Rockwell Automation 1755-OF8 GuardPLC Controller Systems - Manual (Page 233)

Publication 1753-UM001C-EN-P - March 2010 233
Use GuardPLC Controller as an Adapter Chapter 19
This Class 3 example uses the Cache Connections option. A
connection with this flag is opened when the controller switches to
Run mode and the rung condition is true. In our example, the rung
condition is true when Enable value is true and the timer has expired
(DN flag is set). When the rung condition is False, the connection
remains open.
If the Enable tag is changed to false, the connection still remains
open. To maintain the open connection, the Logix controller
periodically sends the last message with the same data sequence
number. This same data sequence number indicates to the GuardPLC
controller that this is just a keep alive message and that the data has
not changed. If this is a write message, the GuardPLC controller still
responds to it, but ignores the data that came with it because it knows
the data has not changed. This periodic frequency is set by default to
7.5 seconds, meaning that every 7.5 seconds a ‘keep alive’ message
will be sent to keep the connection open.
Once Enable is set back to true, the messages are sent with every
transition of the rung condition from false to true. In this example, a
message is sent when the timer times out at 100 ms, and every time it
has a new data sequence count. So, if the write data changes, this
change is communicated to the GuardPLC controller no later than 100
ms past the data change tick.
The connection is closed when the controller transitions to Program
mode.
Download and Go to Run
1. Download to the Logix controller and switch the controller to
Run.
2. Set Enable to true.
Both messages should show the Done flag set. If an Error flag is set,
double-click the message instruction to see the error description.
Verify the Data Exchange
Follow these steps to verify the data exchange.
1. In RSLogix 5000 software, switch to the Controller Tags tab.