Section 7 – Unit Controls - York YCAL0019 Installation, Operation And Maintenance Manual

FORM 150.67-NM1
ISSUE DATE: 4/28/2017
INTRODUCTION
The YORK Millennium MicroComputer Control Cen-
ter is a microprocessor 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 status, and daily schedules. The
MicroComputer Control Center consists of four basic
components:
• IPU II and I/O Boards
• Transformer
• Display
• 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.
JOHNSON CONTROLS
SECTION 7 – UNIT CONTROLS
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
control contactors, solenoids, etc. for Chilled Liquid
Temperature Control and to react to safety conditions.
The I/O board converts logic signals to operate relay
outputs to 115VAC levels used by motor contactors,
fan contactors, solenoid valves, etc. 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.
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