1 clock polarity option, Oscillator part numbers – CANOGA PERKINS 2270 Fiber Optic Modem User Manual
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2270 Fiber Optic Modem
When the modem is the source of the transmit clock, there is a finite time delay before that clock
arrives at the DTE to clock its transmitter. There is another time delay before the data from the
DTE arrives back at the modem.
Since the modem uses its own clock signal to align the data, there is a potential for these delays
to make the data invalid at the point of realignment. This problem only occurs at high data rates
and if the cable to the DTE is very long or has high capacitance.
There are two methods used to compensate for this possible misalignment between the clock and
data. First, the SCT clock may be inverted (refer to Section 2.7.7.1) or the SCT clock may be
returned on the external clock leads (refer to Section 2.7.7.2).
2.7.7.1 Clock Polarity Option
The modem supplies clock to the DTE on the SCT interface line. Due to transmission and DTE
data delays, this clock may not be properly phased to the transmit data.
If the DTE cannot return this clock back toward the modem on the SCTE or equivalent leads, then
the polarity of the SCT clock may be inverted to compensate for the delay.
Table 2-B. Data Rate Switch Position vs. Oscillator Part Number
Oscillator Part Numbers
(C) losed (O) pen M = megabits K = kilobits
Switch
Setting
2200-C-01
2200-C-02
2200-C-03
2200-C-04
2200-C-05
2200-C-06
1
2
3
C
C
C
10.752M
12.352M
8.192M
12.624M
6.912M
460.8K
C
C
O
5.376M
6.176M
4.096M
6.312M
3.456M
230.4K
C
O
C
2.688M
3.088M
2.048M
3.156M
1.728M
115.2K
C
O
O
1.344M
1.544M
1.024M
1.578M
864.0K
57.60K
O
C
C
448.0K
514.7K
341.3K
526.0K
288.0K
19.20K
O
C
O
224.0K
257.3K
170.7K
263.0K
144.0K
9.600K
O
O
C
112.0K
128.7K
85.33K
131.5K
72.00K
4.800K
O
O
O
56.00K
64.33K
42.67K
65.75K
36.00K
2.400K
OSCILLATOR
FREQUENCY
21.504M
24.704M
16.384M
25.248M
13.824M
921.6K
Switch
Setting
2200-C-07
2200-C-08
2200-C-09
2200-C-10
2200-C-11 2200-C-12
2200-C-13
1
2
3
C
C
C
3.686M
20.00M
6.464M
3.072M
12.928M
115.2K
12.288M
C
C
O
1.843M
10.00M
3.232.M
1.536M
6.464M
57.60K
6.144M
C
O
C
921.6K
5.000M
1.616M
768.0K
3.232M
28.80K
3.072M
C
O
O
460.8K
2.500M
808.0K
384.0K
1.616M
14.40K
1.536M
O
C
C
153.6K
833.3K
269.3K
128.0K
538.7K
4.800K
512.0K
O
C
O
76.80K
416.7K
134.6K
64.00K
269.3K
2.400K
256.0K
O
O
C
38.40K
208.3K
67.33K
32.00K
134.7K
1.200K
128.0K
O
O
O
19.20K
104.2K
33.67K
16.00K
67.33K
0.600K
64.0K
OSCILLATOR
FREQUENCY
7.373M
40.00M
12.928M
6.144M
25.856M
230.4K 24.576M