Chapter 4 - AT Commands, S-Registers, and Result Codes

Command:

&Cn

Function:

DCD Control

Values:

n = 0, 1, or 2

Default:

&C1 (DCD normal)

Description: Controls behavior of the DCD (Data Carrier Detect) signal (pin 8 on the

 

RS232E/V.24 interface). Normally, DCD goes high when the TA establishes a

 

connection and drops when the connection is lost. However, you also can

 

force DCD to remain high at all times or to remain high except for a brief

 

drop following a disconnect.

 

&C0 DCD is forced high at all times

 

&C1 DCD goes from low to high when TA establishes a connection (DCD

 

normal)

 

&C2 DCD drops briefly following disconnect, then goes high again. Register

 

S10 defines how long DCD signal remains low after disconnect

Command: &Dn

Function:

DTR Control

Values:

n = 0, 1, 2, 3, or 4

Default:

&D1 (exits data mode and re-enters AT command mode)

Description: Controls how TA responds to DTR (Data Terminal Ready) signal on pin 20 of

 

RS232E/V.24 interface. A high DTR signal tells the TA that the connected

 

computer is ready to communicate.

 

&D0 TA ignores the DTR signal

 

&D1 When DTR goes low, TA exits data mode and re-enters AT command

 

mode

 

&D2 If DTR goes low, when TA is on-line, the TA hangs up, returns to

 

command mode, and disables autoanswer. If TA is offline, it neither

 

answers nor dials while DTR is low.

 

&D3 When DTR goes low, TA resets the data port and disables

 

autoanswer. If DTR goes low when TA is online, TA hangs up, resets

 

active configuration to the stored configuration, and disables

 

autoanswer.

 

Note: If the user wants to accept calls while DTR is low, the TA must

 

be configured to ignore DTR. To do this, enter AT&D0<cr>. With this

 

configuration, the TA is able to accept calls while DTR is low. If this

 

configuration setting is not made, the TA rejects incoming calls until

 

DTR is high while the calls comes in.

 

&D4 Ignores DTR when answering a data call. If DTR is low when an

 

incoming data call is present, the TA will answer the call. If DTR

 

goes high during that call, nothing will happen. However, if DTR

 

goes high and then drops for the minimum time specified by S25,

 

then the call will be disconnected just as it would with &D1. &D4 is

 

the same as &D1, except that &D4 can answer a data call without

 

DTR and DTR can remain low for the duration of the call, but if DTR

 

goes high, then &D4 will behave like &D1.

Command: &En

Function:

Flow Control

Values:

n = 3–7, 12, 13

Default:

&E4, &E6, &E13

Description: Selects method by which the TA controls the flow of data to and from the computer to prevent either device from accepting data faster than it can handle. The TA provides flow control in both directions. When the TA halts data flow, it is called flow control; when the computer halts data flow, it is called pacing.

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