Model 25 Reply PowerBoard
"The Final Frontier*"
I'd highly advise getting the manual at the very least to perform the Reply upgrade planar installation. Having all of the items needed to do the upgrade is not really required--all you really absolutely need is the planar, floppy disk cabling and the riser. A floppy "cable converter" is included with a "new" Reply board, but I didn't have to use it in any of the install scenarios I tried.
I will at some point, though probably not soon, be publishing a guide to installing the Reply board, as well as the PC Enterprises Genesis board.
Upgrading (Video and CPU Upgrade Thoughts)
I have two known attempts to use non-Intel upgrade CPUs on the the Reply Model 25 Powerboard. The first attempt (using a Kingston TurboChip) by David Beem did not succeed. The planar wouldn't even do POST.
The planar doesn't have a POD style 486 socket, so using a Pentium Overdrive chip is not an option. It looks as though the planar tops out at 100MHz, but even then that's a big improvement from whatever your Model 25 had beforehand.
My own attempt with an IBM 5x86 CPU at 100MHz--the system powered up and would display the BIOS copyright, but it froze after that. I suppose that possible CPUID difficulties are the cause of David and I's trouble with non-Intel CPUs.
Purpose of U4? It's a place you can put additional VRAM. Reply seems to have done this on all their boards--they have left this set of pads empty. I would think another 512K VRAM chip at this location would give you a nice 1MB of VRAM, enough to run high color modes with, but nothing beyond 640x480 at 60Hz!!!
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*Well, almost. A company by the name of PC Enterprises marketed a board known as the Genesis\PC. It's a totally different board than the Reply board. In fact, it is a lot more "clone like" with regards to the BIOS and chipset, but it has a lot more of the "original" PS/2 features, like an earphone plug. There's even a place for a game port and a COM2 port.