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5.16 Model 54575 RS-422 Output Driver Card
5.1601
This card provides ten separate RS-422 compatible differential square wave
outputs. The outputs drive any standard RS-422 compatible receiver through
up to 400 feet of cable. An alarm output signal alerts the 54560 Alarm
Interface Card when one or more outputs have failed.
5.1602
The 54575 is provisioned at the factory in one of two configurations:
Model 54575-2 has an output frequency of 1.544 MHz.
Model 54575-3 has an output frequency of 8 kHz.
5.161 Functions
5.1611
The Model 54575 RS-422 Output Driver Card provides the following:
a. Clock selection by a logic array based on HOLD indications from the A
and B track and hold cards. The current tracking card or, if both are in
HOLD, the last track and hold card to go into the HOLD state is selected.
Switching is nonrevertive to minimize the number of phase disturbances.
b. Override lines from the 54550 Information Management Card allowing for
manual clock selection and for automatic switching directly to a reference
input in the event that both track and hold cards fail (Major alarm state).
c. The first output card from the left (any slot) acting as a master selector
and all other output cards slaved to it to ensure that all output cards select
the same clock reference.
d. A synchronization bus connecting all output cards to ensure that all
outputs from the shelf are synchronized.
e. One-wire interface to the 54550 Information Management Card to report
personality and LED states.
5.162 Circuit
Description
The 54575 block diagram appears in Figure 5-16.
5.1621
Clock Select Switch
The clock select switch is a 1.544 MHz clock selector with phase buildout
circuit, frame/byte synchronization system, LED indicators, and output
alarms. The selector output goes to a phase buildout circuit whose function
is to smooth out sudden phase changes (which might be as much as ±1/2 a
unit interval) occurring when the clock selection is changed. The change of
phase rate is limited to under 20 nanoseconds (1/32 unit interval) in 14
milliseconds (21,616 unit intervals).
5.1622 Logic
Block
The logic block provides a frequency divider circuit that offers an output that
