Model 25SX
IBM PS/2 Model 25SX
The PS/2 Model 25SX is the highest performance Model 25 there is. It was sold exclusively to the educational market in two variants.

PS/2 Model 25SX
Though designated as an "85" series PS/2, it's got a lot more in common with the later "95" series "premium line" PS/2 systems. With onboard IDE, SVGA video, and support for up to 12MB installed RAM (non-K models only), this Model 25 is by far the most powerful around, except for the 7386, PC Enterprises and Reply planar upgrades. The monitor assembly is identical to all of the other PS/2 Model 25 systems.

Ports Trivia
386SX Underclocked---or not?

Features (on mine)
Intel 80386SX-20 MHz CPU
1.44 Floppy (possible 2.88MB support, note the 82077 FDC)
No Hard Disk (yet)
3 ISA Slots (2 standard, one "flipped", all 16 bit)

PS/2 Model 25SX Planar Board

U2 - Intel i386SX-20 (Underclocked to 16MHz in most, some have a 16MHz part here, see below)
U5 - 80387SX-16 Socket
U8 - VLSI VL82C304
U9 - VLSI VL82C305-FC
U10 - Intel i8042 Keyboard Controller (Interesting package--never seen that kind before!)
U16 - OKI 92F1173
U25 - Intel 82077AA FDC
U26 - IBM BIOS P/N 87F4794
U33 - Memory Chip solder pads (populated on non "K" models)
U34 - Memory Chip Hitachi HM514900JP8
U32 - Memory Chip solder pads (populated on non "K" models)
U42 - IBM SVGA 84F7985 (same as on the 512K SVGA/A, 40SX and 57sxx planars)
U43 - Toshiba TC511665BZ-80 VRAM ZIP
U44 - Toshiba TC511665BZ-80 VRAM ZIP
U45 - Inmos IMSG171P Video RAMDAC
U48 - Memory chip solder pads (populated on non "K" models)
U49 - Memory chip solder pads (populated on non "K" models)
U51 - Toshiba TC511665BZ-80 VRAM ZIP
U54 - Toshiba TC511665BZ-80 VRAM ZIP

J1 - Possible speaker output? Jumpered on 1-2.
J2 - Power On Password Clear
J10 - IDE
J11 - Floppy Disk Drive Cable
J12 - Power Connector
J13 - Power Connector
J14 - 4 pin Molex male connector for hard disk drive power
J16 - Unknown Function

Y1 - 32.768 kHz Hardware RTC OSC
Y2 - 14.31818 Software RTC OSC
Y3 - 25.175MHz OSC (For i82077AA?)
Y4 - 28.322MHz OSC (For what?)

OS1 - 32.0000MHz OSC for i386SX (it's underclocked, maybe one could use a 40MHz OSC?)
OS2 - 48.0000MHz OSC (for what?)
OS3 - 41.5390MHz OSC (for what?)

Items drawn in grey are not populated on the "K" series planar. MEM 2 is not populated with a SIMM socket on any planar, but on non "K" planars there are two more memory chips at this location for a total of 4MB on the planar. 2x512K chips, in banks of two = 4x1MB onboard RAM.

An attempt by Phil Mallory to install a SIMM socket at this location failed with the machine experiencing odd memory errors and failure of diagnostics. The system memory controller may be designed to take only the soldered memory packages and not two SIMMs. It might lack the decoding hardware to properly handle two SIMMs?

Ports Trivia

The "Ext. Video Pads" in the planar drawing above are for a separate video adapter installed in the system that allowed teachers and students to work on one computer at the same time. Kxx systems don't have anything except soldering pads in this spot.

The "flipped" ISA slot on the other side of the riser card is used for an IBM Ethernet or Token Ring adapter that fits in a special cutout on the back of the system. It might be possible to hack a standard Ethernet or Token Ring card to go in there. You might not even need to hack it if you use a small "half height" card.

Mystery Of U42/IBM Extended VGA or SVGA Controller

This chip is the same as the one that's used on the 512K Server SVGA card. This chip also appears on the 56/57 SX/SLC planars, as well as the Model 40SX. The Model 25 even comes with all the VRAM needed to use the SVGA modes that the chipset provides. I know it can do 640x480x256 and 800x600x16 for sure.

Using the IBM VESA driver for the 512K Server SVGA card, I was able to kick it up into 256 colors at 640x480 and 16 colors at 800x600, at which point the 25SX's built in monitor was unable to sync properly.

Though the chip is willing and the needed video RAM is available, for most people this is a useless feature. IBM didn't write any drivers for other than DOS (VESA) or OS/2. People I've asked within IBM don't seem to know anything of the "enhanced VGA" chip, beyond acknowledging that it existed.

Underclocked 386SX--Or Not???

Long story short, the IBM specs may be lying to you. The 25SX can have a 16MHz CPU clocked at 16MHz, a 20MHz CPU clocked at 16, and finally, 20MHz CPUs clocked at 20MHz. (Gee, I wonder if any were ever overclocked?)

Special thanks to bobwatts, formerly of Diesel Chevette World. He sent me my first and only example of a true 20MHz 25SX.

Why does all this matter to you? It may mean the difference between a CPU upgrade that clips over the soldered i386SX CPU being an option you can use to upgrade the processor in your 25SX or not.

Very few 16MHz parts (and even fewer as used in many PS/2s) have the ability to be shut down with a so-called disable pin on the CPU. Your CPU must be stepping 2308h or higher if it's a 16MHz unit. All 20MHz and greater i386SX CPUs have the disable pin that's required to use an upgrade CPU. 16 MHz 386SX CPUs marked "C-step" should also have the needed disable pin.

It has been suggested, though not proven, that those systems with a 20 MHz CPU actually running at 20 MHz may also be overclocking the ISA bus.

(And who says the 25SX can't compare to the Model 9x with all its CPU complexes?!)
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