8-Bit Expansion Bus
Early IBM PCs and XTs used the 8-bit expansion bus. There were only eight data lines running from the processor to the expansion connectors. It was a single long channel connector with 62 metal "finger connectors" or channels. The bus speed was from 4.77 MHz to 8 MHz. It provided eight interrupts and four DMA channels, all of which were pretty well pre-assigned. It was configured using jumpers and DIP switches.

16-Bit ISA Bus
The PC AT ( I believe it stands for Advanced Technology) hit the markets with a new bus, the Industry Standard Architecture bus (ISA - pronounced "ice-ah"). Also called the AT bus, it had a 16-bit data path for the new 16-bit processors. It provided eight more interrupts (16) and four additional DMA channels (8). Many were pre-assigned, but at least now there were some open for expansion. It had a bus speed of 8 MHz and was capable of using 1 bus-mastering device. The 16-bit ISA slot was characterized by 2 separate channel slots, a shorter one in front of the typical 8-bit slot to create the 16-bit connector. This allowed for backward compatibility with the older technology. A couple of 8-bit slots were often still found on some motherboards. This was perhaps because there were some older cards that had a plastic skirt around the connectors and the second 16-bit slot was in the way.

The 16-bit ISA bus can be found in 286, 386, 486 and Pentium computers. Near the end of the 486's reign, the PCI bus was introduced and adopted. Late 486s and, of course, all Pentiums rely on the PCI bus, but so many legacy ISA cards are out there (and still being made) that most Pentium motherboards still have a couple of 16-bit ISA slots on them for compatibility with older expansion cards.

ISA expansion cards were assigned the proper IRQs and DMA channels through jumpers on the card itself. However, later ISA cards made use of the discovery that you could put jumper settings into an EEPROMM chip, and some devices could be configured using software programs. Some ISA cards being manufactured now are capable of PNP.

ISA architecture also allowed for the fact that faster CPUs were now being developed. The CPU was given an internal clock or multiplier that was dissociated with the bus clock allowing each to run at its rated speed.

Micro-Channel Architecture
IBM came out with a new 32-bit bus architecture that had a speed of 10MHz. The Micro-Channel Architecture (MCA) bus was capable of using multiple bus mastering devices and could be configured using software. A big improvement over jumpers and DIP switches. Actually you needed two disks. One was a "Reference" disk, which allowed access to the configuration program, and the other was an "Options" disk, which provided the options available.

Actually a very good bus, IBM didn't want to share, and the MCA bus was only available on IBM machines. Because of the millions of computers out there with ISA bus architecture, manufacturers probably saw more money in developing a more 'generic' expansion card that would fit in any clone, as opposed to manufacturing a highly proprietary card that would only fit in one type of machine. The number of makes, models, and types of devices for MCA was limited.

Extended ISA Bus (EISA)
As a result of the advanced architecture of the MCA, several different companies put their heads together and came up with their own version, the EISA (pronounced 'ee-sah') bus. It has a 32-bit data path and is capable of using multiple bus mastering devices. The EISA bus has no need for interrupts or DMA channels and is configured with software, using a configuration utility and a device specific program. It still only has an 8 MHz bus speed.

The socket itself is taller than the 16-bit ISA. The EISA expansion cards have two horizontal rows of metal contacts on their edge connector, and there are two corresponding horizontal rows of metal 'fingers' in the socket. If you place an ISA card in an EISA socket, it doesn't fit all the way down, and its contacts only reach the top row of metal 'fingers'. This makes it completely backward compatible with the ISA cards. Although it wasn't completely proprietary, it was found mainly in brand name computers. I can't remember seeing one in a clone, or a custom built computer. As a result, there isn't a large number of EISA devices on the market.

VESA Local Bus (VLB)
The idea of a local bus came from the need for a bus that could keep up with the faster CPUs. A local bus connects directly to the processor and operates at the same speed as the CPU externally (not multiplied). The Video Electronics Standards Association (VESA) developed a local bus that had a 32-bit data path. It was built on the ISA architecture, but was longer, with a third connector that had all its lines running directly to the processor.

It was usually used for video cards, I/O cards and multimedia expansion cards. There couldn't be more than 3 VLB slots on a motherboard because the processors couldn't keep up with the transfer. Often, one of the slots was a shared slot. This meant that if the ISA slot beside it was being used, the VLB slot beside it had to be left empty. If the VESA slot had a card in it, the ISA slot beside it could not be used.

Peripheral Component Interconnect (PCI)
The PCI bus was introduced with the Pentium computer. The 16-bit and 32-bit bus architecture would limit the performance of the 64-bit Pentiums. The PCI bus supports both 32 and 64-bit data paths and uses a chipset that will also support ISA and EISA architectures. This means that the PCI bus can be used for both 486 computers and Pentiums, and motherboards can have a combination of PCI and ISA or EISA slots.

The PCI bus communicates with the processor through a bridge circuit, which acts kind of like an interpreter. This means that it can be processor independent. It can work with CISC or RISC technologies as long as it has the proper bridge circuit to interpret the information.

PCI has a 33 MHz bus speed and can support multiple bus mastering devices. The cards are Plug-and-Play and come in two versions, 5Vdc and 3.3 Vdc. The slots are keyed differently and will not allow the wrong voltage card to be inserted.

USB (Universal Serial Bus)
Universal Serial Bus is a relatively new bus technology. It was designed for low to mid-speed peripherals such as scanners, keyboards, mice, joysticks, printers, modems and some CD-ROMs. USB boasts the ability to daisy-chain up to 127 devices. This means that you could have a joystick plugged in, with a printer plugged into that, and a scanner plugged into the printer, etc...

USB is Plug-and-Play, and is completely "hot-swappable". In other words, devices can be plugged in, and unplugged while the computer is turned on and running

USB was first introduced with new computers around 1997 and the final version of Win95 (SR2) provided very limited support for it. A few problems seemed to develop at first. You had to have a Pentium machine with a BIOS that supported USB, and it had to be enabled in the setup. Your computer had to have USB ports on it, or pins that allowed for the attachment of a USB interface. Aside from that, you could install a USB adapter card in one of your PCI slots.

Some problems did arise at first. One problem was the Operating System. The early versions of Win95 did not support USB. Also, a lot of machines shipped with USB ports or capabilities before the BIOS supported it completely. Updating, or flashing the BIOS could sometimes solve the problem. Despite the growing pains, Windows 98 and computers shipped after 1998 provide excellent support for USB; and the number of devices have increased dramatically.

PCMCIA
PCMCIA stands for Personal Computer Memory Card International Association. It's often just called a PC Card bus and is used in laptops and notebook computers to add external devices such as modems, network cards, memory and removable hard drives. The bus has a 16-bit data path and only supports one IRQ. It is software configurable, using what is called Card and Socket Services software.

There are 3 different types of PC Cards and sockets. Type I is 3.3mm thick and has a single row of connectors. It was used mainly for add-on memory cards and isn't found in newer laptops. Type II is thicker (5mm) and has two rows of connectors. This is the most common and is used for modems and network interface cards. Type III cards aren't found much anymore either. They're thicker still, (up to 10.5mm) and have three or four rows of connectors. They're used mainly for adding an external hard drive to a laptop or notebook computer.

Most laptops today have two type II sockets.