View Full Version : A possible CPU upgrade on a PS/2 Model 25/286 - 12Mhz / Zero wait?

July 12th, 2016, 07:24 PM
I have been doing some pondering over the schematic for the PS/2 Model 30/286. It's about the same thing as my model 25. I have noticed a few interesting things:

The chipset is a VLSI VL82CPCAT-QC and is capable of 12Mhz operation and zero wait states with 80nS RAM. :

The PS/2 Model 25/286 and model 30/286 both operate at 10Mhz with 1WS.

The VL82C101B System controller in these systems is what takes the 20Mhz Osc and splits it to 10Mhz (the speed our PS/2's run at)

The RAM Wait state is enabled by pin 18 on the system controller:

On our PS2 machines pin 18 is hardwired to 5V, thus wait state is always on. FASTMODE is also always off. I'm not 100% How these work, but the datasheet for the chipset says it can run zero wait state at 12Mhz (our ps/2's are 10Mhz) with 80nS RAM. The stock PS2 ram is slow and can't run at that speed, but my computer is expanded to 4Mb with 80nS SIMMS.

I'm considering a few experiments, but I really don't want to destroy my PS/2. Has anyone attempted to travel this path before?

I'd like to try a few things. I have access to micro / mini level component repair and modification.

1) I'd like to switch the system controller osc from 20Mhz to 24Mhz and see if my PS/2 can run at 12Mhz. I'll probably buy the original style CPU at 12Mhz (they seem to be cheap and plentiful)

2) I'd like to make a jumper or switch of the wait state and fast mode setting.

If anyone is wondering "Why" it's kind of a pet project. I love the form factor, but the PS/2 model 25/286 is marginally faster than my 8086 with an NEC V30. With real VGA graphics, my ISA soundblaster 16, and 4MB of RAM I feel like this system is lacking just a touch of "oomph" in the processing department.

I don't want to post links so I don't get auto banned but I found the PS/2 model 30/286 schematic on google and the VLSI chipset info in a PDF document called VTI computer products 1989.

July 12th, 2016, 07:38 PM
I suspect the original 10MHz 80286 will run at 12MHz--the garden-variety 80286s had quite a bit of leeway in the specs.

Getting the RAM to run at 0WS may be a bit trickier, however.

July 12th, 2016, 07:43 PM
A book from another motherboard that used this same chipset would be a huge help. I'd like to see configuration options for boards that did either zero or one wait state. Other than the Ram speed what else would cause a need. The memory controller is part of the same chipset.

March 18th, 2018, 03:53 AM
Hey, I thought I'd post the results of doing some of this.

I have a PS/2 Model 30-286.
I switched Wait-State OFF, and it showed a moderate to large performance improvement in some benchmarks, but CheckIt 3.0 always corrupted then crashed when I tried to navigate its menu. CheckIt works perfectly with Wait-State ON like default.
I also have 4 x 1MB 80ns SIMMs like you.
Mine is the stock Intel 80286 10MHz.
I didn't try FASTMODE because I didn't understand the description: "If FASTMODE is desired, the pin must be held low until after the first memory read cycle."

I can think of some possible reasons for the instability with Wait-State Off:
> It requires 12MHz CPU. I couldn't tell from the datasheet if this was true or not.
> This motherboard isn't stable with Wait-State Off for some reason. The reason it is hard-wired ON can be assumed because of IBM's typical minimal-configuration proprietary nature with the PS/2 line.
> My SIMMs are not stable 80ns SIMMs, even though all the chips on them are 80ns rated. Here's a pic of mine:

I will try the following eventually:
> Add small 6-way DIP switch / crystal board with settings for:
>> 10MHz, 12MHz, 12.5MHz, 13.5MHz, Wait-State, Fast-Mode.
> Try other SIMMs if I can find any of the rare PS/2 type ones (or modify some as seen here [if the modified ones work in a 30-286 and not just the SCSI card]: http://john.ccac.rwth-aachen.de:8000/misc/ps2cache/ ).

March 30th, 2018, 02:12 AM
Follow up to my previous post.

Have made the board with 6-way DIP switch as described above.



Fastmode Enabled: Error 201 (Memory Error) at a random location on boot. Tried 80ns and 70ns memory.

Wait-state Disabled:
10MHz: Performance improved but unstable.
12MHz: FFF0 201 Error

Default fastmode and waitstate (just changing CPU freq, with 10MHz rated CPU):
10MHz: This is stock.
12MHz and 13.5MHz: Expected linear performance increase. Currently testing for stability; seems fine.

The performance increase from disabling the waitstate (until I try CheckIt and make it crash) was similar to going from 10MHz to 13.5MHz, so I imagine the performance would be much greater with waitstate disabled AND 13.5MHz.