Information Technology Reference
In-Depth Information
3
Interfacing Standards
3.1
Introduction
The type of interface card used greatly affects the performance of a PC system. Early models
of PCs relied on expansion options to improve their specification. These expansion options
were cards that plugged into an expansion bus. Eight slots were usually available and these
added memory, video, fixed and floppy disk controllers, printer output, modem ports, serial
communications and so on.
There are eight main types of interface busses available for the PC. The number of data
bits they handle at a time determines their classification. They are:
•
PC (8-bit)
ISA (16-bit)
•
EISA (32-bit)
MCA (32-bit)
•
VL-Local Bus (32-bit)
PCI bus (32/64-bit)
•
SCSI (16/32-bit)
PCMCIA (16-bit)
3.2
PC bus
The PC bus uses the architecture of the Intel 8088 processor which has an external 8-bit data
bus and 20-bit address bus. A PC bus connector has a 62-pin printed circuit card edge con-
nector and a long narrow or half-length plug-in card. As it uses a 20-bit address bus, it can
address a maximum of 1 MB of memory. The transfer rate is fixed at 4.772 727 MHz; thus, a
maximum of 4 772 727 bytes can be transferred every second. Dividing a crystal oscillator
frequency of 14.318 18 MHz by three derives this clock speed. Figure 3.1 shows a PC card.
Figure 3.2 defines the signal connections. The direction of the signal is taken as input if a
signal comes from the ISA bus controller. An output comes from the slave device and in-
put/output identifies that the signal can originate from either the ISA controller or the slave
device.
The following gives the 8-bit PC bus connections:
SA0-SA19
Address bus (input/output). The lower 20 bits of the system address bus.
D0-D7
Data bus (input/output). The eight data bits that allow a transfer between the
busmaster and the slave.
AEN
Address enable (output). The address enable allows for an expansion bus board
to disable its local I/O address decode logic. It is active high. When active, ad-
dress enable indicates that either DMA or refresh are in control of the busses.
