XRT5897
12
Rev. 1.10
SYSTEM DESCRIPTION
This device is a seven channel E1 transceiver that
provides
an
electrical
interface
applications.
Its unique architecture includes seven
receivercircuits thatconvertITU G.703 compliantbipolar
signals to TTL compatible logic levels. Each receiver
includes a LOS (Loss ofSignal) detectioncircuitthatmay
be configured for either a fixed or a user-programmable
LOS response time delay.
direction, seven transmitters convert TTL compatible
logic levels to G.703 compatible bipolar signals. Each
transmittermay be operated eitherwith RZ, orNRZ data
types. In NRZ mode a transmitclock is required as well.
The following description applies to any of the seven
receivers or transmitters contained in the XRT5897.
Therefore, the suffix numbers fora particularchannelare
deletedforsimplicity. i.e.“RTIP”applies toRTIP1through
RTIP7.
for
2.048Mbps
Similarly, in the transmit
Receiver Operation
A bipolar signal is transformer-coupled to the receiver
differentialinputs (RTIP andRRING). Thereceiveris able
totolerate upto12dB ofline loss measuredat1.024MHz.
Itcontains slicing circuitry thatautomatically samples the
incoming data ata fixedpercentage (50% nominal) ofthe
peak signal amplitude. A precision peak detector
maintains the slicing levelaccuracy. The TTL compatible
receiver output data rails appear at the RXPOS and
RXNEG pins. The pulse widthofthis data; whichis inRZ
format, is a function of the amount of the cable loss
present.
Receiver Loss Of Signal Detection (LOS)
Absence ofsignalatany receiverinputis detectedby the
loss ofsignal(LOS) circuit. OneLOS detectioncircuitryis
provisionedforeachreceiver. The LOS signalis asserted
(LOS=1) whena LOS conditionis detectedandis cleared
(LOS=0) when a valid input signal is restored.
Two modes of LOS circuit operation are supported.
These distinct modes are called “automatic” and
“user-programmable”. When LOSSEL (pin 38) is set to
logic “1”, theautomatic modeis selected. Inthis modethe
LOS condition will be declared and cleared in full
compliancewithITU G.775specification. WhenLOSSEL
is connected to logic “0”, the user-programmable delay
mode is enabled. In this mode the userhas the optionof
extending the delay of LOS declaration and clearing
specified in the ITU G.775. This is done by providing a
user-supplied clock to LOSCNT (pin 73).
programmable mode” is provisioned to allow systems
designers to comply with older versions of LOS
specifications inlegacy systems. Itneeds to be stressed
that the delay for declaration and clearing of the LOS
conditionwillneverbe less thanthe rangespecifiedinthe
G.775 specification (10-255 pulse intervals).
The “user
The LOS detection/clearing circuitry of the XRT5897 in
“automatic” mode will detect LOS when the incoming
signalhas “notransitions,”i.e.whenthesignallevelis less
thanorequaltoa signallevelA
D
dB belownominalsignal
level,forNconsecutivepulseintervals, where10
<
N
<
255.
The value of A
D
can vary between 10dB to 32dB
depending on the ones density of the incoming signal
assuming the received data has minimum permissible
ones density. Furthermore LOS detect is cleared when
the incoming signalhas “transitions,” i.e. whenthe signal
level is greater than or equal to a signal level of A
C
dB
below nominal, for N consecutive pulse intervals, where
10
<
N
<
255. The value of A
C
can vary between 9dB to
31dB depending on the ones density of the incoming
signal assuming the received data has minimum
permissible ones density. Eachpulse intervalis 488ns at
E1rates. The absolute value ofA
C
is always smallerthan
A
D
by at least 1dB.
The LOS detection/clearing criteria described above is
fully compliantwithG.775 LOS specification. Inthe “user
programmable” mode the user has the option of
extending the declarationandclearing delay (10
<
N
<
255)
by an amount which is equal to 2048 x T. T is the time
period of the clock supplied to LOSCNT (pin 73) by the
user.
Nominal signal level is defined as 2.37V peak measured
between RTIP or RRING and ground. (This voltage will
be presentin 75
applications using a 1:2 CT ratio input
transformer terminated in 300
with the center tap
grounded with 0dB of cable and a 2.37V peak amplitude
transmit pulse at the cable input.)