AT Commands


Required Settings

For the Internet Gateway to function correctly, the modem or ISDN TA must be set as follows.

Setting AT Command
Fixed baud rate setting AT&B1
RTS/CTS flow control AT&K3
DCD to track the presence of a carrier AT&C1
DTR off to hang-up modem AT&D2
DSR always on AT&S0

If your Modem or ISDN TA uses the commands above, then the Initial String would be as follows:

AT&F&B1&K3&C1&D2&S0X4


Finding the current Initial String

If your modem or ISDN TA is already working correctly using the PC's serial port, but you don’t know what the initialization string is, you can use the following procedure to find out.

  1. Select My Computer, then Dial-Up Networking.
  2. Select the icon for your connection, then Properties.
  3. Click the Configure button, then the Connection tab, as shown below.

Figure 1:- Connection Properties (W95/98)

  1. Select Advanced to see the screen below.

Figure 2:- Advanced Connection (W95/98)

  1. Check the option Record a log file. Then click OK and exit.
  2. Use Dial-up Networking to make your on-line connection normally. A log file MODEMLOG.TXT will be created in your Windows directory.
  3. Use Notepad or another editor to read and print the file MODEMLOG.TXT.
  4. Examine the file to determine the Initial String value.

Standard AT Commands

Most modems use the standard AT commands, as shown in the following tables. Consult the manual for your modem to set what AT commands it supports.

Note that the trailing "n" in many commands indicates a number.
The allowable numbers, and their effect, are listed below the command.

Command Description
<any key> Terminate current connection attempt
+++ Escape sequence code, entered in data state, wait for modem to return to command state
ATA Force answer mode on-line
ATBn Handshake operation
B0 Select ITU-T V.22 for 1200 bps communication
B1 Select Bell 212A for 1200 bps communication
ATD Dial number and options that follow
P Pulse dial
T Tone dial
, Pause for a specified time
; Return to command state after dialing
! Hook flash, call transfer
W Wait for second dial tone
@ Wait for 5-second silence before proceeding, otherwise return O ANSWER"
R Reverse Dial (Originate a call in answer mode)
ATDL Dial last number
ATDSn Dial number stored in NVRAM at position n. (n=0..9)
ATEn Command mode local echo of keyboard commands
E0 Echo off
E1 Echo on
ATHn On/Off hook control
H Hang up modem
H0 Hang up (on hook), same as ATH
H1 Get off hook
ATIn Display inquired information
I0 Display product code
I1 Display product information and ROM checksum
I2 Link status report
ATLn Speaker volume control. n=0-7
ATMn Speaker control
M0 Speaker always off
M1 Speaker on until carrier is detected
M2 Speaker always on
M3 Speaker on after last digit dialed, off at carrier detect
ATNn Ring volume control, n=0 disables ring function. (n=0..7)
ATO Return to on-line state
ATP Pulse dial
ATQn Result code displayed
Q0 Modem returns result code
Q1 Modem does not return result code
Q2 Return result code but quiet in answer mode (will not show in AT&Vn)
ATS0=n Number of rings required before modem answers. n=0 disables auto-answer.
ATSr.b=n Set bit b of S-register r to n. (n=0 or 1)
ATSr.b? Inquiry bit b of S-register r
ATSr=n Set S-register r to value n, where n is a decimal number between 0-255
ATSr? Display value stored in S-register r
ATT Tone dial
ATVn Verbal/Numeric result codes
V0 Display result codes in numeric form
V1 Display result codes in verbose form
ATXn Result code options. (n=0..7)
ATZn Reset the modem and set power-on profile. (n=0..4)
Zn Reset modem and load user profile n (n=0..3)
Z4 Reset modem and load factory settings
AT$ Help, Basic command summary
AT&$ Help, Extended AT& command summary
AT*$ Help, Extended AT* command summary

Extended "AT&" Commands

(Includes RTS/CTS Flow Control Commands)

Command Description
&Bn Data rate, terminal-to-modem
&B1 DTE/DCE rate fixed at DTE setting
&Cn Carrier Detect operations
&C1 Carrier Detect tracks presence of carrier
&Dn Data Terminal Ready (DTR) operations
&D2 DTR off causes modem to hang up
&F Load the default factory settings,
&Kn Data flow control, DTE/DCE, n=0,3,4
&K0 Flow control disabled
&K3 Hardware (RTS/CTS) flow control
&K4 Software (XON/XOFF) flow control
&Sn Data Set Ready (DSR)
&S0 DSR overridden, DSR always on