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cruising
March 15th, 2014, 10:08 AM
I guess my XT has a 8088 4.77Mhz, would it work if i placed a nec V20/30 10Mhz or a INTEL D8088-2 8Mhz in the socket? or does the bios only support 4.77 Mhz?

mbbrutman
March 15th, 2014, 10:15 AM
I guess my XT has a 8088 4.77Mhz, would it work if i placed a nec V20/30 10Mhz or a INTEL D8088-2 8Mhz in the socket? or does the bios only support 4.77 Mhz?

The clock speed of your machine is 4.77Mhz. Unless you overclock it (replacing the clock crystal) the speed of whatever part you put in the machine is going to be 4.77Mhz. Overclocking an XT is not recommended.

The best, easiest upgrade you can do is to find a NEC V20. It would also run at 4.77Mhz, just like your existing Intel 8088. But it is more efficient, and it will give you some improvement (a few percent depending on what you are doing) without altering anything else.

A V30 does not work - that is a replacement for an 8086, which is a different chip. (Same instruction set, but different pins.) A part rated faster (8Mhz) does not do anything unless you actually run it that fast, which requires overclocking.

There are other speed modifications that you can do (PC-Sprint) but they have varying levels of risk and complexity. Enjoy the machine for what it is today, or maybe use a V20 because it has other advantages. But overclocking is generally not a good idea on a 30 year old machine.

cruising
March 15th, 2014, 10:28 AM
I see!
Thats to bad that the clock depends on the crystal and not the cpu rate. i guess i have to find a V20 then and install it.
And if anyone has one for sale, pls tell me. but i still gonna look on Ebay

Compgeke
March 15th, 2014, 11:45 AM
They're quite hard to find but there exist some 286 upgrade cards. I have one of these in my XT making it a 12 MHz 286 over the stock 8088. I haven't run into any compatibility options but then again it also replaces the original processor.

http://goput.it/fjwjj8b

joekster
March 17th, 2014, 06:30 AM
I've got an orchid tiny turbo 286 that i would sell if the price was right.

its a nice board:
+ actual switch on it to go between 8088 and 286 modes
+ 16kb of cache
+ all hardware, no drivers to install

sergey
March 17th, 2014, 11:25 AM
Why would you want to do that? If your goal is getting a "Super XT" when it might be easier to buy an XT clone motherboard with NEC V20 or high speed 8088 (I've seen up to 12 MHz). Or even consider getting a 286 or 386 motherboard instead :-)
Just replacing 8088 with V20 won't give much speedup (anything from 0% to 15%, depending on application)... the more plausible cause for replacement is that V20 supports 80186+ instructions, so that real mode 286 applications might work on it.
Overclocking an XT is a tricky business. Many things are tied to 14.31818 MHz crystal frequency:
- 8253 timer gets 1.19318 MHz (OSC/12)... it used for timing memory refresh, system clock and sound.
- CGA uses system's 14.31818 MHz for timing (MDA does not, but I've seen VGA that have OSC connected).
- 8237 DMA works with < 5 MHz frequency... no more. So if you overclock you can forget about DMA.

So just replacing the crystal is not a good idea, and a better solution is something like this:
- Keep existing 14.31818 MHz crystal
- Use an additional IC to generate clock for 8253 (might be another 8284 clock generator or 7492 divide-by-12 IC fed with OSC signal)
- Connect an oscillator with the frequency of x3 more of the desired CPU turbo frequency (e.g. 24 MHz for 8 MHz CPU) to EFI input of 8284
- Use F/C input of 8284 to switch between normal and turbo frequencies
- Use more additional ICs to monitor DMA requests (e.g. when DMA's HRQ signal goes active) - switch system to normal frequency when it happens.
During 80's a few overclocking boards were available that would do all that is described above. They normally were connected in place of system's 8284 chip (that needs to be unsoldered, and replaced with a socket), and required some additional wires to be connected to DMA or the bus arbitration circuit to monitor DMA requests.

sergey
March 17th, 2014, 11:28 AM
During 80's a few overclocking boards were available that would do all that is described above. They normally were connected in place of system's 8284 chip (that needs to be unsoldered, and replaced with a socket), and required some additional wires to be connected to DMA or the bus arbitration circuit to monitor DMA requests.

I am wondering if there is an interest in such a board... I can see if I can come up with a similar board as my next weekend project :-)
It should be something small, so perhaps $5 per board... :-)

Update: Found the design of PC-Sprint board http://www.brutman.com/pcsprint_annotated.zip that does exactly that.

Update 2: This doesn't seem to monitor DMA activity, so I assume user supposed to switch the system to normal speed manually when he knows that DMA activity will take place (not sure how it works with memory refresh though).

Ole Juul
March 17th, 2014, 05:25 PM
If a normal XT is too slow for what you want to do, then my opinion is that you would be better off using a faster machine. I use a 133MHz Pentium for DOS work and an Ivy Bridge for contemporary processing. When speed and capability is not the primary goal, then I enjoy vintage gear for what it is - as it is.

Caluser2000
March 17th, 2014, 08:01 PM
I find it refreshing seeing new projects that enhance old kit beyond standard.

SpidersWeb
March 17th, 2014, 09:51 PM
I'm of two mindsets.
One part of me thinks 'dont buy an XT and expect speed'
Another part of me says that upgrades like these are still a part of history. I think the board sergey describes fits that criteria quite well - a creative solution that was used when 4Mhz owners badly needed more punch.

Was having this battle with myself a few weeks back - IBM PS/2 Model 50 which had a 386 MCA card installed.

sergey
March 17th, 2014, 10:09 PM
Here is my schematic for such 8284-replacement turbo adapter. Mostly copied from my Xi 8088. It includes a glitchless clock switching circuit, which adds 2 ICs.

17804

Ole Juul
March 18th, 2014, 12:21 AM
I find it refreshing seeing new projects that enhance old kit beyond standard.
Hmm, I'm not sure I see the vintage angle there. :) But then when I think of it, perhaps I do . . .


I'm of two mindsets.
One part of me thinks 'dont buy an XT and expect speed'
Another part of me says that upgrades like these are still a part of history. I think the board sergey describes fits that criteria quite well - a creative solution that was used when 4Mhz owners badly needed more punch.

Upgrades are indeed a part of history, and as you say, perhaps this project has a historical style. That said, I also like treating computer building as an art form. One where the feel that one personally has towards specific parts and how they look and interact is the palette.

I guess I'm of two minds as well. The archivist and the artist.

cruising
March 18th, 2014, 03:31 AM
Guys, calm your horses!

I was simply asking if i could replace the cpu to a faster one, not modify the whole computer, i got a hp jornada and a IBM 380Z 233Mhz laptop and they are not called speedy. So why would i buy a pc almost 20 years older and expect speed? i dont!

I guess better PCs was made bc of peoples needs + softwares got more power hungry etc. Peoples want speed! so do i...but only with upgrades from very same era.


I have been hunting a IBM-XT for years now, and now when i have one shipping home to me, i research info about it, what you can do with it, compatible components and upgrades for it. Bc i know guys in th 80's did the very same... tried to get out as much power they could, and thats the reason (i guess) the companies made better PCs and upgrades for diff computers, or am i wrong about that? i could be :)


And why would i upgrade? because it is fun to see old stuffs perform better then it was made for, not because i need it.


The only thing i will do with my XT is to get a 1.44Mb floppy to work, and also get a nec v20 cpu. Then im done upgrading :)

krebizfan
March 18th, 2014, 06:56 AM
I remember that the upgrade kits didn't always work well, especially with games. The software expected the entire system to behave like a typical system with the higher performing CPU and got confused by the subtle differences. Sometimes switches were available to ease the problem but still it makes for more work. I prefer picking up multiple normal systems for the era which costs less while being more compatible.

Great Hierophant
March 18th, 2014, 08:21 AM
I am wondering if there is an interest in such a board... I can see if I can come up with a similar board as my next weekend project :-)
It should be something small, so perhaps $5 per board... :-)

Update: Found the design of PC-Sprint board http://www.brutman.com/pcsprint_annotated.zip that does exactly that.

Update 2: This doesn't seem to monitor DMA activity, so I assume user supposed to switch the system to normal speed manually when he knows that DMA activity will take place (not sure how it works with memory refresh though).

The upgrade would be impractical in any DMA using PC compatible, because DMA is frequently used during floppy disk access or XT hard disk controller access.

jharre
March 18th, 2014, 09:08 AM
I am wondering if there is an interest in such a board... I can see if I can come up with a similar board as my next weekend project :-)
It should be something small, so perhaps $5 per board... :-)

I'd be interested in a couple boards to play around with - might be interesting in my PCjr.

sergey
March 18th, 2014, 09:13 AM
The upgrade would be impractical in any DMA using PC compatible, because DMA is frequently used during floppy disk access or XT hard disk controller access.

That's what puzzles me. To make things worse DMA is also used in PC and XT to refresh memory. Yet upgrades like PC-Sprint (http://books.google.com/books?id=k3Wu-GT3NdIC&pg=PA38&dq=PC-Sprint&hl=en&sa=X&ei=NX4oU7OZAovcoAT3noGYAg&ved=0CEMQ6AEwAA#v=onepage&q&f=false), MicroSpeed Fast88 (http://books.google.com/books?id=vcqI6sY2e-kC&pg=PA172&dq=Fast88&hl=en&sa=X&ei=Vn0oU8ORJ5HnoATaw4HABQ&ved=0CDAQ6AEwAQ#v=onepage&q&f=false), and MicroWay 87/88 Turbo Board (http://books.google.com/books?id=vcqI6sY2e-kC&pg=PA137&dq=MicroWay+87/88&hl=en&sa=X&ei=yn0oU7afIoSGogTQ4oL4Bw&ved=0CEMQ6AEwAA#v=onepage&q&f=false) were sold in mid-80's.

My design (17811) actually tracks DMA requests and reduces clock speed during DMA operations. But to do that it needs two additional connections to the motherboard, which means soldering a couple of wires to IC pins.

Chuck(G)
March 18th, 2014, 10:05 AM
I've got a couple of clone boards with the (labeled) 5MHz 8237 DMACs. I can verify that they're run at 8MHz in turbo mode. I've wondered if it could be that it was common knowledge among the clone makers that 5MHz labeled parts were either from the same run as the 8MHz parts or simply all 8237s made since a certain date were capable of running in a PC at 8MHz.

The early "turbo" boards even had the defect that the time-of-day clock would run too fast and the tones coming out of the speaker would be too high.

sergey
March 18th, 2014, 02:44 PM
I've got a couple of clone boards with the (labeled) 5MHz 8237 DMACs. I can verify that they're run at 8MHz in turbo mode. I've wondered if it could be that it was common knowledge among the clone makers that 5MHz labeled parts were either from the same run as the 8MHz parts or simply all 8237s made since a certain date were capable of running in a PC at 8MHz.


Please verify. I will be surprised if they run at 8 MHz. 8237 has very strict timing requirements. PCs (and XTs) even include a circuit to change clock duty cycle from 33% to approximately 50%. I checked several schematic diagrams of XT clones and reverse engineered the clock generation circuit used in American Research Corporation Model 10 motherboard (it apparently was used in early Dell computers), they all switch off turbo mode during DMA operations.

I don't know what is so special about 8237 design, but 5 MHz was the fastest part. Harris/Intersil and NEC make/made CMOS variants of 8237 that were rated for 12 MHz and 10 MHz respectively. These use PLCC-44 package instead of DIP-40 and I have never seen ones used in PC compatibles. By the way AT systems run their 8237s (or their equivalent in the chipset) at 4.77 MHz or lower. The original IBM AT was running 8237s at a half of CPU clock, newer ATs run it at exactly 4.77 MHz.

Great Hierophant
March 18th, 2014, 02:58 PM
The Tech Refs for the IBM PC AT and XT/286 indicate that their 8237-5 DMA controllers are operating at 3MHz. I know that the dual 7.16/4.77MHz 8088 Tandys (HX, EX & SX) run their DMA chips at 4.77MHz regardless of CPU speed, but they use custom logic chips.

The PC-SPRINT upgrade is best for a PCjr.

Sergey, I assume that your board maintains a 2.36MHz PCLK and 14.318MHz OSC signal regardless of the speed of CLK signal going to the CPU.

sergey
March 18th, 2014, 03:11 PM
The PC-SPRINT upgrade is best for a PCjr.

PCjr doesn't have DMA - no DMA no problem :-) Well almost no problem... the real time clock will run faster, and sound pitch probably will change (PCjr uses SN76489, but I assume it is still fed with the frequency derived from CPU clock).



Sergey, I assume that your board maintains a 2.36MHz PCLK and 14.318MHz OSC signal regardless of the speed of CLK signal going to the CPU.

Right, that's what it was supposed to do... and you found the first bug! :-) I made PCLK to be OSC/12.... and it actually should be OSC/6.

The frequency of 8284's OSC output is always the same as the crystal frequency (not EFI), so no modification needed there.

Trixter
March 19th, 2014, 11:09 AM
And why would i upgrade? because it is fun to see old stuffs perform better then it was made for, not because i need it.

I share your view that this is fun to play with. In that vein, you can swap in an NEC V20 for about 20% more speed on average, and if you would like to see a "silly" speedup you can run FRACTINT before and after and compare speeds. You can then install an 8087 math coprocessor and run FRACTINT again and notice the 2-5x speedup on some fractals :-) An 8087 will speed up CAD and spreadsheet software, and specialty math programs like MATLAB and FRACTINT. But that's about it, so it's not really necessary if you have trouble sourcing an 8087.