TM 10--3510--221--24 0007 00--2 CONTROL SYSTEM THEORY OF OPERATION -- Continued 0007 00 SYSTEM MONITORING AND CONTROL LADS operation is electronically controlled. This control is accomplished using a central processing unit (PCB9) that contains custom software.  PCB9 interfaces with an analog card (PCB1), discrete input card (PCB2), digital input/output (I/O) card (PCB7), optically isolated input card (PCB8), and solid--state relay boards (PCB3 through PCB8) to control and monitor all automatic operations. +5, +12, --12 VDC FROM TB9 SOLID--STATE RELAY BOARD (PSB3) SOLID--STATE RELAY BOARD (PSB4) SOLID--STATE RELAY BOARD (PSB5) SOLID--STATE RELAY BOARD (PSB6) CENTRAL PROCESSING UNIT (PCB9) ANALOG CARD (PCB1) DISCRETE INPUT CARD (PCB2) DIGITAL I/O CARD (PCB7) OPTICALLY ISOLATED INPUT CARD (PCB8) INTERFACE CARDS (PCB10) VACUUM FLUORESCENT DISPLAYS (VFDs) PUSHBUTTON SWITCHES (PBS1 --5) POWER SWITCHES (SW3) CARD CAGE LEVEL SENSORS PRESSURE SENSORS TEMPERATURE SENSORS 12 VDC FROM TB2 POSITION SWITCHES PROXIMITY SWITCHES EMERGENCY STOP SWITCH FLOW SWITCHES MOTOR PROTECTORS INVERTERS 12 VDC FROM TB2 PRESSURE SWITCHES TEMPERATURE LIMIT SWITCH FLAME PROGRAMMER 110 VAC FROM TB7 +5 VDC FROM TB9 +5 VDC FROM TB9 12 VDC SOLENOID VALVES 110 VAC SOLENOID VALVES 110 VAC MOTOR CONTACTORS INVERTERS AUDIBLE ALARMS INDICATORS 12 VDC FROM TB2 +5, +12, --12 VDC FROM TB9 110 VAC FROM TB2 HEATER CONTROL RELAY ANALOG--TO-- DIGITAL CONVERTER CARD (PCB11) LIMIT SWITCHES