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I/O Expansion Board
The I/O Expansion Board provides multiplexing to allow
additional inputs to be connected to the Microprocessor
Board via a single data line. The additional inputs are
multiplexed according to the selection made by the
Microprocessor through address lines.
Signals routed through the I/O Expansion Board include
Discharge Temperature, Current Transformer outputs
(motor current signals), and Oil Temperature.
Included on the I/O Expansion Board are the outputs for
the slide valve control. This control consists of a Digital
to Analog Converter (DAC) and power transistors to
modulate current through the slide valve solenoids.
Relay Output Boards
One Relay Output Board per system operates the motor
contactors/starters, solenoid valves, and heaters which
control system operation.
The relay boards are located in the logic section of
the control panel(s). The boards convert 0 - 12VDC
logic levels outputs from the Microprocessor Board to
115VAC levels used by the contactors, valves, etc.
The common side of all relays on the Relay Output
Board is connected to +12VDC REG. The open collector
outputs of the Microprocessor Board energize the DC
relays or triacs by pulling the other side of the relay coil
to 0VDC. When not energized, both sides of the relay
coils or triacs will be at +12VDC potential.
1.6 CIRCUIT BREAKERS
Three Circuit Breakers are provided for the 115VAC
controls.
• CB1 allows removal of control power from System
1 for control system circuitry servicing: specifically,
the 115VAC feed to Relay Output Board 1 which
energizes contactors and solenoids.
• CB2 allows removal of control power from System
2 for control system circuitry servicing: specifically,
the 115VAC feed to Relay Output Board 2 which
energizes contactors and solenoids.
JOHNSON CONTROLS
• CB3 allows removal of control power to the Mi-
croprocessor Board, Power Supply Board, I/O
Expansion Board, and Evaporator Heater.
The Circuit Breakers remove 115VAC
control power only. High voltage cir‑
cuitry will still be energized from the
high voltage supply.
REMOVING 115VAC power to CB3 or
opening CB3 removes power from the
evaporator heaters. This could cause
evaporator freeze‑up in low ambient
temperatures.
1.7 CURRENT TRANSFORMERS (C.T.)
C.T.s, located internally in the Motor Protection Mod-
ules on each of the 3 phases of the power wiring of
each compressor motor, send AC signals proportional to
motor current to the Power Supply Board which rectifies
and filters the signals to variable DC Voltage (analog).
These analog levels are then fed to the Microprocessor
Board via the I/O Expansion Board allowing the mi-
croprocessor to monitor motor currents for low current,
high current, unbalanced current, and single phasing.
1.8 TRANSFORMERS
3 Transformers (2T, 3T, and 4T) are located in the
Control Panel. These transformers convert the 115VAC
Control Power Input to 24VAC to operate the micropro-
cessor circuitry.
2T: This 75VA transformer supplies the 24VAC
microprocessor power supply.
3T: Supplies the I/O Expansion Board # 1 voltage
for slide valve control.
4T: Supplies power to the Motor Protector
Modules.
FORM 201.18-NM7
8
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