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80287 question

DonutKing

Experienced Member
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Mar 24, 2011
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96
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Gold Coast, Australia
Hi

I have a Chaintech ELT-286B-1000-SM with a 10MHz 286 processor. I can't find any info about it on the web.
here's a photo, sorry for large size and bad quality.
I have an 8MHz 80287 copro in a DIP package.

I've read that 287's run at a 2/3 divider from the system clock. So the 8MHz copro should be fine in a 10MHz system.
Is this a hard and fast rule?

If not, is it unreasonable to expect the 287 to run at a 2MHz overclock?
 

Agent Orange

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Joined
Sep 24, 2008
Messages
6,207
Location
SE MI
Hi

I have a Chaintech ELT-286B-1000-SM with a 10MHz 286 processor. I can't find any info about it on the web.
here's a photo, sorry for large size and bad quality.
I have an 8MHz 80287 copro in a DIP package.

I've read that 287's run at a 2/3 divider from the system clock. So the 8MHz copro should be fine in a 10MHz system.
Is this a hard and fast rule?

If not, is it unreasonable to expect the 287 to run at a 2MHz overclock?


Wondering what you intend to use the co-processor for. Unless you have a spreadsheet, data base, or some software explicitly designed for use with the co-processor, its virtually useless unless you just want it for 'show & tell'.
 

krebizfan

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May 23, 2009
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Connecticut
The Intel documentation lists two possible methods of 287 clock speed. One runs at 1/3 the speed of the system clock (which is twice the CPU speed); the other uses a seperate clock and runs at the same clock speed as numeric coprocessor. The first method (where the 287 needs to be at 2/3 clock speed of the 286) is common in most AT style systems; the second method is more common with 386 systems since the 287 can't share the 386's clock.

Unless your system is very, very strange, a 8MHz 80287 will be perfect complement to a 10 MHz 80286. With all the CHIPS logos there, I expect you have a budget box and will not have any second clock.

With later 287s (often marked as 80287A or 80287XL), the chip will be able to run at 10MHz or 12MHz with no problem but you won't need it to. Yields got good but Intel wanted to stress the 80387 as the performance choice.
 

DonutKing

Experienced Member
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Mar 24, 2011
Messages
96
Location
Gold Coast, Australia
OK great, so it should be fine then? I poked around and found a datasheet: http://datasheets.chipdb.org/Intel/x86/287/datashts/210920-008.pdf
It says that Pin 39 is low if the copro is to run off a divider of the system bus. So if I stick a multimeter in pins 39 and 30, and don't see any voltage, it should be running at about ~6MHz?

Wondering what you intend to use the co-processor for. Unless you have a spreadsheet, data base, or some software explicitly designed for use with the co-processor, its virtually useless unless you just want it for 'show & tell'.

Sim City makes use of it. Game years advance a bit quicker if you have a co-pro. But apart from that, no real reason other than I have one and might as well stick it in a system :)
 

Chuck(G)

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Jan 11, 2007
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Pacific Northwest, USA
It was very common for faster motherboards to have a couple of headers for running the 287 at full speed or at a reduced speed. How many of the jumpers on your motherboard can you identify?
 

DonutKing

Experienced Member
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Mar 24, 2011
Messages
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Location
Gold Coast, Australia
Basically through trial and error I believe I've identified them all.
Here's a shot of the jumpers:
http://dl.dropbox.com/u/9151127/286/IMG_0550.JPG

J18 seems to be keylock/LED, J19 is speaker. J3 and J4 on the upper right set the installed memory size, experimenting with that gives 512KB, 640KB and 1MB memory (and also a no-boot on one of the settings).
J30 seems to be reset (at least, it restarts if I short it with a screwdriver...) while J26 seems to be Turbo. Using Sysinfo's CPU benchmark the score drops from 7.6 to about 5.2 if this jumper is enabled/disabled.
J25 seems to be enable/disable CPU speed select via keyboard. With this jumper set 1-2 I can use ctrl+alt and +/- to increase or decrease speed. Sysinfo actually reports that the CPU changes from 10MHz to 8MHz using this key combo with a drop in the benchmark score to about 6.0. If J26 is set 2-3, this keyboard combo doesn't have any effect- CPU stays at 10MHz and score doesn't drop.
The only other jumper is on the opposite end of the board and is the color/mono jumper.
 

Chuck(G)

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Jan 11, 2007
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I'd try the 80287. Watch out for it heating up too much--some 287s were shipped with heatsinks. Even when it's operating normally, it'll get pretty warm.
 
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