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Allen-Bradley ControlLogix Ethernet Driver Help

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time > cache timestamp + tag scan rate. If this case holds, another block request is made to the device, the cache is refreshed, and the cycle repeats.

Benefits

  • 1.

    Contents will be retrieved on every read.

  • 2.

    Low device turnaround time because the

Client/Server Tags are specified in the packet with their logical address.

3. Contains the maximum request per Multi-Request Packet because each request is a fixed size.

Detriments

1. Initialization overhead when uploading the project to determine the logical addresses.

2. If the minority of Logix Tags are referenced, it is slower than Logical Non-Blocking Mode (because more data is being accessed from the PLC than referenced in the Client/Server Tags).

Note: This mode is preferred when the majority of Structure Tag members are referenced by a client/server.

See Also: Performance Statistics and Tuning

Symbol Instance vs. Physical Protocol

Symbol Instance reads are CIP requests wherein the CIP Instance ID is used to specify a Native Tag in a read request. In Non-Blocking Mode, the CIP Member ID may be required to fully qualify the path to structure members and array elements. For example, the CIP Instance ID would represent the structure whereas the CIP Member ID represents the member within the structure. Because of the addition of CIP Member IDs required to fully qualify a Client/Server Tag, requests can vary in size. The deeper the nesting of structures, the more CIP Member IDS that are required to specify it, and the less requests that will fit in a single packet. Symbol Instance reads were introduced in FRN V21.

Physical reads are CIP requests wherein the DMA address is used to specify a Native Tag in a read request. In Non-Blocking Mode, the byte offset may be required to fully qualify the path to structure members and array element. For example, the starting DMA address would represent the structure whereas the byte offset represents the member within the structure. Ultimately the start + offset is the DMA address specified in the request: all requests are fixed in size (unlike Symbol Instance reads). No matter how deep structures are nested, the request will be the same size in the packet. Physical reads have been deprecated as of FRN V21.

Detecting a Change in the Controller Project

The Allen-Bradley ControlLogix Ethernet Driver monitors for project changes and can detect downloads in progress, online edits, and offline edits. When the protocol is set to Logical, users have the option to synchronize the driver's project image with that of the controller project. Synchronization ensures that the driver uses the current logical address for each Native Tag when performing reads and writes.

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Downloads in Progress: The driver monitors for both online and offline edits in every request. It will detect if a download occurs while actively reading or writing to Native Tags, and will then follow a project change procedure depending on its mode. To enable this synchronization, right-click on the device and then select Properties. In the Logix Options tab, locate either Online Edits or Offline Edits and select Yes.

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Online Edits: The driver monitors for online edits in every request. It will detect if an online edit occurs with the controller on the following read or write operation, and will then follow a project change procedure depending on its mode. To enable this synchronization, right-click on the device and then select Properties. In the Logix Options tab, locate Online Edits and select Yes.

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Offline Edits: The driver monitors for offline edits in every request. It will detect if an offline edit occurs with the controller on the following read or write operation, and will then follow a project change procedure depending on its mode. To enable this synchronization, right-click on the device and then select Properties. In the Logix Options tab, locate Offline Edits and select Yes.

Project Change Procedure (Symbolic Mode)

  • 1.

    A project change is detected.

  • 2.

    A message is posted to the Event Log indicating that a change is detected.

  • 3.

    During project change, the scenario for downloads is as follows:

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