This is a long story so I hope you have your reading glasses on... I will try to summarize.
I have been collecting PC bios chips for a long while knowing that eventually, I would attempt the homebrew modchip thing.
I attempted this once before and failed because I didn't really know enough then and I used too thick of wire.
First recent attempt, about a week ago, I came across another PC bios socket, the 32-pin rectangular type (PDIP??) and it just hit me to try again (original attempt was about 5 years ago). I still had the original device that I built before to hold the bios chip but I wanted to start fresh. The original device I built used 29 standard IDE wires and I wanted thinner wire. I built a new one using a 3 cell AA battery holder box and for this one, I used wire from an 80 wire IDE cable. Much thinner.
I had a PDIP chip already flashed with an xbox bios (evox m8) from the last time I tried this so all I needed was a holder for the chip. I wanted the ability to swap chips from outside a fully assembled xbox so I used the full length IDE wires.
I wired up the socket according to the various diagrams found on the internet, popped the chip in the socket, and booted the xbox. It booted from the TSOP. Did some more digging on the net and I came to one of 2 conclusions. Either my wires were too long or the chip I was using (I had 2 with M8 on them) add not actually been flashed somehow, so I thought.
I quickly realized that I had no easy way to attempt to reflash this type of chip and to shorten my wires, would have been a crap load of work too. At this point I gave up again.
Now keep in mind 2 things. 1.) When I start a project if I don't finish I will start to obsess over it and it eats at me. 2.) The room in my house where all my PC and xbox parts reside is also the room I have designated for smoking so every time I go have a smoke, I see parts and pieces and then thoughts and ideas and possibilities run through my mind and taunt me, so I started it back up again.
I had a few incomplete PC's laying around that would do it but they both were missing pieces that I decided to start over again fresh using a PLCC chip and socket. I got this setup hot glued right to the bottom of the xbox board eliminating the wire length issue. Next, I got the idea to flash the chip from the xbox itself instead of using a PC. Now, At this phase, I am starting to learn a bit about some of the wires here like the link32, OE, etc. So I softmodded my original kernel, fired up evox, and because my tsop is winbond, I had to use raincoat. Now for some reason raincoat is not showing video when I run it so when I got the right wires twisted together and attempted the flash, I inadvertently flashed and killed my TSOP. So now I can't give up. I at least have to get my TSOP restored.
My idea was that if I could take a network card and "socket" the boot rom chip, I could use it to flash my new modchip and recover my TSOP. So I desoldered the bootrom off a 3com card, soldered on a socket, socketed my chip and attempted to flash it. The card had a 128k chip size limit.
I didn't give up. I forged on.
So then I dug through all of my working PC MoBo's looking for a PLCC bios chip, socketed or not because at this point I now know I can desolder and socket these chips. I found one. A dell.
So, I de-solder that bios chip and soldered on a socket and put in the original bios chip. Now, even tho this is a spare board, I am still nervous as hell. I have never done PC surgery of this caliber before so I crossed my fingers and turned it on. It booted
Long story short, the bios is not write-enabled on this Dell board.
Now I am discouraged because I am running out of sockets and options.
I take a look at the main PC in that room and it is aftermarket but with no socketed bios. So, I thought about whether or I want to sacrifice this PC as it is a pretty good PC and if I screw up, it will be bad. I came to the conclusion that I could do it as I had just done it 2 other times. Got the chip de-soldered, carefully soldered on another socket, plugged in the original PC bios chip and fired it up. It boots! So I hunt down a floppy drive, hook it up and fire up uniflash. 3 drives later and 6 floppies later, I found a working combo only to learn that My PC has a chip limit of 256k
My chip was an am29f 512k chip. I started this project with it, this is the chip I had to stick with.
Right about this time a buddy of mine stopped over who is also an xbox modding enthusiast. I explained my project to him and he said his PC had a socketed bios! So I dropped everything and went to his crib with him to try it. More problems. His PC didn't like my chip and would not flash it!!
Well, we tried a blind write to chip and then I rushed right home to try it in the xbox. At this point I thought I had everything wired up correctly so when the chip didn't boot, I was again discouraged. But I didn't give up because I had a feeling I was close.
Being stuck home too late to go back over to my friends house I decided to use what I had that would work. I dug out a 256k chip. Flashed it using my PC and had a homebrew modchip. So, I take it to my xbox stick it in the socket and fire up the xbox. I see "Loading iNDbios" on the screen and I almost died I was so relieved. So I spent days and hours more trying different combination's of the wiring, running through my options in my head. "Should I restore the TSOP only to permanently disable it in favor of using the chip?" Nope, I'll put in a switch.. "What about flash enable/disable??" I'll put in another switch... "Do I want and LED for status?" Hell yea..
I drew up a schematic designed for 2 DPDT toggles and an LED and here I am today. My buddy who stopped by earlier in my story says he has 2 toggles so that is what I am waiting on. I have installed the LED and am procrastinating cutting a hole in the sheilding and case bottom (my idea is to have a small hole in bottom of the case above the socket so I can hotswap chips without opening the xbox) because I am out of dremel blades.
Long story for sure but I am happy in the end with it and I have learned alot of stuff. I have basically condensed all of the lackluster guides online concerning this mod.