Does anyone have a Mac286 Manual?

[rickrob]

I'm looking for the settings for a 3 pin jumper next to the 80287 math co-processor socket. It is labeled J4.

My guess is it enables the co-processor. I have a 80287 in mine but I'm not sure how to tell if it is being recognized.

 

[Unknown_K]

If you are using DOS 6 run MSD and see if the FPU is detected or not.

 

[rickrob]

I'm still running DOS 3.3.  Guess I could upgrade to DOS 6, but trying to stay 80's vintage correct.

 

[Gorgonops]

Norton SI will tell you.

https://dosbenchmark.wordpress.com/research/norton-si/

I don't recall ever seeing an fpu enable jumper on any real PC motherboard, but I'll grant it's been a long time. I do remember that some 286 machines had a jumper to adjust the clock for the coprocessor between 1:1 and 2:3 of the main CPU clock. (Long story why.)

 

[rickrob]

Norton SI will tell you.

https://dosbenchmark.wordpress.com/research/norton-si/

I don't recall ever seeing an fpu enable jumper on any real PC motherboard, but I'll grant it's been a long time. I do remember that some 286 machines had a jumper to adjust the clock for the coprocessor between 1:1 and 2:3 of the main CPU clock. (Long story why.)

Thanks, I didn't even remember that program anymore....  it's been so long since I ran DOS, I almost have to start over.

That jumper could be for adjusting the clock as you mentioned.

I have to check again, but I think the trace from the jumper goes to pin 39 of the 80287, which is labeled "CKM".  Maybe that is an abbreviation for Clock Multiplier.

I'll verify that again over the weekend.

 

[Gorgonops]

From the 80287 datasheet:

CKM - CLOCK MODE SIGNAL:
Indicates whether CLK input is to be divided by 3 or used directly. A HIGH input will select the latter option. This input may be connected to VCC or VSS as appropriate. This input must be either HIGH or LOW 20 CLK cycles before RESET goes LOW

So, yeah. I wouldn't touch that jumper unless you specifically find some fault in how it's configured now.

The 80287 was sort of a weird beast. The original version basically uses the exact same core as the 8087 designed to pair with the 8086/88 and was limited to lower effective CPU clocks than the 286; by default (IE, with CKM not connected to VCC) it divides the input clock by 3 to run an effective 2/3rds as fast as the main CPU. (The 286 normally divides its input clock in half, IE, an 8mhz 286 is fed with a 16mhz clock.) The CKM pin is there to allow a 287 with a different speed rating to be clocked at some other arbitrary speed with the help of an external clock generator. (The 287 runs asynchronously from the 286, including bus requests, so they can be clocked substantially differently from each other.) Again, I wouldn't try just sticking a jumper on there without knowing exactly how this is implemented on the Mac286 board. (IE, what the effective clock will be if you move the jumper to the other position.) Overclocking a '287 can kill it, the older ones in particular didn't have much headroom.

 

[rickrob]

Thanks for the info.  I'll leave that jumper right where it is...

 

[rickrob]

All good-- Norton SI sees the co-processor.