Onboard 4M crystal oscillator, the socket crystal frequency can be replaced at any time. The 4-bit independent keyboard is connected to RB0 RB1 RB2 RB3. 8 LEDs are connected to the RD port. When the J3 is plugged in, the LED is enabled. J3 is unplugged and the RD port is completely released. Standard RS232 communication interface, microcontroller board and computer communication interface. External 5V DC power interface (send USB power cable without additional purchase).