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Table 3-C.
Standard Internal
Clock Rates
DIP Switches
(C) Closed
Rate Switches
Rate
1
2
0
C
C
1
O
C
2
C
O
3
O
O
4
C
C
5
O
C
6
C
O
7
O
O
8
C
C
9
O
C
* These Data Rates, except 1.544M, have up to 125 ns of jitter
** These Data Rates actually run 0.7% higher than noted and have up to 125 ns
jitter.
3.2 External Clock Modes
The external clock modes are used when it is necessary to have the DTE provide the
transmit clock or when the 2240 is used as a tail circuit connecting to a DCE. In these
modes, the DTE or DCE sends this clock to the modem on the Terminal Timing (TT)
or equivalent signal leads. For an example of a typical complete tail circuit, refer to
Section 3.4. There are two different types of External Clock Modes in the 2240:
Sampled and Locked.
NOTE: Interfaces which extract the clock from a composite signal, such
as T1 or E1, require the use of the Locked External Clock Mode.
3.2.1 Sampled External Clock Mode - Mode 0
In this mode, the 2240 transmits an 8.192 Mbps optical composite signal which is
derived from an internal oscillator. One half of the composite bandwidth is used to
send the clock signal which is sampled at 4.096 MHz. One fourth of the composite
bandwidth is used to send the data signal which is sampled at 2.048 MHz. This
sampling results in 244 nanoseconds of pulse distortion on the clock received at the
other modem. The distortion is a result of the sampling process. The maximum data
rate is limited to 1.544 Mbps where the distortion is 37% of the clock period.
(O) Open
Normal and Alternate Table Switch
(TBL / NORM) set to NORM
3
4
Group 1
C
C
2.048M
C
C
1.024M
C
C
512K
C
C
256K
O
C
115.2K**
O
C
57.6K**
O
C
28.8K**
O
C
14.4K**
C
O
128K
C
O
64K
2240 Fiber Optic Modem
Data Rates
Group 2*
Group 3*
1.536M
1.544M
768K
19.2K
384K
9.6K
192K
4.8K
448K
153.6K
224K
76.8K
112K
38.4K
56K
19.2K
96K
2.4K
48K
1.2K
31
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