Information Technology Reference
In-Depth Information
CL3
4
0
X
YZ11
CL34
0
X
YZ11
CL340
CS220
PC bus connector
ISA extension
ISA card
Figure 3.3
Figure 3.4
ISA bus connections
The Industry Standard Architecture (ISA) bus uses a 16-bit da
ta bus
(D0-D15) a 24-bit
address bus (A0-A24) and the CLK signal is set to 8.33 MHz. The
SMEMR
and
SMEMW
lines
are used to transfer data for the lowest 1 MB (
0h
to
FFFFFh
) o
f me
mory (
where
the S prefix
can be interpreted as small memory model) and the signals
MEMR
and
MEMW
are used to
transfer data between 1 MB (
FFFF
Fh
) an
d 16 MB (
FFFFFFh
). For example if reading from
the address is
001
000h
then the
SMEMR
the line is made active low, while if the address
1F0000h
then the
MEMR
line is made active. For a 16-bit transfer the
M16
and
IO16
lines are
made active.
The extra 16-bit ISA bus connections are:
Address bus (input/output). The upper 7 bits of the address of the
system address bus.
A17-A23
SBHE
System byte high enable (output). The system byte high enable
indicates that data is expected on the upper 8 bits of the data bus
(D8-D15).
D8-D15
Data bus (input/output). The upper 8 bits of the data bus provides
for the second half of the 16-bit data bus.
