Editing Industry Standard Architecture (section)

===Varying bus speeds===
Originally, the bus clock was synchronous with the CPU clock, resulting in varying bus clock frequencies among the many different IBM clones on the market (sometimes as high as 16 or 20&nbsp;MHz), leading to software or electrical timing problems for certain ISA cards at bus speeds they were not designed for. Later motherboards or integrated [[chipset]]s used a separate clock generator, or a clock divider which either fixed the ISA bus frequency at 4, 6, or 8&nbsp;MHz<ref>{{cite book | url=https://books.google.com/books?id=KrE0GGzOQ6oC&dq=isa+slot+speed&pg=PA557 | title=A+: Training Guide | isbn=978-0-7897-3044-2 | last1=Brooks | first1=Charles J. | date=2003 | publisher=Que }}</ref> or allowed the user to adjust the frequency via the [[BIOS]] setup. When used at a higher bus frequency, some ISA cards (certain [[Hercules Graphics Card|Hercules-compatible]] video cards, for instance), could show significant performance improvements.