š Key Concepts
ā¢ RS-232 logic levels and how they differ from standard TTL logic (where 1 is usually a positive voltage)
ā¢ Reading the voltage vs. time diagram in figure "C" carefully, focusing on where the bits labeled 0 and 1 sit relative to +12 V, 0 V, and -12 V
ā¢ Noting which line in the diagram is explicitly marked as "0 level" and which is marked as "1 level"
š Think About
ā¢ On the voltage scale at the left of figure "C", which voltage corresponds to the horizontal line labeled "0 level" and which corresponds to the line labeled "1 level"?
ā¢ Look at the sequence of data bits in the diagram (0 1 1 0 1 0 0 0). For the bits marked "1", are the signal levels above or below the 0 V axis?
ā¢ Compare the actual drawn height of the waveform for the "1" data bits to the labeled voltages (+12 V, 0 V, -12 V). Which of the answer choices matches that level?
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Before You Answer
ā¢ Verify exactly where the "1 level" label is placed on the vertical axis in relation to +12 V, 0 V, and -12 V
ā¢ Confirm that you are not assuming TTL logic; RS-232 inverts the usual idea of 1 and 0 levels
ā¢ Double-check that youāre looking at a data bit, not the start, parity, or stop bits, when matching the voltage level