Allen-Bradley A-B QUALITY DL20 Series User Manual page 88

Chapter 7
The Parallel Port
Reducing Host
Output Requirements
19 18
D0 D1
PC Outputs (3)
7–16
In these examples where MS0 AND MS1 are not strobed, the data lines
change state quickly, or the DL20 could read erroneous data. The DL20
samples the data and strobe lines at the set scan rate. The lines are always
resampled at least twice, depending on the debounce time setting, and
whether AC or DC sampling is selected. All readings must agree, or the
DL20 will continue to sample them until they do. If the data takes longer to
change than the debounce time, then erroneous data could be entered into the
DL20.
Figure 7.5 shows how to reduce output requirements even further. Many
applications do not require the full 1022 message capability of the DL20. If
fewer messages are required, you need only enough data lines to address all
the messages. The number of messages addressed by n data lines is 2 to the
nth power less one. For example, 4 data lines can address 16 - 1, or 15
messages. A total of seven messages may be displayed by the example
shown in Figure 7.5.
Figure 7.5
3 Output Configuration
17 16 15 14
D2 D3 D4 D5
Tie Together
For LOW TRUE
Logic Only
13 12 11B 11A
D6 D7 D8 D9
Tie Together
For HIGH TRUE
Logic Only
Terminal
21 20 23
Number
MS0 MS1 GND Terminal Block
Designation
Power Supply
-
(Observe DC
+
5-30 VDC
Polarity)