In the EIA-232 (RS-232) serial interface, which line coding is specified for representing mark and space at the electrical level?

Introduction:
EIA-232 (often called RS-232) is a classic point-to-point serial interface standard widely used for terminals, modems, and instrumentation. Understanding its electrical signaling and line coding is fundamental for debugging and interfacing legacy serial equipment.

Given Data / Assumptions:

• RS-232 defines voltage levels for logical states called mark and space.
• We are focused on how bits are represented on the wire over time.
• No clock is embedded (as in Manchester) because asynchronous framing supplies timing.

Concept / Approach:
RS-232 uses NRZ-L coding, where a constant voltage level over a bit interval indicates the bit value (mark/space). There is no guaranteed transition each bit period, unlike Manchester or Differential Manchester line codes used for self-clocking links. Because NRZ-L lacks inherent clock recovery, RS-232 relies on start/stop bits and agreed baud rate for timing.

Step-by-Step Solution:
1) Identify logical states: mark and space correspond to specified positive/negative voltage ranges.2) Recognize that bit values are conveyed by sustained levels (NRZ-L), not by transitions (NRZ-I) or mid-bit flips (Manchester).3) Confirm asynchronous framing supplies timing instead of a self-clocking code.

Verification / Alternative check:
Equipment datasheets and scopes show constant-level signaling for each bit interval with start and stop bits bracketing data bytes, consistent with NRZ-L.

Why Other Options Are Wrong:

• NRZ-I: information encoded by transitions; not RS-232.
• Manchester/Differential Manchester: self-clocking codes used in Ethernet/Token Ring, not in RS-232.
• Bipolar-AMI: alternate mark inversion used in T-carrier systems, not RS-232.

Common Pitfalls:
Assuming all serial links embed a clock; RS-232 does not and therefore depends on accurate baud configuration at both ends.

Final Answer:
NRZ-L (Non-Return-to-Zero Level)