Silicon Laboratories SI2493/57/34/15/04, SI2494/39 manual ISOmodem in Epos Applications

AN93

					Isomodem with V.80
		UART					HDLC			TX Bit	PLL
Data							FLAG			Clock
Block							Insertion
Data
Data
Block					V.80
Block	V.80							Zero-Bit		Modulate
					Transparency
	Transparency	Transmit	TXD	MODEM					Stuffing
					Decode
	Encode	Path	CTS*
				UART					g
	Insert V.80
	EM<FLAG>				Frame Check
						Sequence
	between blocks
							Generate
	V.80 Handler									RX Bit Clock
	V.80						HDLC
							FLAG			Recovery
	Transparency
						Detection
	Decode
	EM<FLAG> if good	Receive	RXD	MODEM	V.80			Zero-Bit		Demodulate
					Transparency
		Path		UART				Deletion
	RX frame, EM<ERR>				Encode
Data	if bad RX frame								nCRC
					Add EM<FLAG>
DataBlock									Check
Data					or EM <ERR>
Block
Block

Figure 51. ISOmodem V.80 Protocol HDLC Framing in Framed Sub-Mode

The ISOmodem in EPOS Applications

XTAL

DCE

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.

260 Rev. 1.3