Reply 160 of 613, by keenerb
Is the ISA bus speed on this board a set value, or is it tied directly to CPU speed?
I can't make heads or tails of the schematic in this regard, mostly because I'm not entirely sure what the "ISA bus speed" means. I don't see anything on the standard ISA pinout diagrams that shows any sort of "clock" provided except pin B30 which is the 14.31818mhz signal from the oscillator.
*edit*
Ah, I see that B20 is the bus clock, and ont he schematic that is "CLK", which is an output from the 8284.
http://www.jameco.com/Jameco/Products/ProdDS/52871.pdf
This says it'll be a 5 or 8mhz output from a standard 8284, OR a 10mhz output from a 8284-1.
So that makes me think that the bus speed is independent of the absolute CPu speed, which makes me really wonder why my EGA/CGA cards aren't working at cpu speed 10mhz, but do at 8mhz.
BUT, then it also says that the frequency of the ic is in sync with the CPU, so I have no idea.
*edit2*
OK, so I learned a little more about schematic reading.
The turbo oscillator is connected to the 8284's "External frequency in" pin. In the datasheet EFI is stated to be "three times the CLK output". The CLK output is directly tied to the ISA bus on pin 20.
I think this must mean that the ISA bus runs at TURBO OSCILLATOR/3, which at 32mhz chip is a little over 10mhz bus speed.
This would probably explain why the EGA/CGA cards aren't working properly.
Bloodycactus, that also might explain some of your weirdness since you also installed a 32mhz turbo oscillator as I recall. It might also be worth checking that your 8284 chip is rated for >8mhz; the NEC UPB8284AD I bought says it is OK up to "25mhz" but I'm not sure if that's EFI or CLK out...
I think it would be in the best interests of stability to stick with 24mhz turbo oscillator.