdd' and 'Vss', where Vdd means drain
supply and Vss is the source supply, for CMOS parts. In this case,
Vdd is the same as Vcc.
Dana K6JQ
Dana@Source.Net
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