ISA bus

[falcompsx]

i've always been wondering...when you raise your system bus above spec, we all know the dangers it presents to the pci bus, but what about the ISA bus? some of us still use isa modems so can anyone tell me what it does to the isa bus if anything? thanks

[LJE2]

ISA slots are not affected by the FSB, all ISA slots (8 bit and 16 bit) run at approx. 7 MHz

[Brydon]

I'm not sure if that is correct...I was always led to beleive that the isa bus ran at a %age of the pci bus so if that was way out of spec so would your isa bus. Obviously the newer mobos have a 7/8mhz option but I think that is if the pci bus is at 33mhz. I would like to hear other opinions aswell http://www.sysopt.com/forum/smile.gif

[alpha]

I doubt if ISA slots run at a percentage of the PCI, because there was no PCI in the PC-XT and PC-AT (first machines with ISA) and I know that the original IBM designs are raely strayed from (we even have the 640k base ram limit!). I don't think increased FSB can cause problems, unless your ISA cards are really ancient, 'cause newer cards should have enough tolerences to do, say, 10mhz.

[Bonehead]

ISA bus runs at 7 or 8mhz for most people depending the busspeed and devider. Check with Sisoft Sandra. You won't see a faster modem or netwerkcard, the difference is too small.

[Andy_L]

most M/B divide the fsb to get the frequency for the ISA, but the more the speed is divided, the smaller the actual difference in ISA speeds in the end.

[LJE2]

IBM invented the "XT" slot (now called the ISA slot) back in the days of the 8088 processor to keep things standard so all cards that were plugged into it would work, this slot ran at a constant speed that was regulated by a quarts crystal ocillator, like the one in your watch, That speed was approx 7.16 MHz, at the time it was fast enough, in fact it was faster than the processor. The ISA slot runs on all motherboards the same speed today to keep things standard. This is the reason the PCI slot was invented for the Pentium processor to speed up the expansion slot bus to 33 MHz.

[alpha]

LJE2, you made one slip-up - the original ISA bus ran at 4.77mhz - the speed of the mobo and chip of the XT (i used to have one!) I think it was later sped up on the PC-AT.

[Konan555]

The ISA bus on a modern system is actually counted as a PCI card the same way onboard IDE and video is.

Some systems have an EISA bus which will handle 32MB/sec (via an extra set of connections) Unfortunatly if one card isn't EISA then the hole bus runs at ISA.

[LJE2]

alpha
"On first-generation IBM XT's the 8088 processor had an 8 bit external data bus and ran at a top speed of 4.77 MHz. Therefore, IBM make the expansion slots on the XT with an 8-bit external bus connection. IBM settled on a standard expansion bus speed of 8.33 MHz (maximum), although most machines ran their expansion buses at around 7 MHz. And 7 MHz was fast enough-at the time it was faster than the CPU!"
This was taken verbatium from the book, "all in one A+ certification Exam Guide second edition" by Michael Meyers. Taken from Page 235 under the heading "History of the PC Expansion Buses: Part 1
I also had one of these speedy devils, It's good to talk to another "old timer" LOL

[Peter M]

'nuff guessing ...

Intel chipsets run ISA at PCI/4, no other choices (other than useless PCI/3 which is still there from 50 MHz CPU bus systems with 25 MHz PCI).

Other chipsets have more (and software programmable) dividers, plus, most of the time, a constant 7.14 MHz setting (derived from the 14.318 MHz reference clock).

Regards, Peter

[alpha]

Cool, I just thought that the ISA couldn't be faster than the cpu - just wouldn't sync right - I was wrong. But how could the expansion bus run faster than the mobo? Just seems weird.

Off topic, but what stuff did you have for your XT? I think I had Test drive (1987), Alleycat, Willy the worm, pacman, stargate and spacewars. Those were the days! http://www.sysopt.com/forum/wink.gif