Johnny's home-built 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.
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 !!
