A Look at Electronic Marshalling Reliability

You may or may not have heard about electronic marshalling. It’s a new development with the DeltaV automation system. Emerson’s DeltaV team has developed a new whitepaper, Electronic Marshalling Robustness, to describe this technology and its impact on overall system availability. If you’re not familiar with DCS marshalling panels to see the traditional method of wire marshalling, here’s a Google Image search showing pictures.

The basic purpose of a marshalling cabinet is to collect all the wires coming in from devices throughout the process, and then cross wire them over to the automation system’s I/O terminations. Historically, this has been done to separate the installation efforts of field wiring, from control system wiring. These activities often occur at different stages in a project. With electronic marshalling, this cross-wiring process is eliminated.

The whitepaper describes two new components, characterization modules (CHARMs) and CHARM I/O cards (CIOCs). Each CHARM is a single-channel of I/O for the various I/O types and voltage options including digital input (DI), digital output (DO), HART analog input (AI), HART analog output (AO). Each CHARM has current limiting circuitry to prevent wiring faults such as short circuits and ground faults. Each CHARM is designed to fail open to isolate any voltage or current problems from neighboring CHARMs.

For the field loop with which an individual CHARM connects, it provides signal conditioning, analog to digital (A/D) conversion, loop power, line fault protection, field wiring disconnect capability, and a HART modem (for AI and AO CHARMs). Since CHARMs provide single channel integrity, their theoretical mean time to fail (MTTF) is higher than an I/O card that has multiple I/O channels per card. The whitepaper shows a multiplier of increased reliability ranging from 7.53x for an AO HART CHARM to 3.95x for a DI 24V dry contact CHARM.

Another important reliability measure, mean time between failure (MTBF) which is the average amount of time a system will run between failures. The measure is based upon the hardware components and their respective MTTFs. The whitepaper shares a comparison of similar systems done with classic I/O versus electronic marshalling I/O. It concludes that the electronic marshalling approach has higher availability based on the MTTF differential.

The individual CHARMS connect into redundant CHARM I/O cards, which in turn connect to the redundant network where the DeltaV controller resides in a traditional star network topology. They are segregated through firewalled switches. The whitepaper shows this topology with a diagram.

There’s more including how CHARMs are physically keyed to prevent different types being accidently inserted in a maintenance situation and how these can be reset if different types of I/O are required over time.

Overall, the whitepaper provides a technical understanding of how availability is increased to augment other papers on why this electronic marshalling approach can improve project economics.

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Update: A friend I’ve known for many years reminded me of the Electronic Marshalling Overview whitepaper as good background on this technology. There’s also a YouTube video which is worth at least a thousand words.

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