The MITSUBISHI PLC special auxiliary relay has 256 points, which are used to represent some of the states of the PLC, providing clock pulses and signs (such as the carry, the bit of the bit), setting the running mode of the PLC, or for the step control, the prohibition of interruption, the setting counter is counting or subtracting count. Special auxiliary relays are divided into two types: (1) contact use type The system program of PLC drives the coils of the contact use special auxiliary relay to use the contacts directly in the user program, but not their coils. Here are several examples: M8000 (operation monitoring): when PLC executes user programs, M8000 is ON; when execution stops, M8000 is OFF. M8002 (initialization pulse):M8002 is only ON (see Figure 3-8) in a scanning period of the M8000 changing from OFF to ON state (see Figure 3-8), which can be used to initialize or give initial values to the components with a power failure retention function with the constant open contacts of the M8002. M8011 ~ M8014 are 10ms, 100ms, 1s and 1min clock pulses respectively. M8005 (lithium battery voltage reduction): the battery voltage drops to the specified value to ON, which can be used to drive the output relay and the external indicator to remind the staff to replace the lithium battery. (2) coil drive The coils are driven by user programs to enable PLC to perform specific operations, and users do not use their contacts. For example: After the M8030 coil is energized, the “battery voltage is lowered” and the LED is extinguished. When the coil of M8033 is energized, the state of all output relay remains unchanged when PLC enters STOP state. When the coil of M8034 is energized, all outputs are prohibited. When the coil of M8039 is energized, the PLC operates in the scan time specified in D8039.