2 locked external clock mode - mode 7, 3 internal clock modes - modes 1, 2, 3, 4, Locked external clock mode - mode 7 – CANOGA PERKINS 2240 Fiber Optic Modem User Manual
Page 34: Internal clock modes - modes 1, 2, 3, 4
2240 Fiber Optic Modem User Manual
Chapter 3 Mode and Rate Selection
Internal Clock Modes - Modes 1, 2, 3, 4
34
Sampled External Clock Mode does not use the Rate Switches.
3.2.2 Locked External Clock Mode - Mode 7
When the customer-supplied clock is within certain ranges, this mode allows transmission of
clock and data signals with minimal jitter. In the Locked mode, the entire transmitter section of the
2240 is locked to the clock provided by the DTE. The Locked mode is always used for T1 (1.544
Mbps), E1 (2.048 Mbps), any synchronous data transmission between 1.490 Mbps and 2.060
Mbps and possibly at lower speeds if the customer's equipment cannot tolerate the pulse jitter of
the sampled external clock mode.
NOTE: Since the customer's equipment supplies the transmit clock in Mode 7, the
2240 turns off its ST or equivalent signal leads.
NOTE: The use of front panel DIP switch position 8 to select the LOW frequency
ranges shown in Table 3-2 is an enhancement feature added to the 2240
after mid-summer 1996. Earlier versions of the 2240 do not have this
enhancement.
NOTE: Set the Rate switches to the appropriate setting for your data rate. Refer
to Table 3-2 for the rate switch settings and the range of data rates which
use the Locked External Clock Mode. If the desired data rate falls below
128 kHz, the Sampled External Clock Mode must be used.
3.3 Internal Clock Modes - Modes 1, 2, 3, 4
The internal clock modes are used to provide the Transmit Clock for the DTE. In these modes,
the modem sends the clock to the DTE on the Send Timing (ST), or equivalent, signal leads.
Each of the four modes provides a separate group of clock frequencies. Each of the four modes
provides a separate group of clock frequencies. The first three groups of clock rates are
synthesized from standard frequency references and are shown in Table 3-3. The fourth group
allows for a custom set of frequencies to be provided if an additional oscillator is specified for the
modem prior to purchase. Oscillators can be changed in the field, if necessary. In Group 4, the
Rate Switches select the divider ratio for this oscillator. Refer to Table 3-4 and section 3.3.2 for
more details.
Table 3-4. Group 4 Internal Clock Rate Divide Ratio
Rate
DIP Switches
(C) Closed (O) Open
Rate Switches
Group 4 Divide Ratios
Normal and Alternate
Table
1
2
3
4
NORM (ON)*
TBL (OFF)
0
C
C
C
C
4
16
1
O
C
C
C
8
32
2
C
O
C
C
16
64
3
O
O
C
C
32
128
4
C
C
O
C
48
768
5
O
C
O
C
96
1536