Real-Time Workshop User's Guide

Rate Transition Block

The VxWorks Rate Transition block provides a sample time for blocks connected to an asynchronous function-call subsystem when double buffering is not required. There are two options for connecting I/O to an asynchronous function-call subsystem:

• Use the Rate Transition block, or some other block that requires a sample time to be set, at the input or output of the asynchronous function-call subsystem. This will cause blocks up- or downstream from it, which would otherwise inherit from the function-call subsystem, to use the sample time specified. Note that if the signal width is greater than 1, data consistency is not guaranteed, which may or may not an issue. See next option.

The Rate Transition block does not introduce any system delay. It only specifies the sample time of the downstream blocks. It also informs Simlink to allow a non-buffered asynchronous connection. This block is typically used for scalar signals that do not require double buffering.

• Use the Asynchronous Buffer block pair. This not only will set the sample time of the blocks up or downstream that would otherwise inherit from the function-call subsystem, and also guarantees consistency of the data on the signal. See the Asynchronous Buffer block for more information on data consistency.

Rate Transition Block Parameters

This picture shows the VxWorks Rate Transition block's dialog box.

The Sample time parameter sets the sample time to the desired rate.

Rate Transition Block Example

This picture shows a sample application of the Rate Transition block in an ISR.

In this example, the Rate Transition block on the input to the function-call subsystem causes both the In and Gain1 blocks to run at the 0.1 second rate. The Rate Transition block on the output of the function-call subsystem causes both the Gain2 and Out blocks to run at the 0.2 second rate. Using this scheme informs Simlink to allow non-buffered connections to an asynchronous function-call subsystem.

Asynchronous Buffer Block

Creating a Customized Asynchronous Library