FORM 201.26-NM1
ISSUE DATE: 1/19/2018
JOHNSON CONTROLS
115
7
SECTION 7 – UNIT CONTROLS
INTRODUCTION
The YORK MicroComputer Control Center is a micro-
processor based control system designed to provide the
entire control for the liquid chiller. The control logic
embedded in the microprocessor based control system
will provide control for the chilled liquid temperatures,
as well as sequencing, system safeties, displaying sta-
tus, and daily schedules. The MicroComputer Control
Center consists of four basic components:
1. IPU II and I/O Boards
2. Transformer
3. Display
4. Keypad
The keypad allows programming and accessing set-
points, pressures, temperatures, cutouts, daily sched-
ule, options, and fault information.
Remote cycling, demand limiting and chilled liquid
temperature reset can be accomplished by field sup-
plied contacts.
Compressor starting/stopping and loading/unload-
ing decisions are performed by the microprocessor to
maintain leaving or return chilled liquid temperature.
These decisions are a function of temperature devia-
tion from setpoint.
A Master ON/OFF switch is available to activate or de-
activate the unit.
IPU II and I/O Boards
The IPU and I/O boards are assembled to function as a
single microprocessor controller requiring no addition-
al hardware. The IPU II board contains a coldfire mi-
croprocessor and is the controller and decision maker
in the control panel. The I/O board handles all of the
chiller I/O (Inputs and Outputs). System inputs from
pressure transducers and temperature sensors are con-
nected to the I/O board. The I/O board contains a pro-
cessor capable of reading the inputs and controlling the
outputs. It communicates through the transition header
with the IPU II microprocessor.
The I/O board circuitry multiplexes the analog inputs,
digitizes them, and constantly scans them to keep
watch on the chiller operating conditions. The input
values are transmitted serially to the IPU II micropro-
cessor board. From this information, the IPU II then
issues commands to the I/O board relay outputs to con-
trol contactors, solenoids, and so on, for Chilled Liquid
Temperature Control and to react to safety conditions.
The I/O board converts logic signals to operate relay
outputs to 115 VAC levels used by motor contactors,
fan contactors, solenoid valves, and so on, to control
system operation. The low voltage side of all relay
coils on the I/O board are powered by +12V.
Keypad commands are actuated upon by the micropro-
cessor to change setpoints, cutouts, scheduling, operat-
ing requirements, and to provide displays. The keypad
and display are connected to the I/O board.
The on-board power supply converts 24 VAC from
75 VA, 120/24 VAC 50/60Hz UL listed class 2 power
transformer to +12V, +5V and +3.3V using switching
Summary of Contents for YCWL0056SE
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