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The PS/2 Model 80 was one of the most popular PS/2 models made and available up to a 386DX25 with external cache. However, applications grew quickly and 386-class machines were soon to small for them. In case of chop suey PCs, you simply throw out the mainboard and replace it with something faster. Right also for the Model 80? Yes - if you find a board that mechanically fits into the Model 80's case. Reply was a company known for making such replacement boards, and some of these upgrade planars were even marketed by IBM for some time. Given how expensive the early PS/2 models were, a lot of users bought these upgrade boards to 'protect their investments'...
A look into the machine doesn't show many differences compared to an original Model 80, the case just looks a lot 'emptier': The large, 5,25-inch full height disks are gone, a small 3,5-inch IDE drive now has more capacity then the largest of the ESDI monsters IBM offered. You need an additional adapter frame to hold the small drive in place...the battery holder is also empty, the battery was moved onto the planar, just like in the later IBM PS/2 models.
Gone are also the raiser-styled memory modules that were unique even in the PS/2 line. The planar uses standard 72-pin SIMM modules and can take up to 8 of them. 8-MByte modules are known to work, so you can stuff in at least 64 MBytes, but it is possible that even larger modules work.
The CPU itself is placed in a ZIF socket for easy upgrade. 5V-based models are simple, DX4 CPUs probably need a voltage regulating interposer.
If you build a Microchannel-based machine, it is difficult to get around IBM: the metal-capped chip is from IBM and probably responsible for driving the MCA bus.