Johnny's homebuilt 8080 CPU corner (and other stuff, maybe)

 

74120 - Pulse Syncronizer Driver

 

This is a rare and odd circuit I found in my old Texas Instrument TTL Data Book.

I have had this book since the late 70s and have read it cover to cover countless times, and still managed to miss this little gem.

 

Link to Texas Instrument

 

74120 - Pulse Syncronizer Driver

To the left is a simplified version of the 74120. I only want to start/stop a pulse train so I don't need more than this. The full circuit in a 74120 can do more.

 

Above is my DTL version.

 

Not shown in the schematic:

All base resistors = 15K, all collector resistors = 3K9.

All clamp diodes = 1N4148, all transistors = BC547B/BC548B/2N3094

All input diodes = BAT42.

 

A real 74120 chip is unlikely to be found today, but it can easily be reproduced by two 74HC11 triple NAND's. Like this!

Testdriving my DTL circuit:

Top trace is the gating signal [E]

Lower trace is clock pulses [C]

 

Those signals comes from two free running oscillators.

 

When [E] is high, clock pulses [C] go through the circuit to [Y]. When [E] is low, clock pulses is blocked. Note the last one of the 6 clock pulses - it's not clipped! That what this circuit does!

 

 

Another screenshot.

And, finally, here is two screenshots from LTSpice, showing what happens when the [E] signal moves slightly relative to the [C] signal. Output [Y] always deliver complete clock pulses !!