XRT72L56 Datasheet, PDF (295/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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PRELIMINARY
XRT72L56
SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
REV. P1.1.2
FIGURE 114. ILLUSTRATION OF THE SIGNAL THAT MUST OCCUR BETWEEN THE TERMINAL EQUIPMENT AND THE
XRT72L56 IN ORDER TO CONFIGURE THE XRT72L56 TO TRANSMIT A YELLOW ALARM TO THE REMOTE TERMINAL
EQUIPMENT
Terminal Equipment/XRT72L5x Interface Signals
0
1
4
5
6
7
8
9
10 10-
TxOHClk
TxOHFrame
TxOHIns
TxOH
Remaining Overhead Bits with E3 Frame
A bit = 1
TxOHFrame is sampled “High”
Terminal Equipment asserts TxOHIns and
Data on TxOH line.
XRT72L5x Framer device samples TxOH and
TxOHIns signals
In Figure 114 the Terminal Equipment samples the
TxOHFrame signal being "High" at rising clock edge #
0. From this point, the Terminal Equipment will wait
until it has detected the 10th rising edge of the TxO-
HClk signal. At this point, the Terminal Equipment
knows that the XRT72L56 is just about to process the
“A” bit within a given outbound E3 frame. Additionally,
according to Table 50, the 10th overhead bit to be
processed is the "A" bit. In order to facilitate the
transmission of the Yellow Alarm, the Terminal Equip-
ment must set this "A" bit to "1". Hence, the Terminal
Equipment starts this process by implementing the
following steps concurrently.
a. Assert the TxOHIns input pin by setting it "High".
b. Set the TxOH input pin to "1".
After the Terminal Equipment has applied these sig-
nals, the XRT72L56 will sample the data on both the
TxOHIns and TxOH signals upon the very next falling
edge of TxOHClk (designated as "10-" in Figure 114).
Once the XRT72L56 has sampled this data, it will
then insert a "1" into the "A" bit position, in the out-
bound E3 frame.
Upon detection of the very next rising edge of the Tx-
OHClk clock signal (designated as clock edge 1 in
Figure 114, the Terminal Equipment will negate the
TxOHIns signal (e.g., toggles it "Low") and will cease
inserting data into the Transmit Overhead Data Input
Interface.
After the Terminal Equipment has performed this in-
sertion procedure, it leaves the remaining overhead
bits (within this particular outbound E3 frame) in-tact,
by terminating this Overhead Bit Insertion proce-
dure. The Terminal Equipment should now terminate
this overhead bit insertion, by doing the following.
a. Assert the TxOHIns input pin by setting it "High".
b. Set the TxOH input to "0".
If the Terminal Equipment wishes to continue its
transmission of the Yellow Alarm condition to the Re-
mote Terminal Equipment, then it should resume the
Overhead Bit Insertion procedure (as described
above), at the beginning of each outbound E3 frame
(or each time TxOHFrame is sampled "High”).
5.2.2.2 Method 2 - Using the TxInClk and TxO-
HEnable Signals
Method 1 requires the use of an additional clock sig-
nal, TxOHClk. However, there may be a situation in
which the user does not wish to add this extra clock
signal to their design, in order to use the Transmit
Overhead Data Input Interface. Hence, Method 2 is
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