Information Technology Reference
In-Depth Information
Zero wait states (input). The zero wait states (or no wait state) allows a slave to
shorten the amount of time required for a bus cycle.
0WS
DMA request (input). The DMA request indicates that a slave device is request-
ing a DMA transfer.
DRQ1-DRQ3
DMA acknowledge (output). The DMA acknowledge indicates to the request-
ing slave that the DMA is handling its request.
DACK1-DACK3
T/C
Terminal count (input). The terminal count indicates that the DMA transfer has
been successful and all the bytes have been transferred.
Refresh (output). The refresh signal is used to inform a memory board that it
should perform a refresh cycle.
REF
Interrupt request. The interrupt request signals indicate that the slave device is
requesting service by the processor.
IRQ2-IRQ7
RESET DRV
Reset drive (output). The reset drive resets and plug-in boards connected to the
ISA bus.
Crystal oscillator (output). The crystal oscillator signal is 14.318 18 MHz signal
provided for use by expansion boards. This clock speed is three times the CLK
speed.
OSC
I/O check (input). The I/O check signal indicates that a memory slave has de-
tected a parity error.
IO
CH
CHK
-
5V
, -
12V
and
GND
Power (output).
3.3
ISA bus
IBM developed the ISA (Industry Standard Architecture) for their 80286-based AT (Ad-
vanced Technology) computer. It had the advantage of being able to deal with 16 bits of data
at a time. An extra edge connector gives compatibility with the PC bus. This gives an extra 8
data bits and 4 address lines. Thus, the ISA bus
has
a
16-bit data and a
24-bit
address bus.
This gives
a
maximum
of
16 MB of addressable memory and like the PC bus it uses a fixed
clock rate of 8 MHz. The maximum data rate is thus 2 bytes (16 bits) per clock cycle, giving
a maximum throughput of 16 MB/sec. In machines that run faster than 8 MHz the ISA bus
runs slower than the rest of the computer.
A great advantage of PC bus cards is that they can be plugged into an ISA bus connector.
ISA cards are very popular as they give good performance for most interface applications.
The components used are extremely cheap and it is a well-proven reliable technology. Typi-
cal applications include serial and parallel communications, networking cards and sound
cards. Figure 3.3 illustrates an ISA card and Figure 3.4 gives the pin connections for the bus.
It can be seen that there are four main sets of connections, the A, B, C and D sections (Figure
3.4). The standard PC bus connection contains the A and B sections. The A section includes
the address lines A0-A19 and 8 data lines, D0-D7. The B section contains interrupt lines,
IRQ0-IRQ7, power supplies and various other control signals. The extra ISA lines are added
with the C and D section; these include the address lines, A17-A23, data lines D8-D15 and
interrupt lines IRQ10-IRQ14.
