Serial programmer circuit for AT89Sx MCUs

Simple RS-232 ISP programmer for AT89Sx devices. This device is intended for education purposes and for hobbyists. It has been tested with AT89S2051 and AT89S8253 along with USB to serial port converter cable (with PL2303 chip).
Serial programmer circuit for AT89Sx MCUs - schematic

MCU in programmer can work while computer is turned off or disconnected. Transistors can be replaced with almost any universal low frequency transistors with the same pinout. Bad resistor values may cause malfunction. It might be a good idea to try it in solderless breadboard before making PCB and soldering. RS-232 connector on the board is female. Reset button resets MCU connected to the programmer. Hardware connected to the MCU ports may cause programmer malfunction. Power supply: 8 → 30 V (Programmer work with 5V) LED1: Indicates power on LED2: Indicates PC activity or uninitilized programmer ISP programming cable: Trough this interface programmer should be able to program MCU for instance in breadboard or some PCB. But remember it can comunicate with only one MCU at the time. So MCU placed in the socket on the programmer board must be removed firts.

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