The expansion slots, all mounted on what is called the "bus" move data from the CPU to the cards, and quite a few other signals as well. Details of the busses are given below. Slots in PC's are of the following variety: (see also the pins-out [text] file for details):

• 8 bit slots: 31 contact on each side -- if this is all there is on the board, scrap the computer, it is an XT.

• 16 bit slots (the ISA bus) -- same as the 8-bit slot, but with a shorter second section added which acts as an extension and adds another 18 contacts on each side. This is the AT class computer. Slower machines will transfer data at 6.5 Megs per second. The slots will take 8-bit cards as well as 16-bit cards.

• EISA, a 32 bit slot -- but mostly obsolete; 32 Megs per second transfer rate. The slots will also generally take 8 bit and 16 bit cards.

• The MCA bus -- created by IBM for the PS/2 series, and dropped. About a half inch longer than the ISA slot (58 double contacts?), but without the division found in the ISA slots, although there is an internal index at about contact 56.

• the VLB slots -- Vesa Local Bus, at first mainly used for video, adds yet another three inch extension to the 16-bit ISA slot, and will scream along as fast as the PCI bus, below. Some VLB slots for 486-33 machines are not standard, and are now known as VLB-1. Also used for SCSI cards and HDC cards on 100 megahertz 486's.

• the PCI bus (and slot)-- The new industry standard, moves 32 bits at a crack, and at a speed of 60 megahertz or more. Here is where the interchangeability stops, these will not take cards designed for ISA or ISA-VLB. The slots are much shorter than the ISA slots, often done in white plastic, and set four or more inches away from the back of the motherboard (and, I should note, PCI boards have the solder side apposite of where they are found on the ISA boards).

• The AGP PCI slot - an extension to the PCI slot, which, no, doesn't again doubles things, just speeds up transfer to graphics cards. Looks like a PCI slot but slightly smaller.

New mother boards today often feature a combination of ISA, VLB, and PCI slots. Depending on what expansion cards you already have, and what cards you will buy new, you generally have some range of options.

Here is a quick overview of the various bus architectures available for the PC and some of the strengths and weaknesses of each. Some terms are described in more detail at the bottom. Some of this repeats information given above.

XT bus: used on the first PC/XT computers

• 8 data bits, 20 address bits
• 4.77 MHz

Comments: Obsolete, very similar to ISA bus, many XT cards will work in ISA slots.

ISA bus: Industry Standard Architecture bus (aka. AT bus). Any XT card can be operated on an ISA bus.

• 8/16 data bits, 24 address bits (16Meg addressable)
• 8-8.33MHz, asynchronous
• 5.55M/s burst
• bus master support
• edge triggered TTL interrupts (IRQs) - no sharing
• low cost

Comments: ideal for low to mid bandwidth cards, though lack of enough IRQs can quickly become annoying.

MCA bus: Micro Channel Architecture bus (IBM, for the PS/2) requires cards specifically made for MCA.

• 16/32 data bit, 32 address bits
• 80M/s burst, synchronous
• full bus master capability
• good bus arbitration
• auto configurable
• IBM proprietary (not ISA/EISA/VLB compatible)

Comments: Since MCA was proprietary, EISA was formed to compete with it. EISA gained much more acceptance; MCA is dead.

EISA bus: Enhanced Industry Standard Architecture bus - an industry attempt to equal the MCA, but never caught on. Most XT and ISA cards can be operated on an EISA bus.

• 32 data bits, 32 address bits
• 8-8.33MHz, synchronous
• 32M/s burst (sustained)
• full bus master capability
• good bus arbitration
• auto configurable
• sharable IRQs, DMA channels
• backward compatible with ISA
• some acceptance outside of the PC architecture
• high cost

Comments: EISA is great for high bandwidth bus mastering cards such as SCSI host adaptors, but its high cost limits its usefulness for other types of cards.

VLB: VESA Local Bus -- An extension of the ISA bus slot. Any XT or ISA card can be operated on a VESA bus.

• 32 data bits, 32 address bits
• 25-40MHz, asynchronous
• 130M/s burst (sustained is closer to 32M/s)
• bus master capability
• will coexist with ISA/EISA
• slot limited to 2 or 3 cards typical
• backward compatible with ISA
• moderate cost

Comments: VLB is great for video cards, but its lack of a good bus arbiter limits its usefulness for bus mastering cards and its moderate cost limits its usefulness for low to mid bandwidth cards. Since it can coexist with EISA/ISA, a combination of all three types of cards usually works best.

PCI: Peripheral Component Interconnect local bus - requires "made for" cards.

• 32 data bits (64 bit option), 32 address bits (64 bit option)
• up to 33MHz, synchronous
• 132M/s burst (sustained) (264M/s with 64 bit option)
• full bus master capability
• good bus arbitration
• slot limited to 3 or 4 cards typical
• auto configurable
• will coexist with ISA/EISA/MCA as well as another PCI bus
• strong acceptance outside of the PC architecture (note)
• moderate cost

note: which means MAC started to use them. Comments: The newest of the buses, combining the speed of VLB with the advanced arbitration of EISA. Great for both video cards and bus mastering SCSI/network cards.

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