Allen-Bradley ControlLogix Ethernet Driver Help
As discussed in Logix Communication Parameters, the block size is adjustable and should be chosen based on the project at hand. For example, if array elements 0-26 and element 3839 are tags to be read, then using a block size of 3840 is not only overkill, but detrimental to the driver's performance. This is because all elements between 0 and 3839 will be read on each request, even though only 28 of those elements are of importance. In this case, a block size of 30 is more appropriate. Elements 0-26 would be serviced in one request and element 3839 would be serviced on the next.
In the Logical Addressing modes, a write to STRING.DATA will also write to STRING.LEN with the proper length value.
Automatically Read String Length In this driver, string tags are structures with separate character data and length components. As such, the driver will automatically read a string tag in two transactions: one in Logical Protocol Mode for the string character data (DATA) and one in Symbolic Mode for the string length (LEN). When the "Automatically Read String Length" option is unchecked, a single transaction will be made to read the string character data. In this case, the Symbolic Mode read for string length will be bypassed. In a project with many string tags, this can significantly reduce the time required to read all tags.
Note: For more information on the "Automatically Read String Length" option, refer to Logix Options.
Optimizing Your Application
The Allen-Bradley ControlLogix Ethernet Driver has been designed to provide the best performance with the least amount of impact on the system's overall performance. While the Allen-Bradley ControlLogix Ethernet Driver is fast, there are a couple of guidelines that can be used in order to optimize the application and gain maximum performance.
The server refers to communications protocols like Allen-Bradley ControlLogix Ethernet as a channel. Each channel defined in the application represents a separate path of execution in the server. Once a channel has been defined, a series of devices must then be defined under that channel. Each of these devices represents a single Allen-Bradley Logix CPU from which data will be collected. While this approach to defining the application will provide a high level of performance, it won't take full advantage of the Allen-Bradley ControlLogix Ethernet Driver or the network. An example of how the application may appear when configured using a single channel is shown below.
Each device appears under a single channel, called "CLEthernet_Channel1". In this configuration, the driver must move from one device to the next as quickly as possible in order to gather information at an effective rate. As more devices are added or more information is requested from a single device, the overall update rate begins to suffer.
If the Allen-Bradley ControlLogix Ethernet Driver could only define one single channel, then the example shown above would be the only option available; however, the Allen-Bradley ControlLogix Ethernet Driver can define up to 256 channels. Using multiple channels distributes the data collection workload by simultaneously issuing multiple requests to the network. An example of how the same application may appear when configured using multiple channels to improve performance is shown below.