IO module replacement without disturbing field wiring

I/O replacements can be among the most challenging (and perhaps feared) of upgrades, and, if necessary, may be accomplished by replacing I/O modules, leaving wiring intact, according to Amanda Smith, product marketing specialist, DCS and migration solutions, Invensys Operations Management.


Invensys 247 FBM fits into the standard base plate of its I/A Series DCS and can be programmed with software to manage most standard I/O combinations. Courtesy: Invensys and Paul Maggetti of Maggetti Digital MediaTo decrease potential downtime, distributed control system (DCS) I/O upgrades may be accomplished by replacing I/O modules, leaving wiring intact, according to Amanda Smith, product marketing specialist, DCS and migration solutions, Invensys Operations Management. If you’re considering a full retrofit, including rewiring, then see: “IO module replacements: Intelligent marshalling, integrated networking.”

Control Engineering: I/O wiring can be among the most challenging (and perhaps feared) of upgrades. When replacing input/output connections, what are key considerations?

Amanda Smith, Invensys: One of the biggest challenges of I/O replacement is the associated cost, risk, and potential downtime of having to rewire the field wiring. Since field wiring typically outlasts the DCS system life 3 to 1, choosing a solution in which field wiring can be reused while taking advantage of new I/O made available can have significant benefits.

An example is the Foxboro Plug-in Migration solution, in which Invensys manufactures new I/O modules in the same form factor as most other vendors’ I/O modules. The new cards are standard I/A Series I/O modules that are one-for-one, form-fit replacements for most other vendor legacy I/O cards. These modules plug into existing cabinets and racks without need to move one process wire or to increase the current footprint. They line up exactly with legacy I/O points, eliminating substantial construction costs and lengthy process downtime caused by rewiring to new cabinets, reducing installation often to a matter of hours.

In addition to one-for-one card replacement, Invensys also offers a “nest replacement” for upgrading older generations of its own I/O modules to the latest generation. Invensys’s new Foxboro 200 Series FBM rack-mounted inserts plug directly into existing Foxboro 100 Series cells. This retains field wiring and communicates to the Foxboro 200 Series FBMs via new termination assembly adapters (TAAs) and existing nosecones, retaining much of the existing infrastructure, including all field wiring, without increasing the footprint.

CE: Is there documentation?

Smith: Often companies have little documentation of their legacy I/O subsystems. An installation may, for example, have been acquired by a new owner, and documentation has been lost in the shuffle or field equipment has changed and there were no procedures for updating documentation. This can present a very costly and time-consuming challenge to I/O replacement. This obstacle is avoided with the Foxboro Plug-in Migration solution. Since the existing field wiring remains intact, recreating field wiring drawings is unnecessary and system data is extracted from the running control system by using back-documentation tools, which extract and validate the control and logic database and then create new documentation.

During retrofit projects, documentation plays a critical part. In addition to standard system documentation, best practice is to document all devices, whether in service or not, as well as third-party devices, emulating them if necessary. It is also important to record all alarm points and values and note critical loops prior to cutover.

CE: What types of signals (analog, digital, serial, mixed...), what units (current, volts, resistance, ac, dc...), what range, and how many channels are needed?

Smith: A typical DCS system requires a variety of signals, and caution must be taken to avoid mixing high- and low-voltage signals. Minimum separation and segregation of cable types should be applied. Software configurable I/O points cover the broadest ranges of channel types and best practices dictate keeping a manageable number of control channels on each card to prevent impact on the control system should one of the cards fail. Invensys, for example, maintains only 4 to 8 isolated channels per card.

CE: What network protocols are required, and will they be wired, wireless, serial, digital, or a combination?

Smith: A solution should offer open standards for network protocols; however, users must consider cyber security impacts and plan their security policies accordingly. Depending on the application and desired degree of security, both wired and digital technologies should be applied. Serial protocols have limitations and should not be considered at all these days. The Foxboro I/A Series Mesh Network is designed on open standards and uses standard fiber-optic and switch technology, instead of proprietary cell buses, packaging, networks, etc. It is designed to easily adopt newer network technologies, including wireless, as they become suitable for industrial use. This Ethernet Mesh network also lowers deployment and lifecycle costs by moving away from expensive proprietary networks to a low-cost standard network that can be easily maintained. It also provides an infrastructure for information sharing between manufacturing and back-office systems.

CE: What are the sensing, signal conditioning, distributed control, isolation, and other requirements?

Smith: Requirements are based on the variety of signals in the application. For example, the Foxboro I/A Series system is a distributed architecture by design, which allows control distribution to be localized to the field or remotely depending on the need. Channel isolation is required in areas of high EMF RFI.

CE: Will the application expand, and by how much?

Smith: Yes, inevitably. New or upgraded systems should be inherently designed for future expansion. The Foxboro I/A Series system's open standards and distributed architecture offer this. Invensys adheres to a continually current philosophy for its Foxboro I/A Series system and continually infuses its platform with the latest technologies. This assures easy upgrade and expansion, significantly reducing total cost of ownership and helping customers maintain competitiveness in evolving markets.

CE: What design and how granular should the modules be?

Smith: The modules should take advantage of existing real estate as rack room space is very often limited and costly to reconstruct. Taking advantage of a plug-in approach provides new functionality within the existing infrastructure while keeping field wiring intact—eliminating extra footprint and extra cost.

CE: Will the I/O connections be enclosed or exposed?

Smith: Probably—depending on the application, most connections will be exposed.

CE: What are among the most-overlooked considerations?

Smith: Most people underestimate the amount of planning that is necessary for a successful migration. We say plan, plan, and plan some more. Users must be sure they are working with a supplier that has dedicated migration and upgrade teams. They should also have checkpoints and guidelines to reference during their replacement project and a team that will work with closely with them to ensure coverage and successful implementation in all phases of the project. This begins with documenting initial project scope, preparing P&I's [piping and instrumentation diagrams] and loop drawings and finalizing the control system architecture. It also includes defining installation requirements and hardware and software, designing and building operator interfaces. And it requires checking of third-party engineering services, staging and testing requirements, site installation, plant commissioning, and start-up. Before engaging, they should also ensure documentation is updated, have a system for keeping accurate records of changes (log book), and pre-emulate for third-party compatibility.

- Amanda Smith, product marketing specialist, DCS and migration solutions, Invensys Operations Management,, and Mark T. Hoske, CFE Media, Control Engineering,