Specifications

AN93
Rev. 0.9 189
Figure 51. ISOModem V80 Protocol HDLC Framing in Framed Sub-Mode
ISOmodems in EPOS Applications
* AT:U87[10] must be set when using Rev B silicon.
* A V.80 interface to V29 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. " A V29FastPOS Sample Program"
on page 210 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 sec. 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 sec. For example, to
command the modem to begin transmitting 3 seconds
after the end of dialing, set AT:U80,8708.
Recording Audio
Recording and examining the audio signals on the
phone line is one of the best debugging techniques for
PSTN modems. Virtually all of the relevant signals are
in the audio spectrum and are easy to acquire using
standard PC sound cards and accessory hardware and
software that is especially designed for music creation
and analysis.
The required hardware is a Radio Shack Catalog No.
43-228A "Recorder Control". It can be used with any
PC.
The resulting wave may be recorded in the field without
any special software, but, for analysis, a software
package capable of showing the Spectral contents as
they change over time is recommended. The two most
widely used ones are Adobe Audition, a commercial
product, and Wavesurfer, which is a free open-source
product that runs on both Windows and Linux.
The technique of recording the audio then does not
replace sophisticated test equipment but is quite useful
in showing up some faults in the line and in the
modem's (DUT) negotiation with the device on the
other side of the phone line.
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*