Two PCs communicate via a direct RS-232 null-modem cable over a reasonable distance. What is the maximum standard distance (in metres) generally permitted for a null-modem RS-232 connection under RS-232 specifications?

Introduction / Context:
RS-232 is a classic serial interface used for direct device-to-device connections, including null-modem links between two computers. Cable length affects capacitance, signal attenuation, and noise susceptibility. The formal RS-232 standard specifies a conservative maximum distance to preserve signal integrity at typical data rates. This question asks for that standard limit in metres for a null-modem style RS-232 link.

Given Data / Assumptions:

• Interface: RS-232 electrical levels and timing.
• Typical data rates considered by the standard (e.g., up to 19.2 kbps).
• Standard (conservative) limits, not best-case marketing claims.

Concept / Approach:
The RS-232 specification traditionally cites a maximum cable length of about 50 feet, approximately 15 metres, based on cable capacitance limits (often quoted as 2500 pF). Longer runs can sometimes work at very low baud rates with high-quality cable, but those are non-standard, best-effort scenarios and not the distance “permitted” by the specification. Therefore, when the options list distances such as 30 m, 80 m, 100 m, or 150 m, none corresponds to the canonical ~15 m standard limit, so “None of the above” is the correct response.

Step-by-Step Solution:

Verification / Alternative check:
Vendor references for RS-232 level transceivers consistently cite 15 m as the standard distance; application notes discuss extended runs only with reduced speeds and special cables, which are outside the standard guarantees.

Why Other Options Are Wrong:

• 30 / 80 / 100 / 150 m: exceed typical RS-232 standard limits; may function only under special low-rate, low-noise conditions.

Common Pitfalls:
Confusing RS-422/RS-485 capabilities (hundreds of metres) with RS-232; assuming a successful ad-hoc long cable equals a standards-compliant installation.

Final Answer:
None of the above (standard limit ≈ 15 m).