Test Your EQ (Engineering Quotient)

EQ #22

In an RS-232 interface, why does the “mark” level correspond to a logical “high” level on the TTL side of the interface?

Back in the days when RS-232 was developed, the primary logic families in use were DTL and TTL. Both of these technologies draw significantly less power when a signal is in the high state than in the low state, so the high state is preferred for the inactive state of any signal.


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