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V.25 bis Command Set Operation
V.25 bis Operating Modes
The ITU-T V.25 bis protocol operates in one of two modes, named for the two ITU-T designations
for circuit 108 of the terminal interface: 108.2 mode or 108.1 mode.
•
In 108.2 mode, the circuit is the "Data Terminal Ready" signal.
•
In 108.1 mode, the circuit is the "Connect Data Set to Line" signal.
Note
Circuit 108 is Pin 20 in a DB25 connector; in a 34-Pin V.35 connector it is Pin H.
A brief description of each operating mode is described next. Refer to the
Command Procedures
108.2 and 108.1 modes. Refer to
synchronous dialing protocol formats.
108.2 Mode
The 108.2 operating mode supports convenient outbound calling to multiple locations. When the
DTE instructs the modem to originate a call in 108.2 mode, it also specifies the number to be dialed.
The DTE can either supply the number itself along with the command to dial, or identify which of
the modem's memory cells contains the number to be dialed.
In 108.2 mode the modem supports an ITU-T V.25 bis compatible command set you can use from
the DTE to interrogate and configure the modem, and to control communication and test functions.
Configuration by means of the command set can include operating parameters such as data rate,
handshake mode, and synchronous timing source; as well as storage of phone numbers in the
modem's memory cells.
In order for the V.25 bis command set to function, the modem must be operating in 108.2 mode and
terminal interface circuit 108.2 (Data Terminal Ready) must be On. When circuit 108.2 is Off, the
modem ignores the commands. Circuit 108.2 can be forced On with the &D0 AT command (this
can only be done in asynchronous operations).
108.1 Mode
The 108.1 operating mode supports automatic outbound calling directed primarily to a single
location. The DTE instructs the modem to originate a call by turning On circuit 108.1, Connect
Data Set to Line. The modem then goes off-hook, dials the number stored in its memory cell 0, and
attempts to establish communication.
The modem will disconnect from the line if it does not successfully handshake in the time specified
in the abort timer selected by the S7=x AT command (default is 30 seconds). If you turn Off 108.1
while the modem is in the data mode, the modem will hang up and disconnect.
The ITU-T V.25 bis compatible command set is not available when the modem is operating in 108.1
mode. Storage of a telephone number in memory cell 0 must be performed either in 108.2 mode
(using the PRN command), or by means of AT commands. Any number can be called in 108.1
mode by dialing manually and then turning On circuit 108.1 when answer tone is heard from the
remote modem.
5-2
in this chapter for a full description of the communication procedures for the
V.25 bis 108.2 Mode Interface Specifications
SpectraComm Dual V.34 Modem
Installation and Operation
V.25 bis Protocol Overview
ITU-T V.25 bis
in this chapter for
060R122-000
Issue 15
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