Section 7 - Unit Controls; Introduction; Ipu Ii And I/O Boards - York YLAA Series Installation Operation & Maintenance

FORM 150.72-ICOM6
ISSUE DATE: 12/29/2017

INTRODUCTION

The YORK Control Center 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:

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 activates or deactivates 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. The IPU II board
contains a microprocessor and is the controller. The
I/O board handles all of the chiller I/O (Inputs and
Outputs). System inputs from pressure transducers and
temperature sensors are connected to the I/O board.
The I/O board constantly scans inputs to monitor the
chiller operating conditions. The input values are trans-
mitted the IPU II microprocessor board. From this in-
formation, 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 re-
act 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.
The on-board power supply converts 24VAC from
75VA, 120/24VAC 50/60Hz UL listed class 2 power
transformer to +12V, +5V and +3.3V using switching
and linear voltage regulators located on the I/O and IPU
II boards. These voltages are used to operate integrated
circuitry on the board. The 40 character display and
unit sensors (transducers and temp sensors) are sup-
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