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Table Of Contents  CertiGuide to A+ (Core Hardware)
 9  Chapter 0111:  Peripheral Devices
      9  VII  Modems

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VII  Modems
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AT Command Set

The AT command set is a set of text-based instructions that can be used to control a modem manually. With newer modems, these commands are automatically performed in the background by the modem's software. You can use any of these commands on any modem through a program called Hyper Terminal.

This program comes with any of the Windows operating systems and is a great tool for troubleshooting modem challenges.

The first command is 'AT,' which stands for attention. This command wakes up the modem and readies it for use. AT commands can be sent in a compound fashion.

Another popular command is 'D' for dial. The command structure uses a T for tone dialing and a 'P' for dialing with pulse or rotary dialing.

Given this lets say you wanted to dial the number 555-1212. Using Hyper Terminal or other modem control program, you would enter:

ATDT 555-1212

If the modem were behind a Private Branch Exchange (PBX) it would not be able to dial without getting an outside line and its dial tone.

For most PBX systems this requires dialing 9 and a brief pause to wait for an outside line. The AT command set offers a pause for two seconds by using a comma. So now to dial the example above you would enter:

ATDT 9,555-1212

To get the modem to pick up the phone you would enter:


To hang up the phone you would enter:


Sometimes a modem will go through a considerable amount of reconfiguration, either by the user or a software application in the PC. To reset a modem back to its original configuration, the command ATZ resets it back to its original state.


The reconfiguring of the modem for a particular call allows each modem to find the maximum transfer rate that can be attained between the two modems. This is known as negotiation or training. The X series of commands can add more intelligence to the modem. For example, using X4 adds the ability to detect both dial tone and busy signals.

Table 15: Common AT Modem Commands

AT Command



Attention. Wakes up the modem

ATD(T|P) xxx-xxx-xxxx

Dials the modem in either tone (T) or pulse (P), followed by the number


Automatic answer


Pick up (1), or Hang up (0)


Reset the modem

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