WOMBAT Tidbits
WOMBAT is the name for the motherboard used in the Centris 650, Quadra 650, and Quadra 800 computers. The presence or absence of some surface mount resistors on the bottom of the board, and an LED assembly, change the computers identity. Depending on these variables, the computer identifies itself as a C650, Q650, Q800, or one of two new, unreleased and previously unobserved machines. The conversion between these five machines is reversible and independent of processor speed.

The table below tabulates the variables I have studied. Surface mount resistors were either absent from the pads for R151 and R152, or present on just R151 or R152. The Quadra 800 uses a 5 pin connector at J25 to supply power to a LED power indicator. The Centris and Quadra 650 use an LED at D51 for this purpose (The Quadra 800 also has this diode, but the light it emits is not visible when the motherboard is situated inside the case). The connector that grabs pins 1 and 5 from J25 on the Quadra 800 for the LED also serves a second purpose. It has a jumper wire between pins 2 and 3. This jumper wire identifies the computer as one with an upright board, and hence a Q800. The WOMBAT board can also be fitted with an Apple PPC PDS card, and that accounts for the second half of the table. The numbers listed in the table are mach gestalts; they are numbers that identify the machine type.

WOMBAT mach Gestalts R151 and R152 empty R151 (301) (300 Ohms) R152 (122) (1.2k Ohms)
68040 No Jumper (LED) 51 (new) 30 (C650) 36 (Q650)
Jumper (LED) 59 (new) 30 (C650) 35 (Q800)
PPC PDS No Jumper (LED) 123 (PowerMac 800) 120 (PowerMac C650) 122 (PowerMac Q650)
Jumper (LED) nope 120 (PowerMac C650) 123 (PowerMac 800)

The serial ports fail above 30MHz under the conditions for gestalt 30, but they are fine at 40MHz under the conditions for gestalts 35, 36, 51, 59, 120, 122, and 123.
The modification for converting a Centris 650 to a Quadra 650 is outlined in Marlin Prowell's (mbp@janus.com) C650 Serial Port Modification file.

The current (released) system software for 68040 machines, 7.5.1, has the proper resources for mach gestalt 59, but it does not have them for mach gestalt 51. If for some reason you want to get a WOMBAT board up and running with mach gestalt 51, drop me a line, and I'll send you a patch.

When you run these machines around 40/80MHz, video problems seem to be the limiting factor. If you have a PPC card you can pull out the 68040 if you never switch back to it, and you will be able to squeeze out a few extra MHz. It has also been noted that if you use a video card you can also squeeze out a few additional MHz. One clock chipper dropped a Supermac SpectrumV Series 5 card into his C650/PPC and got it up to 85MHz. His processor was not stable at 86MHz.