- Silicon Laboratories, Inc. Modem User Manual
AN93
260 Rev. 1.3
Figure 51. ISOmodem V.80 Protocol HDLC Framing in Framed Sub-Mode
The ISOmodem in EPOS Applications
The Rev D Si2493/57/34/15/04 and Rev A Si2494/39 include all Rev B and C patches.
AT:U87 [10] must be set when using Rev B silicon.
A V.80 interface to V.29 Fast Connect is not supported on Rev B silicon and can be accomplished only as a
patch on Rev C. Please contact Silicon Laboratories, Inc. for latest patch. "5.8. Firmware Upgrades" on page
121 outlines how to use this patch.
When operating as V.22 Fast Connect (+MS=V22, AT:U7A,3), the register U80 can be modified to account for
unusual server timings. The value in U80 should reflect the expected answer tone duration of the NAC. The
units are in 1/600 s. For example, if the answer tone duration of the server is 500 ms, AT:U80,012C.
When operating as V.22 Fast Connect (+MS=V22, AT:U7A,3), a short answer tone of at least 300 ms is required
for proper operation. This answer tone can be 2100 Hz, 2225 Hz, or a V.22 Unscrambled Binary Ones (USB1).
If the server NAC does not have any of these answer tones prior to scrambled data or HDLC flags, it is possible
to command the modem to operate without these tones by setting bit 15 of U80. The modem then begins
transmitting scrambled data (or HDLC Flags) some time after the end of dialing, based on the value in
U80 [14:0]. The units are in 1/600 s. For example, to command the modem to begin transmitting 3 s after the
end of dialing, set AT:U80,8708.
Modulate
Data
Block
Data
Block
V.80
Transparency
Encode
V.80
Transparency
Decode
MODEM
UART
HDLC
FLAG
Insertion
Zero-Bit
Stuffing
g
Frame Check
Sequence
Generate
Data
Block
Data
Block
Data
Block
V.80
Transparency
Decode
Receive
Path
HDLC
FLAG
Detection
Demodulate
Insert V.80
EM<FLAG>
between blocks
V.80
Transparency
Encode
Zero-Bit
Deletion
n
CRC
Check
EM<FLAG> if good
RX frame, EM<ERR>
if bad RX frame
TX Bit
Clock
TXD
RXD
V.80 Handler
Add EM<FLAG>
or EM <ERR>
DCE
PLL
XTAL
RX Bit Clock
Recovery
Isomodem with V.80
UART
Transmit
Path
Data
Block
MODEM
UART
CTS*