A PLC is to control the solar heating system shown in Figure 12.37. The system has two interrelated parts: (a) A solar thermostat turns on and off the solar heater if the sun sensor says the sun is shining, and (b) a backup thermostat turns on and off a conventional furnace if the solar energy is insufficent. Both heating systems share the same ductwork, so if the backup thermostat closes, the PLC must turn on the backup furnace (and turn off the solar heater if it's on). Draw a PLC ladder diagram for this system.
