Transferring tips on buses

Archived Articles, Computer Upgrades

Question: What are PCI and ISA expansion slots as opposed to VL-Bus and PC/AT Bus? I have a 486 and am planning to upgrade to a Pentium system. I want to know if I can still use my old SoundBlaster 16 and Intel fax-modem, both of which use 8/16, bit expansion slots. Can I still use these cards on a PCI/ISA Pentium motherboard, or would I have to buy new ones? – Mark

Answer: All of those nutty acronyms you’re asking about relate to your computer’s internal bus architecture. A bus in a computer is a data pathway that connects one part of the computer to another.

Think of a bus as just simply a network of roads that carry truckloads of data back and forth. The capacity of each truckload of data moved along that data road is measured in bits. One end of the bus connects to the processor and memory, and the other end connects to slots. Those slots take internal expansion cards, like sound cards, internal modems, drive controllers, etc.

Now for a little history lesson:

  • In the beginning, IBM created the ISA (for “Industry Standard Architecture”) bus. It was the bus architecture used in the IBM PC/XT.
  • Then came the PC/AT bus, which was an improved version of the ISA bus and shuttled 16-bit chunks of data back and forth. The earlier ISA version moved eight bits.
  • The next innovation was EISA (for “Extended Industry Standard Architecture”) bus. That was a 32-bit bus initiative pushed by Compaq, as well as some other computer industry players. It arrived on 486 machines.
  • Not to be outdone, IBM countered with the MCA bus in 1987. It wasn’t very popular, though, because it wasn’t backward-compatible with ISA cards, whereas EISA was.
  • Somewhere in there, Apple came out with the NuBus and built it into the Macintosh II through to the Performa.
  • The VESA Local-Bus, or VL-Bus, was pioneered by VESA, which is the Video Electronics Standard Association. It was invented to transport video data quickly through computers made in 1994 and 1995, although the standard was quickly used for other types of data. You’d find it today on some 486 66 Mhz machines and into the low-end Pentiums up to the 75 Mhz Pentium.
  • In today’s computers, you’ll find the PCI (for “Peripheral Component Interconnect”) bus, invented by Intel. The architecture is innovative, in that it can be found in PC-systems as well as Apple machines. In fact, a PCI expansion card will fit in either type of machine, but will require a system-specific software driver to make it work.

Through many of these innovations, the ISA standard has endured. In today’s Pentiums, you’ll find ISA expansion slots (or more accurately the PC/AT version of ISA) alongside the PCI slots. In fact, when you buy a new machine, its specs will list the number of PCI slots AND the number of ISA slots.

Ah, but will your ISA sound card and fax-modem fit in a new ISA/PCI Pentium machine? Maybe. The ISA card tabs will fit in the ISA slots, but only if there’s a free ISA slot. Some computer makers separate the ISA and PCI slots and some cheat. If the slots are separated, you’ll have no problem. The cheaters put some of the PCI and ISA slots so close together that if you put a PCI card in a PCI slot, there won’t be enough space to plug in an ISA card in an adjacent ISA slot. Check with your computer vendor to find out.