Allen-Bradley A-B QUALITY DL20 Series User Manual page 155

Rung Descriptions (Figure G.1)
Rung 1
Triggers special message #1 to clear the display if no other messages are
being triggered in a later rung. A constant 1 is stored in address 057.
Rung 2
Triggers message 20 if input 112:00 is true and if rungs 3 and 4 are not true.
Rung 3
Triggers message 21 if input 112:01 is true and if rung 4 is not true.
Rung 4
Triggers message 22 if input 112:02 is true.
The ladder diagram (Figure G.1) provides a priority type operation. For
example, rung 4 (message 22) has priority over rung 3 (message 21) because
the later rung (if enabled) will write over any previous message number
placed into word 010.
Bits 14 and 15 of words 057 through 062 must be set. The message numbers
stored in these words are entered while programming the GET instructions as
described below. Next, bit 14 and 15 are set using the "SEARCH 53"
command. Bits 14 and 15 correspond to the strobe terminals MS0 and MS1
on the DL20. Any other available bit addresses can be used for strobes (MS0
and MS1), however, the module outputs must be wired accordingly.
Get and Put Instructions (PLC-2)
The GET instruction transfers 16 bits of the designated word and places it in
the PUT instructions designated word. However, only the lowest 12 bits are
displayed on the industrial terminal in BCD form (such as 001).
The GET instructions shown in Figure G.1 have two additional bits set which
are not visible on the industrial terminal ladder diagram. These are bits 14
and 15. These outputs are connected to the MS0 and MS1 strobe bits on the
DL20 parallel port.
These outputs must be turned on along with any message number being
triggered. In this example, the message numbers stored in addresses 057,
060, 061, and 062 must also include bits 14 and 15 being set in each of those
words. This is done using the SEARCH 53 Data Manipulation instruction
after entering the GET instruction and the message number.
Appendix G
Application Notes
G–3