XRT59L91 Datasheet, PDF (18/28 Pages) Exar Corporation – Single-Chip E1 Line Interface Unit

XRT59L91
Preliminary
3.0 Diagnostic Features
In order to support diagnostic operations, the
XRT59L91 supports the following loopback modes:
l Local Loopback
l Remote Loopback
Each of these loopback modes will be discussed
below.
3.1 The Local Loop-Back Mode
When the XRT59L91 device is configured to operate in
the “Local Loop-back” Mode, the XRT59L91 device will
ignore any signals that are input to the RTIP and RRing
input pins. The “Transmitting Terminal Equipment” will
transmit data (and clock, for “Clocked”
Mode) into the XRT59L91 device via the TxPOS,
TxNEG and TxCLK input pins. This data will be
processed through the “Transmit Terminal Input Inter-
face” and the “Pulse Shaping” circuit. Finally, this data
will be output to the line via the TTIP and TRing output
pins. Additionally, this data (which is being output via
the TTIP and TRing output pins) will be looped back into
the “Receive Equalizer” block. As a consequence, this
data will also be processed through the entire “Receive
Section” of the XRT59L91 device. After this “post-loop-
back” data has been processed through the “Receive
Section” it will output, to the “Near-End Receiving
Terminal Equipment” via the “RxPOS and RxNEG
output pins.
Figure 12, illustrates the path that the data takes
(within the XRT59L91 device), when the chip is config-
ured to operate in the “Local Loop-back” Mode.
R T IP
R R in g
LLoop
R eceive
E q u a liz e r
Peak Detector/
S lic e r
RxLOS
LOS
D etector
Local
Loop Back
MUX
Local Loop Back
Path
TTIP
T R in g
Pulse Shaping
C ircuit
R eceive O utput
Interface
RxPOS
RxNEG
R em ote
Loop Back
MUX
RLoop
Transmit Input
Interface
TxPOS
TxNEG
T x C lk
Figure 12. Illustration of the “Local Loop-back” within the XRT59L91 Device
Rev. P2.00
18