In serial interface standards, which of the following is NOT a standard RS-232-C control/handshake signal line?

Introduction / Context:
RS-232-C is a classic interface standard that specifies electrical characteristics and signal assignments between Data Terminal Equipment (DTE) and Data Communication Equipment (DCE). Knowing the common control and handshake lines is essential when wiring serial links or troubleshooting modem connections. This item asks you to pick the one label that does not belong to the RS-232-C family of well-known signals.

Given Data / Assumptions:

• Common RS-232-C control lines include RTS (Request To Send), CTS (Clear To Send), DSR (Data Set Ready), DTR (Data Terminal Ready), RI (Ring Indicator), and DCD (Carrier Detect).
• The distractor uses an unfamiliar mnemonic.
• We are focusing on canonical signal names, not vendor-specific aliases.

Concept / Approach:
By recalling standard RS-232-C signal mnemonics, you can quickly reject unfamiliar or nonstandard labels. RTS/CTS are used for hardware flow control, DSR indicates the DCE is ready, and other standard pins include DTR, DCD, and RI. “VDR” is not part of the RS-232-C signal set and therefore is the correct choice as the nonstandard entry.

Step-by-Step Solution:

Verification / Alternative check:
Any RS-232-C pinout reference confirms the absence of “VDR” and the presence of RTS, CTS, and DSR among the control lines, reinforcing the identification by elimination.

Why Other Options Are Wrong:

• RTS: standard outbound flow-control request from DTE.
• CTS: standard inbound permission from DCE to send.
• DSR: standard DCE-ready status.
• None of the above: incorrect because one option (VDR) is indeed nonstandard.

Common Pitfalls:
Confusing DSR with DTR; assuming unfamiliar mnemonics might be vendor extensions; overlooking that RS-232 naming is quite consistent across documentation.

Final Answer:
VDR.