Intel 536EX manual 1.3Modem Connection Overview, Intel Confidential, Introduction

Introduction

1.3Modem Connection Overview

The DCE (modem) operates in one of two states: command or online. In each state, both data and commands (including DCE responses) are transferred through the UART THR (Transmit Holding register) and the RBR (Receiver Buffer register).

The modem defaults to the command state. In the command state, the DTE (host) communicates to the modem through AT commands and S-registers. AT commands are character strings that help guide modem operation. S-registers are internal modem registers that the DTE can access. The S- registers contain modem status and configuration information. Many of the AT commands indirectly affect the contents of the S-registers. The AT command set and S-registers are divided into the following modes: Group 3 fax, data, V.44/V.42 bis/V.42/MNP, and voice. Note that supported AT commands are firmware revision-dependent (that is, not all commands are supported by all modem models or all firmware revisions).

All command lines sent to the modem, except for A/, must be preceded by an ‘AT’ (which stands for ‘attention’) and terminated by the contents of S-register S3 (typically a carriage return <CR>). The ‘AT’ prompts the modem to receive a command line from the DTE. A <CR> informs the modem that the entire command string has been transmitted and that the modem should start processing all the commands within the command line.

A command line may include one or more AT commands that may or may not be separated by a space. AT commands may be either upper- or lower-case characters, but all characters for a given command must use the same case. If there are multiple commands in a line, a semicolon (;) must be placed after each fax or voice command. The modem can be configured to send back (echo) to the DTE any data that the DTE sends to the modem (while in command state only). The last command may be repeated by typing A/ without using a carriage return. Each command line may include up to 80 characters and spaces.

Examples of AT command strings:

ATS1?<CR>

A/

AT &C1 &D2 +FCLASS=? <CR>

AT &C1 &D2 +FCLASS=?; S0=1 <CR>

The modem provides status information to the DTE in the form of response codes. These response codes can be expressed in text or numeric form. The supported response codes are provided in Table 4 on page 12.

Examples of modem responses:

+MCR: V90

+MRR: 28800

CONNECT 50666

In the online state, the DCE is off-hook and communicating with a remote modem. Any data sent from the DTE to the DCE is transmitted to the remote modem. Similarly, any data that the DCE receives from the remote modem is transmitted to the DTE.

Note: In the online state, the DCE does not ‘echo-back’ any of the data that the DTE sent to the DCE.

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