User manual

21
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
123
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
123
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