CANOGA PERKINS 2240 Fiber Optic Modem - Manual (Page 24)

2240 Fiber Optic Modem User Manual Chapter 2 Installation and Setup
Setup
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2.1.3.2 Internal Clock Option Switches
There are two switches on the Internal switch block which affect the operation of the Clock
circuits:
• TBL/NORM
• CLK/EXT
2.1.3.3 TBL/NORM Switch
The TBL/NORM switch controls the Data Rate Table as indicated in Table 3-4.
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
2.1.3.4 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.
Factory setting: OFF
2.1.4 Signal Ground Strap
The jumper selects whether chassis ground is connected directly to signal ground (CHASSIS
position) or signal ground is separated from chassis ground (FLOAT position).
NOTE: Float can be overridden by chassis ground jumpers on interface cards or
by a jumper in the 2201 Rack Chassis.
NOTE: When installed in the 2201 rack chassis, any modem main board,
interface, or rack chassis jumper being set to SHORT will override the
FLOAT and 100_OHM positions on all of the other modems. CONSIDER
THIS JUMPER CAREFULLY.
Factory setting: FLOAT
2.1.5 SCT Normal/Invert Jumper
This jumper allows the SCT output from the 2240 to be normal phase or inverted phase. The
purpose of this jumper is to allow compensation for round trip transmit clock/transmit data phase
delays in situations where the customer equipment can not return SCT as SCTE. For more
information about transmit clock, data phasing and SCTE use, see section 3.6 and section 3.7.