Ok, just re-read this thread, my current 286 Motherboard has a Harris 286-16, and a 287-6, there's also a large 32MHz Crystal which I'm assuming is to clock rate the 286-16, plus there's two other small Crystals, a 16MHz, and a 14.31818MHz, which I've no real idea what they control, but presumably at least one clocks the 287-6.
Now I've bought a Harris 286-25, and a replacement large 50MHz Crystal, which I can only assume will control the faster CPU.
However, I've just bought an IIT 2C87-12, how do I get the right crystals to run that?
My assumption here is the 16MHz Crystal currently steps down the clock rate of the existing 286-16, although that doesn't make any sense, so possibly its the crystal selected to supply a harmonic frequency combined with the 14.31818MHz crystal, and must somehow deliver a 6MHz clockrate to the existing 287-6, but I'm also aware that 286 ISA ran at either 6MHz or 8MHz on these boards, with the original speed on 8086/88 boards at 4.77MHz, tell me if I'm barking up the wrong tree here.
Ah, 3x 4.77MHz is 14.31, so this is an ISA reference crystal, so I can't touch this one (14.31818 MHz).
To get to 6MHz requires a leap of faith, but see what you think
4.77 MHz X 16 MHz = 76.32 MHz
then
76.32 MHz / 14.31818 MHz = 5.33 MHz
or 6 MHz near as damn it supplied to the 287-6
So I'm thinking a 32 MHz Crystal to replace the 16 MHz
giving me 10.66 MHz for the 287-12, which is at least twice as quick as the old 287-6 IF my assumptions are correct
Although I still have no idea how the ISA standard is supplied either 6 MHz or 8 MHz, since the NTSC normally use a reference of 14.31818 MHz for ISA, perhaps this is an older motherboard and ISA still runs at the old 4.77 MHz Bus.
Does this sound anything like on the right track?
Any ideas gentlemen/ladies?
8086-8, 286-16, 386DX-40, 486DX4-100, K5 PR166, K6-2 550, K6-3 450, 3x XP 3200+, 64 3700+,
2x 64 X2 4400+, Phenom II X2 220, Phenom II X6 1100T, Athlon X4 845, FX-8370.
Laptops 1110, 600E, 2200, C640, 1520, D830, 3558. Sinclairs + Playstations.