Design standards and their application to the digital retrofit of existing analog safety-critical systems

The functional requirements for an analog safety-critical system are typically similar to those found in its replacement digital system. However, the overall system specifications, especially those regarding maintenance and testability, must drastically change to accommodate the new digital technology. Attempting to re-create a digital version of the current analog safety system without making significant modifications is not a reasonable goal. Such a reverse engineering process only serves to increase the number of system failure modes by adding digital failure modes to the existing system. In addition, a reverse engineering process fails to take advantage of the true capabilities of digital technology. A safe and effective digital retrofit can be accomplished by ensuring the modification design process includes complete execution of all design steps beginning with the requirements phase.