Configurable ControlLogix Protocol Modes for Optimal Performance

Flexible Connectivity to Remote Rockwell AB Controllers Using ControlLogix as a Gateway

The ControlLogix Ethernet driver has an advanced "Protocol Mode" setting that determines how Logix Tag data is read from the controller.  There are three options:

  1. Symbolic
  2. Logical Non-Blocking
  3. Logical Blocking

General Guidelines for Selecting Logix Protocol Mode in TOP Server

Selecting the right protocol mode when using the TOP Server ControlLogix Ethernet driver is dependent on certain aspects of your project and what data you need to retrieve from your controller.

  • Symbolic Mode - Most efficient when only a few tags are being read - this mode packs the entire ASCII symbol tag name in the request so tag name length is a limiting factor. This mode is recommended for best performance over telemetry networks such as cell modems/radios.

  • Logical Non-Blocking Mode - Generally speaking, this is the most efficient mode. If you are accessing less than 30 percent of the tags within a structure, this is the recommended method.

  • Logical Blocking Mode - This mode is most efficient when you need to access the majority of tag members within a structure since it pulls the entire structure into the driver, and allows the driver to parse the tag addresses internally.

Symbolic Mode Details

Each Logix tag address is represented in the packet by its ASCII character name. Symbolic mode is convenient in that all of the information needed to make a data request lies in the Logix tag's address.  It also doesn't require the logical mapping operation when the driver first connects to the device.

To take advantage of the Multi-Tag Request Packet optimization, you want as many tags represented in a single packet as possible. Since tag addresses are represented by their ASCII character name in the packet, this implies tag addresses should be made as short as possible. For example, "MyTag" is preferred over "MyVeryLongTagNameThatContains36Chars".

Logical Non-Blocking Mode Details (Default)

Logical Non-Blocking mode is identical to symbolic in that all Logix tags are requested individually, utilizing the Multi-Tag Request Packet. They differ in that the Client/Server tags are specified in the packet with their physical address, not their symbolic address.

Because the physical pointer for a Logix tag is so much smaller than a symbolic tag name, more addresses can be returned in a single roundtrip request, making this logical non-blocking mode considerably faster than symbolic mode.

Logical Blocking Mode Details

With Logical Blocking mode, all data for a Logix structure is retrieved in a single request. Blocking is possible because each Logix tag has a base physical address from which to work with.

Logical Blocking mode is ideal when most or all members/elements for a given Logix structure are being referenced by a client.

Logix Protocol Mode Support by Firmware Revision

The TOP Server ControlLogix Ethernet driver queries the controller when initializing communications to determine key information about the controller, including the version of firmware currently installed.

As such, for logical/physical modes, the driver can intelligently select the proper protocol to be used based on the confirmed firmware revision and what it supports.

Important Notes and Resources for the ControlLogix Ethernet Protocol:

  • IMPORTANT NOTE: When using either Logical Protocol Mode, it imperative that each device be configured under a separate channel (see Multiple Connections). Failure to do so will result in slow performance due to the TOP Server process of polling devices sequentially when defined under a single channel, combined with the pre-mapping of addresses that occurs with the Logical Protocol Modes.
  • Communicating over Cellular? Switch your device's Protocol Mode to Symbolic for best performance - the logical mapping process doesn't typically work well over telemetry and may result in communication issues.
  • Need to Configure More than One Device per Channel due to Telemetry communications or Other Reasons - Switch your device's Protocol Mode to Symbolic - using any of the Logical/Physical modes with more than one device on a channel will result in the Logical/Physical address mapping process to have to occur for every device every time you communicate, severely impact performance.
  • Guides for Optimizing ControlLogix Configuration:

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The demo is the full product once licensed. Once a client application connects to the  TOP Server, the TOP Server runtime will operate for 2 hours at a time. At the end of the 2 hour demo period, the demo timer must be reset by restarting the TOP Server runtime service.

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