I chipped hundreds of PSX back in the day, depending on the models the wiring was different.
Now bear in mind that "the day" was about 10 years ago or thereabouts, and my brain has grown considerable rust since then, so the following could be flaky. It also pertains to the Australian PAL models, but I guess all the PALs were pretty much the same.
1002 - the original with RCA jacks on the back as well as the standard video output. Needed a crystal installed to display NTSC, and had lots of laser issues. Repaired uite a few (parts etc)
5502 - lost the RCA jacks and also needed the crystal. This gave a more reliable laser but seemed to fail more often.
7002 - reliable unit, able to run NTSC games by bridging a couple of solder points.
7502 - very reliable unit, also ran a bridge to run NTSC
9002 - reliable unit, easy to mod, NTSC bridge as above. Had the parallel port removed due to the popularity of the game shark cartridge.
So in summary the 1002 and 5502 were the originals, and the later models released for minor improvements, possibly integration of PAL/NTSC together to save making two entirely different boards, and I guess each upgrade was designed to thwart piracy. (which was a waste of time)
I'd still have my pic programmer and modchip source code lying around somewhere, with about 50 blank pics and one erasable pic. Perhaps its time to get it out again... I picked up an original PSX a couple of months ago and havent gotten around to modding it yet. Just need to find my diagrams.