B.2 Modems

Common modems have their own power supply, or are powered from the PC bus; they do not depend on the serial leads for power.

Serial Data Leads

Common modems connect to the TXD and RXD leads, and use asynchronously-framed character-string "AT" commands and responses (e.g., "Hayes (tm) -compatible, TIA-602, etc) for control.

Common modems can adaptively detect the asynchronous serial rate, over a range of supported speeds, by measuring the start bit time in the AT (or "at") command string prefix. This method is called "Autobauding". All but very old modems can support 1200 bit/s; some can autobaud to 57,600 bit/s. TIA-602 requires that all compliant modems be able to detect commands at 1200 or 9600 bit/s.

Control Leads

Common modems will also respond to the two control leads, DTR and RTS. The modem's response to these leads is typically programmable using in-band AT commands. For example, the common AT&D command selects one of the following responses to a negative transition on DTR:

Table B-1.  AT&D responses

&D setting

response to DTR-OFF transition

0

Modem ignores DTR

1

Modem escapes from ON-LINE state to ON-LINE COMMAND state.

2

Modem exits ON-LINE state and goes On-Hook (IDLE).

3

Modem resets (not in TIA-602, but common)

See Annex A.4 above for further information.


The "RTS" lead is commonly used to represent the Ready-to-Receive function, for use in flow control of data from the modem, during On-Line state.

Because modems try to interpret DTR and RTS for traditional uses, the COM Enumerator is constrained to use those leads very distinctly from those traditional uses. The method described in section 2.1 depends on using these leads while the modem is IDLE (On-hook) and with a distinct time-signature.

Note that the implementation of the &D3 command poses special problems. See Annex A.4.

Status Leads

Common modems will drive four serial status leads, DSR, CTS, RLSD and RI (see details in section 1.5). Some modems echo DTR to DSR; other assert it automatically when powered. The AT&C command conditions RLSD behavior. The RI lead is typically driven only in response to actually ringing.