XRT59L921 Datasheet, PDF (17/32 Pages) Exar Corporation – TWENTY-ONE CHANNEL E1 LINE INTERFACE UNIT

XRT59L921
REV. 1.2.1
TWENTY-ONE CHANNEL E1 LINE INTERFACE UNIT
SYSTEM DESCRIPTION
The XRT59L921 is a Twenty-One (21) channel E1 transceiver that provides an electrical interface for
2.048Mbps applications. Each of the twenty-one channels includes a receive circuit that converts an ITU-T
G.703 compliant bipolar signal into a TTL compatible logic levels. Each receiver also includes an LOS (Loss of
Signal) detection circuit. Similarly, in the Transmit Direction, each Transmitter converts TTL compatible logic
levels into a G.703 compatible bipolar signal. The Transmitter may be operated in either a Clock or Clockless
Mode.
Each channel within the XRT59L921 LIU consists of both a Transmit Section and a Receive Section, each of
these sections will be discussed in detail below.
1.0 THE TRANSMIT SECTION
In general, the purpose of the Transmit Section (within each channel of the XRT59L921) is to accept TTL/
CMOS level digital data (from the Terminal Equipment), and to encode it into a format such that it can:
1. Be efficiently transmitted over coaxial- or twistedpair cable at the E1 data rate and
2. Be reliably received by the Remote Terminal Equipment at the other end of the E1 data link.
3. Comply with the ITU-T G.703 pulse template requirements, for E1 applications.
The circuitry that the Transmit Section (within the XRT59L921) uses to accomplish this goal is discussed
below. The Transmit Section of the XRT59L921 consists of the following blocks:
• Transmit Input Interface
• Pulse Shaping Block
1.1 The Transmit Input Interface
The Transmit Input Interface accepts either Clock or clockless data from the Terminal Equipment. The manner
in how the Terminal Equipment should apply data to a given channel within the XRT59L921 depends upon
whether the channel is being operated in the Clock or clockless mode.
1.2 Operating the Transmitter in the Clock Mode
The user can configure a given channel (within the XRT59L921) to operate in the Clock mode by simply
applying a 2.048MHz clock signal to the TxClk_n input pin (where x denotes a given channel within the
XRT59L921). The XRT59L921 contains circuitry that senses activity on the TxClk_n line. If this circuit senses
activity on the TxClk_n line, then the corresponding channel will automatically be operating in the Clock Mode.
In the Clock Mode, the Terminal Equipment is expected to apply a 2.048MHz clock signal at the TxClk_n input
pin and NRZ data at the TxPOS_n and TxNEG_n input pins. The Transmit Input Interface circuit will sample
the data, at the TxPOS_n and TxNEG_n input pins, upon the falling edge of TxClk_n, as illustrated in Figure 4.
15