Three known versions of the Discs of Tron roms exist:  
1.  The earliest version is what is referred to as the "Environmental" romset since it 
was the version released with the environmental cabinet version of the game.  I say 
this is the earliest version because it has a bug that the other two versions don't have 
that fixes an issue with upright cabinet operation.

2.  The middle aged version is what is commonly available on the internet as the "Upright" 
version. This version was loaded on the first Upright machines, and fixed the bug in the 
"Environmental" version that prevents a 1 player game from being started in an upright cabinet.  

3.  The final version added scoring display for "derezz a disc 2000 pts" to attract mode.  
It also fixed a possible issue (not a severe bug) that could result if you lean and fall 
off of a disc when the 2nd sark in a level has been defeated.  In this version you cannot 
fall off of a disc unless there is a living sark, no matter how much you lean.  Other 
changes may exist, but I have not run across them yet.


NOTE:  All 3 versions were designed to run in either cabinet.  The "Environmental" version 
could not do this due to the bug preventing 1-p games from starting, but it did have all 
of the code present to work in an upright cabinet.  The other 2 versions work fine in either cab.