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IBM XT-286 5162 speedup

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mkemp

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My XT-286 has a 10mHz 80286 chip in it plus a patched BIOS without the 'speed bump' that should allow for clock speeds greater than 6mHz.

Has anyone tried this and, if so, what kind of speed could you achieve?

Note: the original power supply was dead and I didn't have a spare XT supply (ATs are too tall), so I transplanted the innards from a 230W ATX supply, did a bit of soldering to the Big Red Switch and used a ATX-to-AT cable. Works fine.

The other fun fact is that I found a Seagate ST31220A drive to run in it - 1.080gb that can be jumpered to act like two 540mb drives to get around OS limitations.
 
Curious - what do you mean by the 'speed bump' in the BIOS? I don't have a 5162, but generally machines of this era have fixed clock rates.
Interesting about the drive, I knew some models had an option to limit to 540, but haven't seen an IDE drive do a split before, pretty neat.
 
That's the way it was described on an IBM internal repository when I worked there. From the wiki about the 5170 AT: "Many customers replaced the 12 MHz crystal (which ran the processor at 6 MHz) with a 16 MHz crystal, so IBM introduced the PC AT 239 which would not boot the computer at any speed faster than 6 MHz, by adding a speed loop in the ROM," so I guess they did the same thing to the original XT-286.

Heck, knowing that it's highly improbable that anybody with the original BIOS has been successful. Note to self: get off top-dead-center and into the vintage stuff again.
 
modem7 said:
Diagram at [here].
Presumeably, in addition to the changes that the OP indicated, the motherboard's 12 MHz crystal has been replaced with a faster version.
Click to expand...
I haven't attempted changing the xtal yet. The only real changes in system are the 10mhZ 80286 and the patched BIOS. Well, and the power supply but that was just fixing what was broken.

I did find out something irritating: those 256k 30-pin SIMMs aren't JDEC and trying to use anything else causes the system not to start. After I found that out I put the originals back and grovelled over the techref. It turns out that the sockets don't have the address lines for anything larger even if the rest were compatible. It'd be so kewel to install 1m or even 4m SIMMs. Probably would have to disable that high 128k and come up with a modified decoder PROM along with whatever other mods might be required.
 
In the IBM 5162's predecessor, the IBM 5170, when IBM moved from a type 2 motherboard to type 3, IBM not only upped the crystal from 12 MHz to 16 MHz, but also upped the speed rating of certain chips (shown at the bottom of [here]) to compensate.

The clock circuitry of the IBM 5162, shown [here], is very similar to that in the IBM 5170. It shows that the main chips that will be directly affected by the 12 MHz crystal change are:
* 82284 clock chip
* CPU
* 80287 NPU
* 82288 bus controller
* Keyboard controller (programmed 8042)

As for the 80287, its clocking arrangement is different to that done in the IBM 5170. When the subject crystal change is done on a 5162, the 80287's internal clock will remain at 4.77 MHZ (because its CLK pin remains the same), however the clock signal to the CLK286 pin will rise from 12 MHz to 16 MHz. I do not know if such a change on the CLK286 pin is within spec for a 5 MHz (internal) rated 80287 (80287-3).
 
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