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.
Don't miss out on upcoming issues of Circuit Cellar. Subscribe today!
Would you like to write for Circuit Cellar? We are always accepting articles/posts from the technical community. Get in touch with us and let's discuss your ideas.