Worksheet 2 - AT Commands

This worksheet is, in fact, a logical continuation of the Worksheet 1 in that it focuses on basic AT commands as they are described in the manual for my modem SupraExpress 336i PnP - produced by Diamond Multimedia. Most of these can usually be used for other modems too.

Each of these commands should be preceded by AT, except for the A/ and AT command:

+++ Escape Code

A Answer Phone Line

A/ Re - execute Last Command

AT "Attention" Command Line Prefix

B Selects ITU-TSS protocols at 300 or 1200 bps

B1 Selects Bell protocols at 300 or 1200 bps

Dn Dial Telephone Number

E Inhibit command echo. If you see two characters for everyone you type (i.e. AT looks like AATT), issue E.

E1 Enable command echo. If your computer does not display the characters you type, issue E1.

H On - hook (hang up).

H1 Off - hook (pick up phone line).

L Low speaker volume.

L2 Medium speaker volume.

L3 High speaker volume.

M Speaker always off.

M1 Speaker on until carrier is received.

M2 Speaker always on.

M3 Speaker off while dialing and after carrier is received; on when answering

N Commands - Connection Rate: Note that this command is affected by the setting of register S37:

• N Connect only at the rate specified in S37, or, if S37=0, connect at the rate at which the last AT command was issued. Only attempts connection protocols specified by the ATB command.
• N1 Connect at the highest rate supported by both modems. The ATB command setting is ignored, and the modem attempts the best possible connection.

W Result codes not reported and CONNECT XXXX reports computer (DTE) rate.

W1 Reports Error correction mode. (Carrier: DCE, Protocol, Connect: DTE)

W2 Result codes not reported and CONNECT XXXX reports connection (DCE) speed.

X Blind dial (ignore dial tone and busy signal) and send CONNECT when connected.

X1 Blind dial; send CONNECT XXXX result codes when connected.

X2 Wait for dial tone before dialing; send CONNECT XXXX result codes; do not detect busy signal

X3 Ignore dial tone; send CONNECT XXXX result codes; send BUSY if busy signal detected

X4 Wait for dial tone before dialing; send CONNECT XXXX result codes when connected; send NO DIALTONE code if dialtone is not received in 5 seconds; send BUSY if busy signal detected

&F Direct mode operation without flow control. This command includes these settings: \N1 &K0 &Q0. Your computer rate and connection rate must match.

&F2 Reliable mode operation with flow control. This command includes these settings: \N3 &K3 &Q5. Flow control lets you computer rate be different from your connection rate. Flow control is useful if you are using your telecom software's data compression (MNP or V.42bis), because your throughput is higher than the connection rate and because the throughput changes depending on the compressibility of the data being sent.

&K Disable serial port flow control.

&K3 Enable bidirectional hardware (RTS/CTS) flow control.

&K4 Enable software (XON/XOFF) flow control.

&K5 Enable transparent software flow control.

\N Normal mode, with flow control.

\N1 Direct mode, without flow control.

\N2 Reliable Mode.

\N3 AutoReliable Mode.

\N4 LAPM Mode.

\N5 MNP Mode.

%C Data compression disabled.

%C1 MNP 5 enabled.

%C2 V.42bis enabled.

%C3 V.42bis and MNP 5 enabled.

%E Do not request a retrain.

%E1 Monitor line quality and automatically request a retrain if line conditions are bad.

%G Do not request a rate renegotiation.

%G1 Monitor line quality and automatically request a rate renegotiation if line conditions are bad.

This worksheet lists only the most frequently used commands which also appear to be most crucial in modem communication. They are used either in a telecom application as direct commands or in the modem's initialization string.

After reading through the text, try to test yourself.