Modem commands (Hayes AT set): What is the standard command to hang up (drop the line) on a dial-up modem?

Introduction / Context:
Classic dial-up modems use the Hayes “AT” command set for control. Technicians and terminal users issue commands to dial, configure, and terminate calls. Knowing the hang-up command is essential when automating scripts or troubleshooting.

Given Data / Assumptions:

• Modem supports the standard Hayes AT command set.
• Commands are issued from a terminal program after returning to command mode (escape sequence +++ if needed).

Concept / Approach:

The hang-up command is “ATH”. The prefix “AT” signals “attention” to the modem; “H” stands for hook control (hang up). Variants like “ATH0” explicitly force on-hook (disconnect), while “ATH1” can force off-hook.

Step-by-Step Solution:

Verification / Alternative check:

Telephone line monitor or terminal status shows carrier detect dropping. Subsequent “AT” commands receive “OK” without a live connection.

Why Other Options Are Wrong:

• ATHU: Not a standard Hayes command.
• ATZ: Resets modem to stored profile; may incidentally drop the line but is not the explicit hang-up command.
• ATX: Sets result code and call progress options; not hang-up.
• None of the above: Incorrect because ATH is correct.

Common Pitfalls:

Issuing ATH while still in data mode (use +++ first); modem S-register settings that alter escape timing; misunderstanding ATH1 vs ATH0 behavior.

Final Answer:

ATH