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Chapter 1 – Introduction

CDMA 1xEV-D0 AT Commands Reference Guide (Document S000482B) 11

In the online condition, the modem can be:

• In data state, which passes data through the modem between the host (DTE) and the

remote terminal or network.

• In pass through state (IS-95B), which exchanges commands and replies between the host

(DTE) and the IWF modem (DCE). The data is not sent all the way through the active

connection to the remote terminal. The remote connection is still active (carrier is

maintained) but data is not forwarded to the remote end.

State Transitions

On power up the modem is in command state.

Command to Data State

The modem changes to data state when a session is opened either by dialing a connection (or

answering a data or fax call). When a session opens, the modem issues the CONNECT message and

asserts the DCD control signal.

Note: Due to multi-processing in the modem, the DCD signal may be asserted slightly before the

modem has completed the transition to data state. The host device should allow a delay of 100

ms after receiving the complete CONNECT message and DCD signal before beginning

transmission; otherwise some data may be lost.

Data to Command State

When the modem changes to command state, an OK result is issued. This may be preceded by another

result (such as NO CARRIER) to indicate that the session was closed by an event outside the modem. A

closed session requires the modem to return to command state.

Several events can cause the modem to transition from data to command state based on mode.

SLIP mode transitions:

• DTR is deasserted, with a configuration to use DTR (&D1 or &D2)

• The modem receives the escape sequence as a SLIP frame:

(0xC0 +++ 0xC0)

• The modem is reset or power-cycled

PPP mode transitions:

• DTR is deasserted, with a configuration to use DTR (&D1 or &D2)

• PPP negotiates command state

• The modem is reset or power-cycled

Modem Buffers

Communication with the modem is buffered to allow the modem to provide a variety of features and speed

configurations. This section provides an introduction to the types of buffering performed by the modem.

Command Buffer

When in command state, the modem buffers the input from the host until a <CR> is entered. The

buffered data can be edited using the backspace <BS>. The modem (with Echo enabled) may echo the

sequence <BS><space><BS> for human readability.

There is a limit of 518 characters to one command line, excluding the AT prefix and the <CR>

termination. If the command buffer length is exceeded, the modem continues to echo input (which is not

buffered) until the <CR> is received. When the <CR> is entered, the modem returns the ERROR result

code without executing any commands in the line. Once over the limit, the <BS> does not bring you back

under the limit; the ERROR code is still returned.

The command buffer is distinct from the data receive and transmit buffers. The command buffer retains

the contents of the last issued command until the AT command prefix is received for the next command.

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