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V.25 bis Command Set Operation
Synchronous Format Selection
The V. 34 supports both the bit-oriented and the byte-oriented modes of synchronous operation.
Synchronous Bit-Oriented Operation
The modem uses the HDLC format in the bit-oriented synchronous mode (%V2 AT command).
The structure of each message in this format is shown below:
Time
F
F
= Idle Flag (01111110)
FCS
= Frame Check Sequence (CRC-ITU-T, x16+x12+x5+1)
Message
= Command/Response (7-bit ASCII data and 1 odd parity)
C
= Control (00010011*)
A
= Address (11111111*)
*The modem does not recognize the C or A fields unless verified as shown above.
The modem uses the BISYNC (BSC) format in the byte-oriented synchronous mode (%V3 AT
command). The structure of each message in this format is shown below:
Time
BCC
BCC
ETX
Message
STX
SYN
Note
BCC is always transmitted in the responses of the modem, but optionally included in the DTE commands.
In this way, the modem conforms to BSC conventions without deviating from the ITU-T V.25 bis Recom-
mendations.
Data Rate
The V. 34 does not support 300 bps in the synchronous mode. Also, in the synchronous mode, the
modem does not handshake with an incoming call at 300 bps FSK.
060R122-000
Issue 15
FCS
Message
C
ETX
Message
STX
= LRC Block Check Character for ASCII
= ASCII ETX Character (10000011)
= Command/Response (7-bit ASCII data and 1 odd parity)
= ASCII STX Character (00000010)
= ASCII SYN Character (00010110)
SpectraComm Dual V.34 Modem
Installation and Operation
V.25 bis 108.2 Mode Interface Specifications
A
SYN
F
SYN
5-13
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