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1.3 UNIT (CHILLER) ON / OFF SWITCH
A master UNIT (Chiller) ON / OFF switch is located
just below the keypad. This switch allows the operator
to turn the entire chiller OFF, if desired. The switch must
be placed in the ON position for the chiller to operate.
Any time the switch is in the OFF position, a Status
message indication will be displayed. See page 120 for
the location of this switch.
1.4 MICROPROCESSOR BOARD
The Microprocessor Board controls and makes decisions
for the chiller. Information inputs from transducers and
sensors around the chiller are either connected directly
to the Microprocessor Board or are connected to the I/O
Expansion Board and multiplexed before being sent to
the Microprocessor Board. The Microprocessor Board
circuitry multiplexes all of these analog inputs, digitiz-
es them, and constantly scans them to monitor chiller
operating conditions. Based on this information, the
Microprocessor issues commands to the Relay Boards
to activate and deactivate contactors, solenoids, etc.
for chilled liquid, operating control, and safety control.
Commands are sent from the Microprocessor Board to
the I/O Expansion Board to control the slide valves for
chilled liquid control.
Keypad commands are acted upon by the micro to
change setpoints, cutouts, scheduling, operating require-
ments, and to provide displays.
A +12VDC REG supply voltage from the Power Supply
Board is converted to +5V REG by a voltage regulator
located on the Microprocessor Board. This voltage is
used to operate the integrated circuitry on the board.
System Switches 1 - 4
System Switches for each system are located on the
Microprocessor Board (Section 1.11, Item 5). These
switches allow the operator to selectively turn a given
system on or off as desired.
Internal Clock & Memory Backup Battery
The Microprocessor Board contains a Real Time Clock
integrated circuit chip (Section 1.11, Item 2) with an
internal battery backup. The battery backup assures that
any programmed values (setpoints, clock, cutouts, etc.)
are not lost during a power failure or shutdown period
regardless of the time involved.
124
The battery is a 10 year lithium type, but life will de-
pend upon whether the Real Time Clock's internal clock
circuit is energized. With the clock OFF, a rated life of
approximately 10 years can be expected. With the clock
ON, approximately 5 years. The clock is enabled and
disabled using a jumper on the microprocessor board.
If the chiller is shut down or power failure is expected for
extended periods, it may be desirable to disable the clock
to save battery life. The clock can then be reactivated and
reprogrammed when the chiller is returned to service.
This will not affect the maintenance of programmed
values and stored data by the backup battery.
While a chiller is operating, the clock must be ON
(Section 1.11, Item 1) or the internal clock on the micro-
processor will not be active and the micro cannot keep
track of time, although all other functions will operate
normally. Failure to turn the Clock ON could result in
the chiller not starting due to the time "frozen" on the
clock falling outside the Start/Stop time programmed in
the Daily Schedule, see Section 7.3.
1.5 ANCILLARY CIRCUIT BOARDS
Power Supply Board
The on-board switching power supply is fuse protected
and converts 24VAC from the logic transformer 2T to
+12V REG which is supplied to the Microprocessor
Board, Relay Output Boards, and the 40 character dis-
play to operate the integrated circuitry.
24VAC is filtered, but not regulated, to provide unregu-
lated +24VDC to supply the flow switch, PWM remote
temperature reset, PWM remote current reset, lead / lag
select, and remote print circuitry which may be utilized
with user supplied contacts.
24VAC is also filtered and regulated to +24VDC to
be used by the optional EMS/BAS Circuit Boards for
remote temperature or remote current reset.
Individual rectifier and filtering circuits are present
which receive the Current Transformer signals for each
phase of motor current on each compressor. These
circuits rectify and filter the signals to variable DC. A
phase rotation circuit for each compressor is also present
to assure that the screw compressors do not run in the
wrong direction. All of these signals are sent to the I/O
Expansion Board which multiplexes them and then feeds
them to the Microprocessor Board.
JOHNSON CONTROLS
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