XRT83L34_05 Datasheet, PDF (41/99 Pages) Exar Corporation – QUAD T1/E1/J1 LH/SH TRANSCEIVER WITH CLOCK RECOVERY AND JITTER ATTENUATOR

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XRT83L34
QUAD T1/E1/J1 LH/SH TRANSCEIVER WITH CLOCK RECOVERY AND JITTER ATTENUATOR REV. 1.0.1
FIGURE 15. SIMPLIFIED DIAGRAM FOR E1 IN EXTERNAL TERMINATION MODE (RXTSEL= 0)
X R T 83L 34 L IU
T T IP
9 .1 Ω
TR IN G
R TIP
R R IN G
9 .1 Ω
75Ω
1 :2 .4 5
1 :1
75Ω
75Ω
TRANSMITTER (CHANNELS 0 - 3)
TRANSMIT TERMINATION MODE
In Hardware mode, TXTSEL (Pin 84) can be tied “High” to select internal termination mode for all transmit
channels or tied “Low” for external termination. Individual channel control can be done only in Host mode. In
Host mode, bit 6 in the appropriate register for a given channel is set “High” to select the internal termination
mode for that specific transmit channel, see Table 21, “Microprocessor Register #1, Bit Description,” on
page 64.
TABLE 8: TRANSMIT TERMINATION CONTROL
TXTSEL
0
1
TX TERMINATION
EXTERNAL
INTERNAL
TX TRANSFORMER RATIO
1:2.45
1:2
For internal termination, the transformer turns ratio is always 1:2. In internal mode, no external resistors are
used. An external capacitor of 0.68µF is used for proper operation of the internal termination circuitry, see
Figure 13.
TABLE 9: TERMINATION SELECT CONTROL
TERSEL1
0
0
1
1
TERSEL0
0
1
0
1
TERMINATION
100Ω
110Ω
75Ω
120Ω
EXTERNAL TRANSMIT TERMINATION MODE
By default the XRT83L34 is set for external termination mode at power up or at Hardware reset.
When external transmit termination mode is selected, the internal termination circuitry is disabled. The value of
the external resistors is chosen for a specific application according to the turns ratio selected by TRATIO (Pin
127) in Hardware mode or bit 0 in the appropriate register for a specific channel in Host mode, see Table 10
and Table 23, “Microprocessor Register #3, Bit Description,” on page 68. Figure 14 is a simplified block
diagram for T1 (100Ω) in the external termination mode. Figure 15 is a simplified block diagram for E1 (75Ω) in
the external termination mode.
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