Use a button to run the two kinds of mode selection of fan control PLC system design, for example

Fan has two kinds of operation mode, circulation and exhaust, use a switch to switch. The interlock between the two patterns by the ALT command to complete. When the button PB1, X001) Is activated, the auxiliary relay M100 approximately through a program scan cycle. M100 approximately normally open contacts connected when starting the ALT command. ALT instruction invert output element, in this case, the M101. According PB1 will make M101 ON for the first time, the second press the PB1 are OFF. Use this feature, the ALT command corresponding relay often opened normally closed contacts between the two 'pattern' can choose. In this case, when M101 to ON, the fan is in exhaust mode, Y002 work; When M101 is OFF, the fan is in circulation mode, Y003 work.