CANOGA PERKINS 2240 Fiber Optic Modem - Manual (Page 38)

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Canoga Perkins
3.7 Internal Clock Option Switches
There are two switches on the Internal switch block which affect the operation of
the Clock circuits: TBL / NORM and CLK / EXT (see Figures 2-3 and 3-4 for
the locations of these switches).
3.7.1 TBL / NORM Switch
The TBL / NORM switch controls the Data Rate Table as indicated in Table 3-D.
It is configured as ON when shipped from the factory. If it is switched to OFF, the
alternate Divide Ratios become active.
Factory Setting = ON
3.7.2 CLK / EXT Switch
The CLK / EXT switch controls which clock is used for synchronous input. If it is
switched to ON, any mode which sources Send Timing (Internal or Slave) will use a
turned-around clock coming in on Terminal Timing from the user's equipment. This
compensates for round-trip delays in the sourced clock which could otherwise shift
the clock-data phasing of the transmit signal and cause errors. This setting can
only be used where leads for both are available, and if the user's equipment can
turn the Send Timing back around onto the Terminal Timing leads, either internally
or at the other end of the cable.
NOTE: The ON setting of the CLK / EXT switch is required for
operating redundant modems using either internal or slave clocking.
NOTE: On standalone models, these switches can only be accessed
after the top cover has been removed. The cover is fastened by
screws on the sides of the case. If the modem is mounted in a 2202
Modem Shelf, it must first be removed from the shelf. Be sure to
disconnect power before removing the cover.
Factory Setting = OFF