Planning Your Add-On Instruction Design (Part 1)

Take time to plan your instruction design. Advance planning can identify issues that need to be addressed. When defining the requirements of an instruction, you are also determining the interface. Keep the following aspects in mind when defining your instruction requirements and creating your Add-On Instruction.

Intended Behavior

· What is the purpose for creating the Add-On Instruction?
· What problem is it expected to solve?
· How is it intended to function?
· Do you need to higher level of integrity on your Add-On Instruction?

If so, you can generate an instruction signature as a means to verify that your Add-On Instruction has not been modified.

· Do you need to use safety application instructions and certify your safety Add-On Instruction to SIL-3 integrity?

Parameters

· What data needs to be passed to the instruction?
· What information needs to be accessible outside of the instruction?
· Will alias parameters need to be defined for data from local tags that needs to be accessible from outside the Add-On Instruction?
· How will the parameters display? The order of the parameters defines the appearance of instruction.
· Which parameters should be required or visible?

Naming Conventions

The instruction name will be used as the mnemonic for your instruction. Although the name can be up to 40 characters long, you will typically want to use shorter, more manageable names.

Source Protection

· What type of source protection needs to be defined, if any?
· Who will have access to the source key?
· Will you need to manage source protection and an instruction signature?

Source protection can be used to provide read-only access of the Add-On Instruction or to completely lock or hide the Add-On Instruction and local tags.

Source protection must be applied prior to generating an instruction signature.

Nesting - Reuse Instructions

· Are there other Add-On Instructions that you can reuse?
· Do you need to design your instructions to share common code?