Information Technology Reference
In-Depth Information
no handshaking and when there are four, or less, data bits to be read in. If there are more, or
if there is handshaking, then extra circuitry is required.
PtrBusy (BUSY)
AckDataReq (PE)
XFlag (SELECT)
DataAvail (ERROR)
Data
input
b
7
b
6
b
5
b
4
b
3
b
2
b
1
b
0
Status port (379h)
inval=(_inp(0x379) & 0xf0) >> 4
Figure 17.3
Nibble mode interfacing
Figure 17.4 shows how the nibble mode can be used to read-in eight bits at a time. For
this one bit of the data output lines (D0) is used to select either the upper four bits or the
lower four bits of the 8-bit data byte. If D0 is a low (0) then the lower four bits are selected,
else if it is a high (1) then the upper four bits are selected. The D0 output line connects to a
multiplexor which will select the lower or the upper four bits. If A is the multiplexor selector
line, X[1:4] are the input data bits and Z[1:4] the output from the multiplexor, then the equa-
tion for the multiplexor is
Z
[
=
AX
[
+
A
X
[
Z
[
2
=
AX
[
]
+
A
[
2
]
A
X[1]
Z
[
=
AX
[
7
]
+
A
X
[
Z[1]
X[5]
Z
[
4
=
AX
[
+
A
[
4
]
