Electronic – Is this circuit from a textbook wrong


(This is not a homework question.)

While reading "Digital Computer Electronics" by Albert P. Malvino, I found this circuit on page 121, figure 8-22(a). It claims to be an example of a discrete TTL tri-state buffer, but I believe it to be wrong.

Best I can tell, the output will always be either high or tristated: it will never be low. I also simulated it online, and the results agreed with this assessment. I googled for published errata, but found none.

Would people say this circuit was in error, or am I missing something crucial?

photograph of textbook

Best Answer

For true TTL tri-state operation the output must be able to pull up and down. The circuit in your book can only pull up, which is not compatible with TTL logic.

Here is a real tri-state TTL (inverting) buffer:-


simulate this circuit – Schematic created using CircuitLab

When Enable is high Q3 is turned on, which turns off Q5 via D1, and Q4 and Q6 via Q1B. Q6 provides the essential pull down when not tri-stated.

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