In its basic form, abackplanemainboard contains very little in the way of intelli-
gence andstorage capabilities. It is merely a receptacle board into which processor cards,
memory cards, and other component boards are inserted to add capability to a PC.
This type of backplane board is called apassive backplane. Processor cards, containing
the CPU and its support chips, and I/O (input/output) cards, with bus and deviceinter-
faces, which are referred to asdaughterboards, are plugged into open slots on the
backplane board. The backplane provides the bus and data buffering that intercon-
nects the daughterboards. The passive backplane design is popular on network servers
because it is easily upgraded or repaired. With this type of mainboard, a server can be
back online much quicker since typically only a single card needed to be replaced,
instead of an entire mainboard!
The other type of backplane is theactive backplane. This mainboard design is also
referred to as an intelligent backplane because it adds capability to the main backplane
board to help speed up processing. The CPU, controllers, and other components are still
ontheirowndaughterboards.Actually,thistypeofbackplaneisbeingreplacedbynewer
motherboards that have Slot 1 or Slot 2 sockets for Pentium II and Pentium III Xeon
processors. The motherboard is much like the active backplane with the PII and PIII
Singe Edge Connector packaging acting much like a daughterboard, which also offers the
same utility and advantage of the active backplane.
In this book, whenever I refer to the mainboard, the system board, or the mother-
board, I will be referring to the motherboard type of PC mainboard. When I am discussing
a backplane board, which I rarely do, I will specifically say so.Motherboard Form Factors
The original IBM PC desktop computer, which was introduced in 1981, had a simple
motherboard (compared to today’s designs) that featured an 8-bit processor (the Intel 8088),
five expansion slots, a keyboard connector, memory banks for 64K to 256K of RAM, a
chipset, a BIOS ROM chip, and a cassette tape I/O adapter. The layout and size of theIBM
PC’s system case controlled the size of its motherboard and set the first form factor. Essen-
tially, aformfactordefines a motherboard’s size, shape, and how it is mounted to the case.
However, form factors now include the size, shape, and function of the system case; the
type, placement, and size of the power supply; the system’s power requirements; the
location and type of external connectors, and the case’s airflow and cooling systems.
Table 4-1 lists the more common PC form factors used in PCs.
In case you are wondering why Apple Computer motherboards aren’t listed, Apple
was never anopen architecture, which means that its designs weren’t shared with other
manufacturers. Apple motherboards only worked in Apple computers and each successive
model of the Apple II and Macintosh computers had its own distinctive motherboard.
Apple computers may have had form factors, but they were not industry standards.Chapter 4: Motherboards^73