Modem control with AT commands: Which command from the standard Hayes “AT command set” resets the modem to its default configuration?

Introduction / Context:
Hayes-compatible modems use a standardized “AT” command set for dialing, configuration, and diagnostics. A reset command is frequently needed to clear prior settings and return the modem to a known good state before establishing a connection.

Given Data / Assumptions:

• You have a Hayes-compatible modem.
• You want to reset the modem via serial commands.
• Basic control signals (e.g., DTR) and terms (e.g., DCE) may appear as distractors.

Concept / Approach:
“ATZ” resets the modem to stored default settings (often profile 0) and clears active configurations. Other commands: “ATD” dials, “ATH” hangs up, “ATI” queries identification, “AT&F” loads factory defaults, and “AT&F” differs from “ATZ” by sourcing defaults from factory rather than stored profile. “DTR” is a hardware control line; “DCE” denotes the modem device type.

Step-by-Step Solution:

Verification / Alternative check:
Sending “ATZ” in a terminal emulator typically returns “OK,” after which querying “ATI” or “AT&V” shows a default profile.

Why Other Options Are Wrong:
ATR: not a standard reset command. DTR: a signal line, not an AT command. DCE: device role term, not a command. None: incorrect because ATZ is valid.

Common Pitfalls:
Confusing “ATZ” with “AT&F”; both reset, but from different default sources (stored vs factory).

Final Answer:
ATZ