“What the heck is that?”
Has that thought ever crossed your mind when you’ve cracked the case on an older system and noticed the expansion slots on the motherboard? Even though PCI and AGP are today’s standards for card slots on motherboards today, organizations continue to make use of capable older systems in a variety of ways. These older systems may have expansion slots you’ve never dealt with before. For support techs in these companies, the ability to quickly and accurately identify the bus types in use on these systems can be important, especially when older systems need service.
The magic bus
The bus is how components talk to each other in a PC. While there are multiple buses, the one that I will be discussing in this article is the Input/Output or I/O bus. In this Daily Drill Down, I’ll be discussing the various expansion slot options that have appeared over the years and explaining a bit about them.
ISA: Industry standard architecture
The first system bus of note in PCs is the industry standard architecture (ISA) bus. Over the years, ISA enjoyed very widespread support and was available in both 8- and 16-bit varieties. In 1993, ISA was extended to support plug-n-play (PnP) which was often jokingly referred to as “plug and pray” because of its unreliability. Today’s PCI- and USB-based PnP systems are a far cry from these early efforts towards ease of use.
The first ISA bus supported 8-bits of data transfer at a clock rate of 4.77 MHz. However, in 1984, ISA was extended to support 16-bits of data transfer with clock speeds of 8 MHz. Because of the very large numbers of expansion cards that supported the ISA bus, it enjoyed widespread usage until the very late 1990s, at which time it coexisted with PCI on the motherboard. Now all but gone on new motherboards, if you do happen to have to work with an old PC, chances are that you will be working with ISA at some point.
It is easy to determine whether an ISA card is an 8- or 16-bit unit by simply looking at it. An ISA slot is broken into two pieces. If the edge of the ISA card uses only the first slot, it is an 8-bit card, while a card that uses both slots is 16-bit. You can see an example of both 8- and 16-bit slots and card edges in Figure A.
An 8-bit card is not able to make use of IRQs (interrupt request lines) 9-16 whereas a 16-bit card can. This can become a major problem in older PCs with a number of expansion cards that don’t support PnP. Before PnP, it was necessary to use jumpers on each ISA card to assign its IRQ, and care had to be taken to make sure that the IRQ did not conflict with another device as ISA devices are not capable of IRQ-sharing.
EISA: Extended industry standard architecture
In 1988, EISA was introduced as a competitor to IBM’s micro channel architecture (see below) and offered both improvements over ISA as well as backward compatibility with 8- and 16-bit ISA cards. EISA boasted 32-bit slots on an 8.33 MHz bus and offered both bus mastering and PnP options as a standard part of the specification.
EISA never really took off in the marketplace and was mostly relegated to server applications that could better take advantage of its improved speed. Today, you will be hard pressed to find a new system that actually supports EISA. If you need to work on an older server system, however, you may run across this short-lived, unsuccessful bus.
The EISA bus slot and cards look somewhat different than ISA cards, as you can see in Figure B. If you look closely, you will see that EISA cards look similar to today’s AGP cards. Don’t confuse them, however; an EISA card won’t work in anything other than an EISA slot. Nor will your AGP cards work in an old EISA slot.
MCA: Micro channel architecture
In 1987, IBM introduced the first 32-bit bus to the PC world with the unleashing of the ill-fated Micro channel architecture. MCA was first developed for the IBM PS/2 line of desktop computers. Upset that the ISA bus became an open standard and that it was losing control of the PC market, IBM attempted to use its position in the industry to introduce a superior, but proprietary, bus system and impose royalty fees on clone vendors.
In a classic example of superior technology losing out due to an inferior marketing scheme, IBM’s strategy backfired. With a very few exceptions, among them the Tandy 5000, no clone vendors licensed MCA. Today, MCA is all but dead as a bus. Even worse, MCA offered no backward compatibility with the plethora of already existing ISA cards.
Even though it was a marketing failure, MCA introduced many improvements over ISA. Some of these improvements found their way into EISA, which was introduced the next year in order to counter the MCA threat. Featuring bus mastering, a 10-MHz clock and supporting software-configurable IRQ and DMA (direct memory access) assignments, MCA handily beat ISA in the features department.
MCA slots come in five varieties. First, there are standard 16-bit and 32-bit slots, which each look different. The third slot type is referred to as the “8514/A slot” or MCA 16-bit with AVE (auxiliary video extension). The fourth type of MCA slot allowed for the addition of system RAM by way of the MCA bus and was called the “memory card slot” or MCA 32-bit with MME (matched memory extension). Finally, the fifth MCA slot you may run across is called the “video slot” and is a 32-bit MCA slot with MME and BE (base video extension).
All MCA slots are broken up into separate sections by use of a spacer—much like 16-bit ISA. These 16-bits cards and slots can be quickly identified because the slots on the motherboard and the pins on the card use a short section and a long section, while 32-bit cards have two longer sections. Cards and slots with AVE or BVE also include a third section that is always short. Figure C below shows a representation of standard 16-bit and 32-bit MCA cards.
If you ever find yourself working with an MCA card, be sure to have its reference disk handy. These disks are used to configure MCA devices. For more information on MCA, take a look at this Web site, which is devoted to all things MCA.
VLB: VESA local bus
The immediate predecessor to today’s powerful buses, VLB was introduced in 1992. It appeared primarily in 486-class machines although some very early Pentiums also sport this slot. VLB was the first popular “local bus,” which meant that it was placed near the processor’s much faster memory bus. This placement allowed it to completely bypass the ISA bus, thereby delivering a significant boost in performance. VLB provided 32-bits of data transfer, which also provided a big speed boost over the then prevalent 16-bit ISA bus slots.
VLB was designed primarily as a solution to the ever-increasing need for faster video performance on newer computers using graphical user interfaces. Therefore, VLB was almost exclusively used by video card vendors. Besides being used for video, VLB was occasionally used for other high-performance applications such as IDE and even network cards. However, because most motherboards that supported VLB only had one VLB slot, more often than not VLB was used solely for video.
VLB was somewhat short-lived. It was somewhat problematic in its implementation, did not support bus mastering, and didn’t even support PnP.
VLB slots can be identified by their sheer size. The slot looks like a combination of a 16-bit ISA slot and a PCI slot, as you can see in Figure D.
PCI: Peripheral component interconnect
By far, the most popular bus in use today, PCI is also a local bus with direct access to certain components of the system for fast performance. Initially only available in 32-bits running at 33 MHz, the PCI bus has improved over the past few years and now supports 64-bit applications and a variety of speeds up to 133 MHz and faster. PCI fully supports PnP and bus mastering and allows seamless sharing of IRQs. PCI slots are found in almost every workstation and server machine available.
If you’ve never seen a PCI slot or card, you haven’t worked on anything modern in a long time! Take a look at Figure E for a quick refresher.
PCI slots are generally white, although that is not a rule. I’ve seen them in pretty much every color. Also, 32-bit PCI slots are in two sections, although both sections do not have to be used by an expansion card, and 64-bit cards have three sections.
AGP: Accelerated graphics port
Last on today’s tour is the powerhouse of today’s video card world: AGP. Currently at revision 3.0, AGP now runs at speeds of four times and eight times the original specifications. AGP is mostly backward compatible with the older specifications.
Whereas other buses have been used for multiple purposes, AGP is solely designed for high performance, graphics-intensive applications and nothing else. With 32-bits for the bus and AGP 8X supporting speeds of 533 MHz, AGP is definitely fast. For comparison, the original AGP 1X has a clock rate of 66 MHz.
While AGP is mostly backward compatible, there are times when it is not. Most of today’s motherboards require the use of 1.5-volt AGP cards while the newest AGP 3.0 specification (AGP8X) uses 0.8V cards. While AGP8X slots are supposed to support 1.5V cards and AGP 4X systems should support the 0.8V cards, older AGP systems use different voltages. Blindly putting a new AGP card into an older slot that has a different voltage can quickly lead to problems on your system. Figure F below shows AGP cards.
There is typically only one AGP slot in a system. Brown in color, it is generally the closest slot to the processors and lies between it and the PCI slots.
As you can see, there have been a lot of different bus types through the years. These won’t be the last, and they certainly aren’t the only buses out there. USB 1.0 and 2.0 are also widely used on today’s systems, extending the system bus outside of the case. Firewire is doing the same thing. Additionally, new system buses such as PCI-X are on the horizon, and will add new features and power to your systems in the future.