>    Using a voltmeter, the Vcc pin shows 5 volts with respect to GND.
> And a output pin at logic high shows 4 volts with respect to GND. The
> same pin shows 0 volts with respect to the Vcc pin. The same is
> observed when i used a CRO to look at the waveforms.

Hah - yes fallen into that one myself ...

TTL output driving an LED which is connected through resistor to +5V.

Output of TTL gate goes low, LED turns on.

Output of TTL gate goes high, but still measure 2.5V across LED. Much head 
scratching, but someone else pointed out that you cannot get current flow 
around a loop through the LED resistor and upper transistor of TTL gate when 
output is high ... The fact that a TTL gate never reaches 5V doesn't matter 
... 

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