Unit installation

OMRCPMICRO 7
Component Data
Microprocessor control board
The Model 250 Microprocessor Control Board contains the electronic hardware and software necessary to
monitor and control the unit. The microprocessor control board receives input from the Analog/Digital Input
Board (ADI) and sends commands to the output board, maintaining the unit's optimum operating mode for the
current conditions. Status lights mounted on the control board indicate operating condition of the
microprocessor.
Analog/digital input board (ADI)
The ADI board receives analog/digital signals from sensors and switches. The ADI board also provides
optical isolation between the microprocessor control board and all 24 volt switch inputs. LEDs on the ADI
board provide visual indication of the status of all digital inputs. All analog and digital signals from sensors,
transducers, and switches received by the ADI board and sent to the microprocessor control board for
interpretation.
Output board
The output board contains up to 16 solid-state relays and provides optical isolation between the control board
and Vac load volts. These relays control all compressors, condenser fans, solenoid valves, and alarm
annunciation. The output board receives control signals from the microprocessor control board through a 50
conductor ribbon cable.
Keypad and display
The keypad and display is the primary operator interface to the unit. Operating conditions, system alarms, and
set points can be monitored from this display. All adjustable set points can be modified from this keypad after
the operator has entered a valid password.
Thermistor sensors
MicroTech panels use a negative temperature coefficient thermistor for temperature sensing. A sensor
operating correctly will measure 3,392 ohmsat 72°F. See Table 1 for temperature conversion information.
Pressure transducers
Pressure transducers are selected for a specific operating range and provide an output signal proportional to
the sensed pressure. Typical range for evaporator sensor is 5 to 145 psig with a resolution of 0.1 psig.
Condenser pressure sensors have a range of 20 to 450 psig and a resolution of 0.5 psig. The transducer output
characteristics are shown in Figure 3 and 4 on page 9.
Table 1, MicroTech thermistors (resistance and voltage vs. temperature)
Temp
°F
Temp
°C
Resistance
Ohms
VDC
Input
Temp
°F
Temp
°C
Resistance
Ohms
VDC
Input
Temp
°F
Temp
°C
Resistance
Ohms
VDC
Input
0 -17.8 25,617 4.426 50 10.0 5,971 3.213 100 37.8 1,747 1.724
1 -17.2 24,817 4.410 51 10.6 5,814 3.183 101 38.3 1,708 1.698
2 -16.7 24,044 4.393 52 11.1 5,662 3,152 102 38.9 1,670 1.673
3 -16.1 23,299 4.376 53 11.7 5,514 3.121 103 39.4 1,633 1.648
4 -15.6 22,579 4.359 54 12.2 5,371 3.090 104 40.0 1,597 1.624
5 -15.0 21,883 4.341 55 12.8 5,231 3.059 105 40.6 1,562 1.600
6 -14.4 21,212 4.323 56 13.3 5,096 3.028 106 41.1 1,528 1.576
7 -13.9 20,563 4.305 57 13.9 4,965 2.996 107 41.7 1,494 1.552
8 -13.3 19,937 4.286 58 14.4 4,838 2.965 108 42.2 1,461 1.528
Table continued on next page.