Counting is the simplest and most basic operation. A counter is a logic circuit that implements this operation. In a digital system, a counter mainly counts the number of pulses to achieve measurement, counting and control. function, and also has a frequency division function. The counter is composed of a basic counting unit and some control gates. The counting unit is composed of a series of various types of flip-flops with the function of storing information. These flip-flops include RS flip-flops and T triggers. trigger, D flip-flop and JK flip-flop, etc. Counters are widely used in digital systems, such as counting instruction addresses in the controller of electronic computers in order to sequentially fetch the next instruction, recording the number of additions and subtractions when performing multiplication and division operations in the arithmetic unit, and in digital counting of pulses in the instrument, etc. The counter can be used to display the working status of the product. Generally speaking, it is mainly used to indicate how many pieces of folding and plating work the product has completed. Its main indicator is the number of digits in the counter, the common ones are 3 digits and 4 digits. Obviously, the 3-digit counter can display up to 999, and the 4-digit counter can display up to 9999.