In automatic control, clock and time control are often used. For example, the automatic bell ringing and automatic control equipment in schools are controlled according to the H cycle and time. Some PLCs have real-time clock function. This paper gives some simple opinions on how to set and display time and how to realize time control with various PLCs, such as 1 clock calibration and time data call 1. 1 PLC with clock function and clock instruction. Many PLCs have clock functions and related instructions, such as Siemens LOGO! Mitsubishi's A controller and other microcomputers, this machine automatically enters the running state after being powered on, and can display and set the time value on the display screen of this machine. The figure 1 is the LoGO! A program completed in castration programming language. The box marked in the picture is in LOGO! The instruction is called a time switch, which can set multiple groups of time at the same time. The setting value of the time switch in this procedure is 09:00- Friday; From 18: 30. On Saturday 14: 30, it will turn on and turn off at 22: 30; ; It opens at 7: 30 on Sunday and closes at 2: 30. This is an example of a switch used in three different time periods. Besides, LOOO! There is also an annual clock instruction, which can be used to control the time of month and destination. In this kind of PLC, the setting of time period and the correction of internal clock can use the programming button provided by this machine, which is as convenient as adjusting the electronic watch. Siemens PLC series $7-200 and S7-300 also have clock function. With LOGO! The difference is that when using the clock function, the real-time time value should be transmitted to a certain data unit first, and these storage units should be designated as special units of the clock year, month, day, hour, minute, second and week by using the set clock instruction. After this setting, the values of these units change according to the law of time change. When time information needs to be used, it is also necessary to read the time value into a storage unit by using a read time instruction. Fig. 2 is an example of clock establishment and reading of PLC. In the program, 8 memory cells of VB2 10~VB2 17 have been fed with certain time data in advance. The timing of this clock can be realized by sending new time data to these storage units. In the time control involving only hours, minutes and seconds, a time adjustment button can be connected to the input end of the machine by the method of constant tolerance time, and a program for transmitting time adjustment data can be written in the software. This button can be used as the execution condition of the program. It is stipulated that 8 am is the proofreading time. When the standard time is 8: 00 am, press the proofreading button to complete the proofreading function. To realize the function of randomly adjusting time, it is necessary to add special data input devices, such as programmer matrix switch, data unit or graphic unit. 1.2 PLC clock without clock function The machine with inorganic built-in clock can obtain clock control function. 1 I should make a clock program myself. If you want to program, you should first choose the storage unit for storing germanium year, month, day, hour, minute and second, and then use the internal time base as the standard timing pulse. For example, you can use the second pulse to add 1 to the second work unit and return to zero in 60s. The clock program is designed according to the rules of 1 1 hour and 60 minutes, 1 24 hours a day, 3l days in lunar calendar, porridge days in abortion month, the most poisonous dance soup, and February 1 day and 1 2 months in leap year. This self-made clock is similar to the built-in clock in time calibration and time data access, so I won't go into details. When QF is tripped by relay protection, a buzzer is specially installed to attract the attention of the personnel on duty. In order to avoid starting the emergency audio device during manual tripping, closing and automatic reclosing, the method of connecting two pairs of contacts in the control switch in series is adopted in practice, that is, the principle that 1-3 and 19- 17 are only turned on after closing and all their states are turned on at the same time is realized. The purpose of early warning signal early warning letter is to help the personnel on duty to judge the faulty equipment and its nature, so as to take timely measures to deal with it and prevent the accident from further expanding. In order to save the contacts of PLC, the contacts of the early warning signal output relay are used to drive the eight terminals X(x) and output terminals Y (Y Y.) and Y to drive or indirectly drive the light plate and alarm bell. As shown in Figure 2, at present, three flash signals are provided by flash relays, but after switching to PLC, only two internal timers are needed to form an oscillation circuit to realize the flash signals, which saves the trouble of external wiring. The concrete implementation is shown in ladder diagram 3, and the I/O allocation of PLC is shown in table 14. Conclusion The excellent performance of PLC ensures the accuracy and reliability of central signal action. Although the price of PLC is higher than that of ZC-23 impulse relay currently used on site, it is easy to use, with less maintenance workload and simple secondary circuit. Although this experiment failed to input A and output more accident and alarm signals, the input letters were verified, and the output results fully met the field requirements. If it is put into application, it is only necessary to expand the contacts of PLC through expansion units and expansion modules, or directly select multiple I/O PLCs.