Standard RS232 communication interface, microcontroller board and computer communication interface. ^^ 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. ^^ External 5V DC power interface (send USB power cable without additional purchase). ^^ The 4 bits independent keyboard is connected to RB0 RB1 RB2 RB3. ^^ Onboard 4M crystal oscillator, the socket crystal frequency can be replaced at any time.