XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
á
349
Mode 2 Operation of the Terminal Equipment
As shown in
Figure 147
, both the Terminal Equipment and the XRT72L50 will be driven by an external
34.368MHz clock signal. The Terminal Equipment will receive the 34.368MHz clock signal via its E3_Clock_In
input pin, and the XRT72L50 Framer IC will receive the 34.368MHz clock signal via the TxInClk input pin.
The Terminal Equipment will serially output the payload data of the Outbound E3 data stream, via the
E3_Data_Out output pin, upon the rising edge of the signal at the E3_Clock_In input pin. (Note: The
E3_Data_Out output pin of the Terminal Equipment is electrically connected to the TxSer input pin). The
XRT72L50 Framer IC will latch the data, residing on the TxSer input line, on the rising edge of the TxInClk
signal.
In this case, the Terminal Equipment has the responsibility of providing the framing reference signal by pulsing
its Tx_Start_of_Frame output signal (and in turn, the TxFrameRef input pin of the XRT72L50), "High" for one-
bit period, coincident with the first bit of a new E3 frame. Once the XRT72L50 detects the rising edge of the
input at its TxFrameRef input pin, it will begin generation of a new E3 frame.
N
OTES
:
1.
In this case, the Terminal Equipment is controlling the start of Frame Generation, and is therefore referred to as
the Frame Master. Conversely, since the XRT72L50 does not control the generationi of a new E3 frame, but is
rather driven by the Terminal Equipment, the XRT72L50 is referred to as the Frame Slave.
2.
If the user opts to configure the XRT72L50 to operate in Mode 2, it is imperative that the Tx_Start_of_Frame (or
TxFrameRef) signal is synchronized to the TxInClk input clock signal.
Finally, the XRT72L50 will pulse its TxOH_Ind output pin, one bit-period prior to it processing a given overhead
bit, within the Outbound E3 frame. Since the TxOH_Ind output pin (of the XRT72L50) is electrically connected
to the E3_Overhead_Ind, whenever the XRT72L50 pulses the TxOH_Ind output pin "High", it will also be
driving the E3_Overhead_Ind input pin (of the Terminal Equipment) "High". Whenever the Terminal Equipment
detects this pin toggling "High", it should delay transmission of the very next E3 frame payload bit by one clock
cycle.
The behavior of the signals between the XRT72L50 and the Terminal Equipment for E3 Mode 2 Operation is
illustrated in
Figure 148
.
F
IGURE
147. T
HE
T
ERMINAL
E
QUIPMENT
BEING
INTERFACED
TO
THE
T
RANSMIT
P
AYLOAD
D
ATA
I
NPUT
I
NTERFACE
BLOCK
FOR
M
ODE
2 (S
ERIAL
/L
OCAL
-T
IMED
/F
RAME
-S
LAVE
) O
PERATION
E3_Clock_In
E3_Data_Out
Tx_Start_of_Frame
E3_Overhead_Ind
TxInClk
TxSer
TxFrameRef
TxOH_Ind
NibIntf
Terminal Equipment
E3 Framer
34.368MHz
Clock Source