This Recommendation describes modem procedures to follow for automatic interworking with text telephones. Text telephones use various modem technologies. This Recommendation specifies the signal analysis, signal transmission and logic needed to determine what kind of text telephone there is on a connection. It also specifies the actions needed to communicate in the mode supported by each terminal type.

This Recommendation is intended for use in text telephones, in interworking units, in text relay services, in emergency centers, and in computers to be used for text telephony in the PSTN.

This Recommendation specifies transmission of identification signals to determine when the connection is between two V.18-equipped terminals. For that case, V.21 is the default modulation used. For interworking in text conversation between humans, not only the modulation must be specified. Therefore, this Recommendation specifies that when the connection in V.18 mode is established, the presentation protocol specified in Recommendation T.140 should be used, including an internationally useful character set.

The text telephone types supported by this Recommendation are: EDT, 5-bit (or Baudot), DTMF, V.21, V.23, Bell 103 and V.18-based devices.

In originate mode, V.18 identification signals and V.23 stimulation signals are transmitted until a recognized text telephone signal is received and connection can be established in that mode.

In answer mode, this Recommendation specifies stimulation to connection by transmission of probing signals for the different kinds of text telephones while monitoring for text telephone signals. Once determined, the mode of communication is entered.

For cases when it is not obvious if the connection should be made in originate or answer mode, procedures are provided to resolve that and reach communication.

An automode monitor mode is provided for cases when it is desired to have a text telephone device prepared on the same line as a voice telephone and indicate when there is an active text telephone on the connection.

For DTMF and 5-bit text telephone types using character coding not commonly used with modems, conversion is specified in this Recommendation between these codes and T.50.

For selection between multimedia protocols and this Recommendation, modem connection procedures based on Recommendation V.8 bis are provided.