Specifications

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 are 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.

Optimizing Strings

In the Logical Addressing modes, a write to STRING.DATA also writes 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 automatically reads 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 is made to read the string character data. In this case, the Symbolic

Mode read for string length is 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 the 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 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 is collected. While this approach to defining the application provides a

high level of performance, it doesn'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 moves from one device to

the next as quickly as possible 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 a single channel, the example above would be

the only option available; however, the Allen-Bradley ControlLogix Ethernet Driver can define up to 1024

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.