what speed does ISA run at?

[TuffGuy]

I want to buy an ISA modem that is not a winmodem, but i don't know what frequency the ISA slots run at. is it 33MHz, just like PCI? Does anybody know? Mikewarrior2..?

 

[Mikewarrior2]

if i'm not mistaken, 8mhz or 16mhz... probably 8



Mike

 

[TuffGuy]

...hmmm...how does ISA handle overclocking?

 

[TerryMathews]

I think that the ISA divider is controlled by the same clockgen that produces the PCI clock. (That is, if the ISA clock is synced to the FSB, I don't think it is). Spec is 8.333 MHz. This is 100/12. Or, PCI/4. (PCI on 100MHz is 100/3=33.333). On 133, PCI is 133/4. So, PCI/4 still stands for ISA.

In conclusion since I am very drunk, it's :30 past midnight and I'm watching Seinfeld, I believe now that the ISA bus is simply the PCI bus divided by 4.

 

[Alerious]

Depends on how far you plan on overclocking your system, but I've never read or heard of anyone having any type of problems... of course... not many people use ISA devices anymore (Thank God!!). Plus, ISA slots stay relatively close to their spec when overclocked. They do, however, make non-winmodems for PCI... U.S. Robotics has one that I know of, not sure about Lucent.

 

[Mikewarrior2]

I always thought ISA speed was locked irregardless of FSB?

Eck, but I wouldn' tknow anyways.. i haven't had an ISA card in over 3 years.



Mike

 

[|TOAST|]

I'm almost sure it 16MHz. Currently, I run a Sound Blaster 16 and a Linksys Ether16 10BaseT NIC in NE2000 mode with assigned IRQ and I/O (seems to be faster than the plug and play option in NT4). These are not a problem with overclocking since I am using a BX board (SOYO 6BA+IV) with the FSB at 150MHz.

 

[Peter]

Enough guessing, folks 🙂

ISA clock is generated by the chipset's south bridge. Intel south bridges can only produce PCI/4. Older ones had PCI/3 too for 25 MHz PCI as used with Pentium-75.

Other vendors' south bridges can also do other dividers from the PCI clock, and/or a constant 7.16 MHz derived from the 14.318 MHz AT reference clock. Some BIOS setups offer a choice, some pick the fastest suitable divider automatically, and some are lazy and always set to 7.16 MHz.

The maximum speed allowed by specification is 8.333 MHz (which happens to be one fourth of the maximum allowed PCI bus frequency).

Regards, Peter

PS: Irregardless is not a word.

 

[mcbiff]

I'm 99% sure the ISA bus isn't affected by overclocking. I've never heard of anyone having problems with ISA cards when running the PCI out of spec.

 

[Peter]

As for overclocking - if it's a non-Intel chipset, the BIOS may well have secretly adapted ISA speed to legal values.

Even if this didn't happen, you might well succeed. At those low frequencies of around 8 MHz, signal quality issues aren't likely to happen, and most ISA cards have enough headroom to run well at 9 or 10 MHz as well.

For the stats, many ISA ISDN adapters can't cope with illegal ISA speeds. Game port cards also tend to freak out.

Regards, Peter