8525 SX Planar

Content by William R. Walsh, original here.

PS/2 Model 25 SX Planar
Ports Trivia
Mystery Of IBM Extended VGA or SVGA Controller
386SX Underclocked or Not?

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

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 12 MB 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.

PS/2 Model 25 SX Planar (FRU 87F4769) * * *

25 XT Planar

J1 Speaker output? Jumpered on 1-2.
J2 Power On Password Clear
J11 Floppy Disk Drive Cable
J12 Power Connector
J13 Power Connector
J14 4 pin Molex for HDD power
J16 Unknown Function
OS1 32.0000 MHz OSC (for i386SX)
OS2 48.0000 MHz OSC
OS3 41.5390 MHz OSC
U2 Intel i386SX-20 or -16
U5 80387SX-16 Socket
U8 VLSI VL82C304
U10 Intel i8042 Keyboard Controller
U16 OKI 92F1173
U25 Intel 82077AA FDC
U26 IBM BIOS P/N 87F4794
U32, U33, U48, U49 solder pads
U34 Memory Chip Hitachi HM514900JP8
U42 IBM SVGA 84F7985
U43 Toshiba TC511665BZ-80 VRAM ZIP
U44 Toshiba TC511665BZ-80 VRAM ZIP
U45 Inmos IMSG171P Video RAMDAC
U51 Toshiba TC511665BZ-80 VRAM ZIP
U54 Toshiba TC511665BZ-80 VRAM ZIP
Y1 32.768 kHz Hardware RTC OSC
Y2 14.31818 Software RTC OSC
Y3 25.175 MHz OSC
Y4 28.322 MHz OSC


U32, U33, U48, U49 Memory chip solder pads - populated on non-"K" models.

U42 IBM SVGA 84F7985 - This same chip is used on the 512K SVGA/A adapter, 40SX, and 57sxx planars.

The CPU is actually a 20 MHz part underclocked to 16 MHz in most machines, but some have a 16MHz chip (see below). Maybe one could replace the 32 MHz crystal oscillator (OS1) with a 40 MHz one?

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 IBM Extended VGA or SVGA Controller

The U42 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 16 MHz CPU clocked at 16 MHz, a 20 MHz CPU clocked at 16, and finally, 20 MHz CPUs clocked at 20 MHz. (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 20 MHz 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 16 MHz 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 16 MHz unit. All 20 MHz 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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