2 external clock modes, 1 sampled external clock mode - mode 0, External clock modes – CANOGA PERKINS 2240 Fiber Optic Modem User Manual
Page 33: Sampled external clock mode - mode 0
Chapter 3 Mode and Rate Selection
External Clock Modes
33
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.
NOTE: The pulse distortion is 37% of the bit period at a data rate of 1.544 Mbps.
When using this operating mode, it is important to consider the effect of
this large distortion on the connected equipment.
Table 3-3. Standard Internal Clock Rates
Rate
DIP Switches
(C) Closed (O) Open
Data Rates Normal and
Alternate Table Switch
(TBL/NORM) set to NORM
Rate Switches
Group 1
Group 2*
Group 3*
1
2
3
4
0
C
C
C
C
2.048M
1.536M
1.544M
1
O
C
C
C
1.024M
768K
19.2K
2
C
O
C
C
512K
384K
9.6K
3
O
O
C
C
256K
192K
4.8K
4
C
C
O
C
115.2K**
448K
153.6K
5
O
C
O
C
57.6K**
224K
76.8K
6
C
O
O
C
28.8K**
112K
38.4K
7
O
O
O
C
14.4K**
56K
19.2K
8
C
C
C
O
128K
96K
2.4K
9
O
C
C
O
64K
48K
1.2K
* 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.