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Level Shifter options
The Acorn Tube interface uses 5V levels, where as the Raspberry Pi's GPIO header uses 3.3V levels, therefore some kind of a level shifter is required, or else damage will result.
There are currently four options:
Kjell's Level Shifter board (or kit)
Kjell Sundby has published a design of an interface board here and is also selling this board and another board at a reasonable price - see here. This design is popular and has been successfully used by many people. You will need to decide whether you want a cable-connected universal version, a compact under-Beeb version, or an in-Master version.
Purchase a Tube Silencer and some Jumper Cables
The Tube Silencer is made by John Kortink, and can be purchased here. (Also shown is a serial connection, needed only for debugging during development.)
Breadboard a Level Shifter
This uses two 74LVC245A chips, which cost very little from Farnell.
Wait for our Level Shifter Pi Hat PCB
This is currently being designed on KiCAD v4. The design files are in Github: https://github.com/hoglet67/PiTubeDirect/tree/master/kicad
It is essentially the same as the breadboard design, with some optional components for signal conditioning, that hopefully will not need to be fitted.
Simon Inns has also designed a board and shared his design files here: http://www.stardot.org.uk/forums/viewtopic.php?p=143605#p143605