Allen-Bradley A-B QUALITY DL20 Series User Manual page 162

Appendix G
Application Notes
PLC-5 Programming
Examples
G–10
When no inputs are true, the display clears since the 1st rung shown always
triggers special message #1. If your application requires that the last
message remains displayed indefinitely, do not program the first rung.
The DL20 Auto Clear message feature must be disabled for this type of
operation.
In this example, one rung is required for each unique message to be
triggered.
The DL20 examines 10 bits (D0-D9) on the parallel port when a message
number is strobed in. If binary message data is selected, a range of unique
message numbers from 1 to 1022 is allowed. If BCD message data is
selected, a range of unique message numbers from 1 to 399 is allowed.
Rung Descriptions (Figure G.3)
Rung 1
Triggers special message number 1 to clear the display if no other
messages are being triggered in a later rung. A constant 1 is stored in
address N7:0.
Rung 2
Triggers message number 20 if input B3:1 is true and if rungs 3 and 4 are
not true.
Rung 3
Triggers message number 21 if input B3:2 is true and if rung 4 is not true.
Rung 4
Triggers message number 22 if input B3:3 is true.
This example provides a priority type of operation. For example, rung 4
(message 22) has priority over rung 3 (message 21) because the later rung (if
true) will write over any previous message number put into output word
0:000.
Compute Instruction
The CPT instruction will get all 16 bits of the designation source word and
move it into the designated destination word. This instruction may also be
substituted with a MOV (move) or MVM (move with mask) instruction.