Uncreative Labs

Joined: 20 Nov 2004 Posts: 589 Location: Shandong, China

It's been a while since I've worked on the AT, but a recent finding has allowed me to continue on with this project. My goal is to run an original 5170-339 planar at 16 or 20MHz. As you may recall, I was able to get my AT run well at 12MHz by replacing the BIOS, Crystal, Memory and CPU. That in itself is already quite an accomplishment when you consider that most people with ATs were lucky to be able to get to 10MHz.

Basically I had to stop at 12MHz, because my next highest crystal was 32MHz (for 16MHz CPU clock). They system would not boot at that speed. After reading some documentation I found this is mainly because of two other support chips. The Intel 82288 (bus controller), and 82284 (Clock generator and ready interface). Those chips found in the AT are only rated for 8MHz. I have found a 10MHz 82284 and 12MHz 82288 which I hope will get me closer to the 16MHz goal. I think a little compound and heatsink should do the trick.

I am planning to install IC sockets on my AT motherboard incase I replace the chips again in the future (like if I ever find a 12MHz 82284). I think one or both of these parts may also be produced in 16MHz version, but I am not certain. Anyway, when I have some time I'll have to see if an alternative part is available.

Joined: 03 Feb 2006 Posts: 488 Location: pennsylvanai

harris semiconductor made a 20 mhz 80286 it is a
different process chip, no reason why it would not work at lower speeds.
at least you replaced the bios, IBM put in a block so the later AT's could not be speeded up.
rumor when i was aT intel (DIV division) was that there was a newer AST made in the uk that was faster and had the st-251 drive in the drive table,
better off with a newer bios anyway. I know that will work. we did that then.
try changing the video card and other cards.

Joined: 20 Nov 2004 Posts: 589 Location: Shandong, China

I have a 20Mhz and 25MHz Harris 286 chip, but I am currently not using them. At the moment I have a Make it 486 (486SLC) installed. I may switch to the Harris later if I can get a good memory manager for the 286.

I've seen these faster IBM AT motherboards on TH99, and I thought it must be a mistake. It seems that they were probably for the european market. I believe I saw 10 and 12MHz versions based on the Type I planar with 1MB RAM. It would be interesting to find out how they got the 1MB of RAM on there...

Joined: 20 Nov 2004 Posts: 589 Location: Shandong, China

Well, it seems that I might have to replace additional chips in my AT. I will definetly have to look at the technical reference to figure out how everything is clocked. For example, know that the DMA chips are clocked at exactly half the ISA bus speed. The chips themselves are rated for 5MHz. I have heard that the ISA bus on the AT is decoupled and always runs at 8MHz regardless of what CPU crystal is installed, but as I have not read the technical manual I cannot be 100% certain this is true. Without a manual, I would need an oscilloscope to find out. I don't have a scope at the moment, but I am thinking to get one soon.

Until then, does anyone in here know for certain if the ISA bus is locked at 8MHz? I have been running my AT motherboard at 12MHz and didn't have any problems with the ISA cards, so I would guess this is correct.

Joined: 20 Nov 2004 Posts: 589 Location: Shandong, China

Well, I made some progress with my AT though I ended up clocking it back to 8MHz. While it works fine at 12MHz with my "Make it 486", it seems to have a problem with the buffalo 486 upgrade. The buffalo upgrade is ideal, because it can run the clock at 3X or 4X and has 8kb L1 instead of 1kb. The speedup with this chip is impressive, especially when properly configured. PCTools SI reports the CPU power as slightly faster than a PS/2 model 80 (or about 24 times fater than an XT). Real world performance seems to indicate this is true. I was able to run some fairly demanding DOS games. While they didn't run really fast, they were surpringly usable. I tried both SC2000 and PQ4, both of which ask for at least a 386.

I also upgraded the disk system to an AHA 1540 and an HP C2247 1GB scsi drive. That added DMA disk access and sped up throughput considerably. I guess with the Quantum ELS drives I was only getting about 300kb/sec...but now 1.6mb/sec.