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Marvair MGA3090-3180 Wall Mount AC I&O Manual 05/2023 Rev.14
3. MAINT –
(Maintenance) flashes whenever you insert a memory card. The CPU then changes to
STOP mode. After the CPU has changed to STOP mode, perform one of the following functions to
initiate the evaluation of the memory card:
a. Change the CPU to RUN mode
b. Perform a memory reset (MRES)
c. Power-cycle the CPU
The PLC has indicator LED’s that show the status of all thermostat inputs and outputs. For example, if
the “G” LED is on, this means that voltage is present from the “G” terminal on the thermostat.
1.7 Unit Operation
Modes of Operation
• Normal Start-Up
– On a call for cooling, and with the high pressure switch closed, the cooling
system
(compressor, indoor blower motor and outdoor fan motor) will be energized. (Note: See
the Delay on Make feature). The cooling system will remain energized during the three minute low
pressure switch bypass cycle. If the low pressure is closed, the cooling system will continue to operate
after the three-minute bypass. If the low pressure switch is open after the three-minute bypass, the
cooling system will be de-energized.
• Lockout Mode
– If either the high or low pressure switch opens on the same call for cooling, the
PLC system enters into and indicates the lockout mode. In the lockout mode, the compressor is
turned off, the alarm output is energized and the status LED’s will blink to indicate which fault has
occurred. If there is a call for air flow, the indoor blower will remain energized. When the lockout
condition has cleared, the unit will reset if the demand of the thermostat is removed or when power
is reset. The lockout circuit has a 3-second delay to prevent premature activation and is factory wired
for normally open contacts. The user can select either normally closed or normally open remote
alarm dry contacts.
PLC External LED Indicator Lights
Type
LED Indicator
Color
Description
Power
RUN/STOP
Green
PLC is in run mode
Yellow
PLC is in stop mode
ERROR
Red
PLC has encountered an error
MAINT
Yellow
Maintanence is required
Inputs
DI a .0
Green
High Pressure Switch A
DI a .1
Green
Low Pressure Switch A
DI a .2
Green
High Pressure Switch B
DI a .3
Green
Low Pressure Switch B
DI a .4
Green
Emergency Management System
DI b .1
Green
Phase Monitor
DI b .2
Green
Call for first stage cooling from thermostat
DI b .3
Green
Call for second stage cooling from thermostat
DI b .4
Green
Call for heating from thermostat
DI b .5
Green
Call for indoor blower from thermostat
Outputs
DQ a .0
Green
Output to call for compressor A
DQ a .1
Green
Output to call for for the indoor blower
DQ a .2
Green
Output to call for the outdoor fan
DQ a .4
Green
Output to call for compressor B
DQ a .5
Green
Output to call for the heater
DQ a .6
Green
Flashing lockout indicator (See below for flash code)
DQ a .7
Green
Fault relay
