Version 1.0 Nanocomp uses same Monitor as 1981 version

A project log for Nanocomp 6809 8 Bit Retro Computer on Breadboards

Wireless World Magazine in January and June 1981 published this design. Reproduced on solderless breadboards using the original Monitor ROM

dave-henryDave Henry 11/03/2022 at 00:130 Comments

Version 1.0 of the Nanocomp was built to re-use the original Monitor ROM (though copied onto a new EEPROM) and uses almost the same design.

There have been a few updates. The original RAM used 2 x 2114 1Kx4 bit Static RAM. These have been replaced with a modern 1278KB static RAM. In Version 1.0 we only use 4KB of the 128KB.  The original EPROM required UV erasing so these have been replaced with more recent EEPROM such as the ATMEL 28C64B 8KB EEPROM.

The MC1413 Darlington driver chip was not available and was replaced with a ULN2803A Darlington Driver which has the added benefit of being TTL compatible, so it does not need the additional 5K6 resistors the original design had.

The 7442 BCD Decoder/Driver was not available and was replaced with the 74LS145 which is capable of driving higher currents as well.

The original crystal was about 3.5Mhz due to the RAM and ROM having slower 450ns access times. This is not necessary with the more recent chips so the speed was increased to 4Mhz.

The modern 7 Segment LED Displays have the E & F segments reversed compared to the 1981 design so this has to be considered when viewing the old schematics. 

Videos 1-9 in the Nanocomp 6809 Playlist cover the construction of version 1.0

Videos 10, 11 and 12 cover the testing and exploring the Monitor, simple Machine Code programming and disassembling the original Monitor binary code into assembler source which can be re-assembled back to the same binary image,

Videos 13,14 & 15 cover Nanocomp Version 2.0 which includes a Serial Port and updating the Monitor Save and Load to/from tape to work with the serial port using the Motorola SREC text format for transferring executable code.