By chance I came across THIS thread in the german Sinclair forum while browsing the internet. There it was about an interface which allows the connection of a Commodore 1541 floppy drive to a Sinclair ZX-81.
This has directly aroused my interest, because I had just recently brought a 1999 bought Sinclair ZX-81 back to life. So the opportunity was good to have a closer look at it.
The interface was developed in 2004 by Luis Claudio Grosso for the Czerweny CZ1500, which is probably an Argentinian offshoot of the Timex 1500, which in turn is an offshoot of the legendary Sinclair ZX-81.
Two years later Jarek Adamski adapted the interface for the Sinclair ZX-81. This interface was available for sale on the Internet for some time. Unfortunately, Jarek Adamski is probably no longer available and his website is no longer maintained. Also most of the downloads are no longer available there.
Therefore I decided to simply rebuild the interface. I was able to find the schematic in the Sinclair forum in the thread mentioned above.
With the help of this community and some tips and suggestions I was able to create a new schematic, which later became the circuit board.
The board was then designed to fit into an old case of a 16k memory expansion from the Sinclair.
For the interface to be useful, an internal memory expansion is needed in the ZX-81. Unfortunately the standard RAM is just sufficient to store a ’10 PRINT “TEST”;’. An additional ’20 GOTO 10′ is already too much.
I had extended the RAM in my ZX-81 to 32k. With this the interface works perfectly.
After the ZX-81 has been “switched on”, the following can be done with the command
RAND USR 8192
the internal firmware, which is located on an 8k EEPROM, can be called.
From here the directory can be read or files can be loaded or saved. Commands can also be sent to the floppy drive, for example to format a floppy disk.
Here the syntax is analogous to the C64. For example, a floppy disk is formatted with:
The 32k RAM memory installed on the interface is used solely for the interface, and is not integrated as a memory expansion of the ZX-81.
If you want to rebuild the whole thing, you can find everything you need on my GitHub account: https://github.com/DL2DW/ZX1541-Interface