Rockwell Automation 1761-HHP-B30 MicroLogix 1000 with Hand-Held Programmer (HHP) - Manual (Page 301)

Chapter 15
Using Communication Protocols
15–17
Rung 2:2
Write message with report by exception (RE) or change of state (COS)
logic. COS is used to only transmit data/information when some type of
change has occurred (time, data, etc.). This type of communications is
extremely beneficial on networks because any time you can limit
unnecessary traffic on a network, your ability to send information without
delays is improved. It is STRONGLY recommended that bit S2:0/11 (active
protocol bit) be used to condition all message instructions. This bit
only allows the message instruction to operate when the correct protocol
is active. NOTE: If DF1 was the protocol used by the message
instruction, use –]/[– S2:0/11 for the preceding logic.
| DH-485 Active Write Message Ins. |
| S2:0 +NEQ–––––––––––––––+ +MSG–––––––––––––––––––––+ |
|–––] [–––––––+Not Equal |–––+Read/Write Message +–(EN)+–|
| 11 |Source A I:0.0| |Read/Write Write+–(DN) |
| | 96<| |Target Device 500CPU+–(ER) |
| |Source B N7:25| |Control Block N7:50| |
| | 0<| |Control Block Length 7| |
| +––––––––––––––––––+ | Setup Screen | |
| +––––––––––––––––––––––––+ |
Rung 2:3
This rung monitors the message instruction for “Done” conditions. When
the message is done, the enable bit is cleared allowing communication to
run again. The error retry bit is also used to reset the message if an
error or lockup condition is encountered.
| Write Message Write Message |
| Done Enabled |
| N7:50 N7:50 |
|–+––––] [––––––––––––––––+––––––––––––––––––––––––––––(U)––––––––––––|
| | 13 | 15 |
| | Message Error | |
| | Retry Timer | |
| | T4:9 | |
| +––––] [––––––––––––––––+ |
| DN |
Rung 2:4
This MOV instruction is used to set N7:25 to I:0.0 after the message is
successfully transmitted. This stops the message instruction in rung 2.2
from executing. If I:0.0 changes, the NEQ instruction in rung 2.2
triggers the message instruction to write data to the target device.
(N7:25 is used in the comparison in rung 2.2 to enable or disable the
message instruction from being processed.)
| Write Message |
| Done COS Buffer Register |
| N7:50 +MOV–––––––––––––––+ |
|––––] [–––––––––––––––––––––––––––––––––––––––|Move +–––|
| 13 |Source I:0.0| |
| | 96| |
| |Dest N7:25| |
| | 0<| |
| +––––––––––––––––––+ |
Rung 2.5
| |
|–––––––––––––––––––––––––––––––––––––+END+–––––––––––––––––––––––––––|
| |