AT Commands

General Modem Commands 

ATI0 is an information command, often returning the modem's speed.  
  
ATI3 and ATI4 are also information commands, often returning the modem's manufacturer 
and model.  
  
ATI7 will often provide advanced modem information, such as ROM updates.  
  
ATZ will return the modem to the default settings. 
  
ATDT*70 will disable call waiting.  

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How to Use These Commands 

The PC communicates with the modem using a command set, often referred to as the AT Command Set. These commands 
can be used to configure, diagnose and receive information from the modem. Modem commands 
can either be used as part of an initialization string in a specific program, or can be 
issued directly to the modem using one of the following programs, depending on your 
operating system.  

Windows 95/NT Users 

Click Start, Programs, Accessories, then select Hyperterminal.  
  
Double-click hypertrm.exe.  
  
Enter Terminal Mode in the Name field, then click OK.  
  
Do not enter a phone number in the next box. Instead, click on the drop-down arrow on the 
Connect Using field.  
  
Select Direct to COM X, where X is the port the modem is on.  
  
Click OK.  
  
Click OK once more, and a terminal screen will appear, where modem commands can be typed.  
Windows 3.x Users 
Open Accessories, then double-click on the Terminal icon.  
  
Choose Communications from the Settings menu.  
  
Select the COM port the modem is on, and change the baud rate to 19200. 
  
Click OK.  
  
Now modem commands may be typed. 

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Error Correction Commands 

Error correction helps cut down on noise and other phone line problems, which will result 
in less data loss. The packet design of error correction can also lead to greater throughput

Despite the benefits, in some cases, it is necessary to disable error correction. Connecting 
to an old 2400 modem, which cannot perform error correction, would be one such case.  

To disable error correction:  

For US Robotics Modems, use the command AT&M0  
  
For Rockwell Modems, use the command AT\N0 
  
For Hayes Modems, use the command &Q6 

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Data Compression Commands 

Besides error control protocols, all current high-speed modems also support data compression 
protocols. That means the sending modem will compress the data on-the-fly and the receiving 
modem will decompress the data to its original form.  

In some cases, it is necessary to disable data compression.  

For US Robotics Modems, use the command &K0 
  
For Rockwell Modems, use the command %C0 
  
For Hayes Modems, use the command ATS110=0 

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