The ISOmodem in EPOS Applications. Silicon Laboratories Si2493, Si2404, Si2415, Si2434, Si2494, Si2439, Si2457, Si2443

AN93

Data

Data

Data

Block

V.80

Transparency

Encode

Insert V.80

EM<FLAG> between blocks

V.80 Handler

V.80

Transparency

Decode

Data

Data

Block

Block

EM<FLAG> if good

RX frame, EM<ERR> if bad RX frame

UART

Transmit

Path

TXD

CTS*

Receive

Path

RXD

MODEM

UART

MODEM

UART

Isomodem with V.80

HDLC

FLAG

Insertion

V.80

Transparency

Decode

Zero-Bit

Stuffing g

Frame Check

Sequence

Generate

HDLC

FLAG

Detection

V.80

Transparency

Encode

Add EM<FLAG> or EM <ERR>

Zero-Bit

Deletion

Check

TX Bit

Clock

Modulate

RX Bit Clock

Recovery

Demodulate

Figure 53. ISOmodem V.80 Protocol HDLC Framing in Framed Sub-Mode

PLL

XTAL

DCE

The ISOmodem in EPOS Applications

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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

133 outlines how to use this patch. The Si2494/39 and Revision D of the Si2493/57/34/15 do not require a patch

for V.29 Fast Connect.

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.

When operating as V.29 Fast Connect, UD3 sets the minimum and maximum time for the duration of the

Answer Tone. The allowable range is 50 ms to 180 ms. UD3 is in units of milliseconds. The default is 0x0000.

U1DA allows the user to add a delay to the time the modem waits before responding to an answer tone. This is useful in dealing with non-standard answering modems. The units of this register are 1/600 seconds. The default is 0x0078, or 200 milliseconds.

Rev. 1.41

273