This is a module based on 8051 series microcontroller (was tested with 80C31, 80C32 and AT89C51). It was designed and produced by me in 1998 to be used on many applications, including automation systems, RTU, printers, fiscal modules, data recorders, remote control, traffic lights systems, flight recorders, home appliances etc.

Specification:
- power supply voltage: 5VCC regulated (additional 5-24V for command one relay)
- supported CPUs: 80C31, 80C32, 87C31, 87C32, AT89C51, AT89C52
- CPU clock: 11.0592 MHz for easy implementation of serial transfer rates
- external RAM memory capacity can be from 32kx8 to 512kx8. The address range of CPU for external RAM is limited. For overcome this limitation, I implemented a logic division of RAM memory in pages, controlled by three CPU output ports: P1.5, P1.6 and P1.7. Also the use of external RAM memory is controlled by port P1.0 of CPU (in zero).
- external EPROM or parallel EEPROM
- automatic switch of external RAM memory on battery when the power supply goes down
- the serial interface of the CPU can be switched by software to communicate with two devices (only one is bidirectional).
- output for LCD display controlled with 4 bits
- I2C serial EEPROM memory. I2C protocol must be implemented with software.
- buzzer
- one medium power open collector output (24V max)
- 8 dip switches for various settings (e.g. serial transfer rate)

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Links:
[1] - 8051 development system: Software development board