Hello,

I recently took (and passed) the associate ISCET exam. There is one question that still bothers me. It is:

If a TTL output is providing 0V, what is this called?:
A: Current sourcing
B: Current sinking
c: Asserted
D: Unasserted

It seems to me that both B and D are correct. Is this so? What would be the best answer?

 

It's B: current sinking.

 

Yes, it's B. Neither C or D is correct.

A signal may be asserted (or deasserted) either high or low depending upon the operation required in the design.

For instance, a xstr that enables a relay by sinking current would be a low assertion at the relay coil terminal (other terminal would be at voltage), and would typically use a NOT indicator in the signal/net name; such as ~RELAYEN, RELAYEN*, /RELAYEN, or have an overbar (you could also call it RELAYOFF without the negation, but to the same effect -- in this case high, or open, would mean off).

 

Excellent! That was my answer.

Thanks!