mkarcher wrote on 2023-01-27, 18:18:

appiah4 wrote on 2023-01-27, 12:47:

The DT in ATDT is DIAL TONE, so when the modem receives ATDT it opens the line and looks for a dial tone. If the dial tone is found, it then TONE DIALs the string of numbers proceeding it.

You are completely correct that D means "dial" and T means "by sending tones". The use of ATD (uses default dialing method), ATDT (uses tone dialing) and ATDP (uses pulse dialing) is independent of the action of waiting for a dial tone. Whether a dial tone is required when the modem starts executing any ATD command is set using ATX: ATX3 disables dial tone detection and dials straight away after a short delay after going off-hook, whereas ATX4 enables dial ton detection and instructions the modem to wait for a dial tone after going off-hook before starting to process the number in the command.

You can also add later "wait for dialtone" instructions into the phone number by adding the letter "W" to it. On PBXes with custom dial tones, dialing a number using ATDT0W7121234567 was not unusual. The first digit 0 instructed the PBX to request a public line (with a standard code), and the modem then waits for a dial tone. The letter W can be used independent of the ATX setting. If ATX4 mode is enabled, the modem behaves as if there was a "W" in front of the pohone number.

Isn't ATX3 default? For a modem with no dialtone unless you specify ATX4 ATDT will return NO DIALTONE or some such.

appiah4 wrote on 2023-01-28, 12:58:

Isn't ATX3 default? For a modem with no dialtone unless you specify ATX4 ATDT will return NO DIALTONE or some such.

Probably on modems without EEPROM, ATX3 (or an even lower X mode) is default for compatibility reasons, but newer modems allow saving of the "current configuration" to EEPROM. On those newer modems, the command ATZ restores EEPROM settings, which might contain ATX4. To retrieve factory settings (with ATX3), you would have to use AT&F.

ATX3 disables dial-tone detection, so if the modem is in default ATX3 configuration, it will never report NO DIALTONE unless the phone number contains a "W". Requiring the "dial tone" (a continous tone provided by the public phone network to indicate readiness to accept a phone number digits) before the first digit is dialed is a feature that get enabled only on ATX4. If ATX4 is the active X mode setting, all ATD commands, whether ATD, ATDP or ATDT can return NO DIALTONE if waiting for a dial tone timed out.