Capturing M0-M1 File Data; Specialty I/O Modules With Retentive Memory - Allen-Bradley 1747-SN User Manual

B–8 M0–M1 Files and G Files
Publication 1747 6.6 - July 1996

Capturing M0-M1 File Data

The first two ladder diagrams in the last section illustrate a technique
allowing you to capture and use M0 or M1 data as it exists at a
particular time. In the first figure, bit M0:2.1/1 could change state
between rungs 1 and 2. This could interfere with the logic applied in
rung 2. The second figure avoids the problem. If rung 1 is true, bit
B3/10 captures this information and places it in rung 2.
In the second example of the last section, a COP instruction is used
to monitor the contents of an M1 file. When the instruction goes
true, the 6 words of data in file #M1:4.3 is captured as it exists at that
time and placed in file #N10.0.

Specialty I/O Modules with Retentive Memory

Certain specialty I/O modules retain the status of M0-M1 data after
power is removed. See your specialty I/O module user's manual.
This means that an OTE instruction having an M0 or M1 address
remains on if it is on when power is removed. A "hold-in" rung as
shown below will not function as it would if the OTE instruction
were non-retentive on power loss. If the rung is true at the time
power is removed, the OTE instruction latches instead of dropping
out; when power is again applied, the rung will be evaluated as true
instead of false.
B3
M0:2.1
ATTENTION: When used with a speciality I/O
module having retentive outputs, this rung can cause
!
unexpected start-up on powerup.
You can achieve non-retentive operation by unlatching the retentive
output with the first pass bit at powerup:
S:1
] [
15
B3
] [
0
M0:2.1
] [
1
] [
0
] [
1
M0:2.1
(U)
1
M0:2.1
( )
1
M0:2.1
( )
1
This rung is true
for the first scan
after powerup to
unlatch M0:2.1/1.