Rockwell Automation 1761 MicroLogix 1000 Programmable Controllers - Manual (Page 265)

Programming
Using the Message Instruction
13–17
Example 4
Application example 4 shows you how to use the timeout bit to disable an active
message instruction. In this example, an output is energized after five unsuccessful
attempts (two seconds duration) to transmit a message.
The timeout bit is latched (rung 4) after a period of 2 seconds.
This clears the message instruction from processor control on
the next scan. The message instruction is then re-enabled for a
second attempt at transmission. After 5 attempts, O:1/0 is
latched and B3/1 is unlatched. A successful attempt at transmission resets the counter, unlatches
O:1/0, and unlatches B3/1.
END
0 (EN)
(DN)
(ER)
MSG
READ/WRITE MESSAGE
Read/write
WRITE
Target Device SLC500/ML1000
Control Block N7:0
Control Block Length 7
] [
N7:0
14
(EN)
(DN)
TON
TIMER ON DELAY
Timer
T4:0
Time Base 0.01
Preset 200
Accum 0
] [
B3
1
(CU)
(DN)
CTU
COUNT UP
Counter
C5:0
Preset 5
Accum 0
N7:0
8
(RES)
C5:0
(L)
N7:0
(U)
B3
1
(L)
O:1.0
0
] [
B3
1
] [
T4:0
DN
] [
C5:0
DN
] [
N7:0
13*
1
2
3
5
6
7
* MSG instruction
status bits:
8 = TO
12 = ER
13 = DN
2-second timer. Each
attempt at transmission
has a 2-second duration.
Counter allows 5
attempts.
N7:0/8* is the message
instruction timeout bit.
After timeout error,
unlatch the MSG EN bit to
retrigger for another
attempt.
The fifth attempt latches
O0:1/0 and unlatches the
initiate message
instruction bit.
B3/1 is latched
(external to this
example) to initiate the
message instruction.
] [
T4:0
DN
4
] [
Operation Notes
8
(U)
O:1.0
0
*
] [
N7:0
12
] [
S:0
11 DH-485 Active
Protocol Bit
(U)
B3
1
N7:0
15
(U)