A cornerstone of I/O on Demand is electronic marshalling, a new approach to an industry practice that until now has changed little over the past several decades.
Indeed, classical marshalling is at the heart of a labor-intensive, relatively inflexible work practice that also is subject to the whims of late-stage process design modifications. Changes in process design drive changes in control system inputs and outputs required, and proceed to cascade through all that detailed engineering work—from reworking drawings to control system partitioning to building new cabinets. Late design changes are inevitable, but they add cost, time, and most important, risk to any project. The practice of wired marshalling only intensifies these problems.
But what if the nature of any single I/O channel could be changed at will, at any time during a project? What if a new pair of wires needed only a place to land and could be digitally bound to any controller in the system? What if all marshalling cabinets and junction boxes were of a "standard" design and need not be engineered beyond knowing an approximate total I/O count?
Electronic marshalling does all these things. As a result, it effectively removes I/O from the critical path of many projects—decoupling process design from I/O architecture decisions, as well as eliminating the rework costs and project delays that were once the inevitable consequence of late-stage design modifications.
"The new I/O on Demand capability of Emerson's DeltaV S-series allows users to add or change I/O types whenever they make project design changes, no matter where the I/O is located," notes Larry O'Brien, analyst for the ARC Advisory Group. "This reduces project costs and, even more important, reduces time to startup."
Out with the Old
In the typical project of today, field home run wires are landed on the right-hand terminal strips in the marshalling cabinet shown in Figure 1. The terminal blocks then must be cross-marshalled to the appropriate I/O card and controller on the left-hand side of the diagram, resulting in a rat's nest of wiring that is both difficult to manage and difficult to modify.
Figure 1. Traditional marshalling involves the landing of field wiring on terminal blocks followed by the manual cross-marshalling of each signal pair to the appropriate I/O card and controller on the other side of the cabinet.
With electronic marshalling, wires from the field still are landed on the right-hand set of terminal blocks as shown in Figure 2. But there are no cross wires. All of that work, design and engineering simply goes away. That's because with electronic marshalling, each individual channel is characterized to become the appropriate I/O type—analog input, digital output, etc.—simply by plugging in the appropriate A/D converter module called a characterization module, or CHARM.
Figure 2. Electronic marshalling allows field wiring to be landed on any available terminal block in the cabinet; each channel is then individually characterized and digitally mapped to the desired controller.
So when it comes to instrumentation installation, land the wires anywhere, characterize the signal with a CHARM, drag and drop it to the appropriate controller in the host system and off you go! Any I/O can be used with any controller in the system—meaning that all of the I/O can be bound to the control system much later in the project. Overall system costs are lower because internal cabinet cross-wiring is eliminated, cabinet footprint is reduced, I/O channel assignments are simplified and factory-acceptance test (FAT) activities are trimmed.
An Inside Look
The DeltaV CHARM IO card (CIOC) itself has been designed for ease of use, both in physical installation (Figure 3) and its software tools. Components snap together with secure DIN-rail latches and interlocking carrier connectors; a series of 96 I/O channels can be connected to a DIN-rail in a matter of minutes. Each I/O card can serve I/O signals to any four controllers in the system with 50 ms updates for fast, reliable control.
Figure 3. Detail shows how a series of characterization modules, or CHARMs,
are used to individually condition and convert each I/O signal—HART
The CIOC architecture is fully redundant, starting with the two I/O cards on a carrier. The carrier has redundant communication modules for primary and secondary network connections. There are two 24-VDC input power connections. The carrier connects to the CHARMS base plates and provides redundant power and communication buses to the CHARMs. Everything is redundant down to the individual channel.
No tools are needed to remove a CHARM or CHARM terminal block from the CIOC (Figure 3). Upon initial insertion, CHARMs are sensed by the system, automatically creating the I/O definition in the DeltaV configuration database. Also, upon initial insertion of a CHARM, each terminal block is "self-keyed" so that the wrong type of CHARM cannot be mistakenly inserted. CHARMs also can be partially ejected to a locked position, disconnecting the field wiring from the system to perform field maintenance actions or to remove power to a field device. Activating the CHARM latch ejects the CHARM to the stand-by position. Closing the latch locks the CHARM in place and isolates the field wiring for field work.
Figure 4. Redundancy is built into each CHARM IO card, including the power supply and communication link.
What Cabinet Design?
Because electronic marshalling senses the individual character of each I/O channel to its plug-in CHARM, the design of marshalling cabinets can be greatly simplified. Indeed, Emerson's standard electronic marshalling cabinet comes with no options (none are required) and with all rails and components installed. The only missing parts are the CHARMS and corresponding CIOCs, which means that I/O from field can be wired up at any time—and be electronically marshalled later.
And because all components of the CIOC are rated for installation in Class 1/Division 2 or Zone 2 hazardous locations and feature extended operating temperature ranges and G3 environmental ratings, electronic marshalling can be done in field-mounted junction boxes. Using standard Ethernet infrastructure hardware, I/O can be added to a remote enclosure located miles away from the controllers and control room. This further reduces the footprint of the central equipment room as well as reducing the overall wiring infrastructure of traditional multi-core instrumentation cable.
Like its electronic marshalling cabinets, Emerson's electronic marshalling junction boxes also come in a one-size-fits-all design: Simply run a CAT5 or fiber-optic Ethernet back to the controller cabinet, and now the marshalling cabinets disappear altogether. This means even fewer design tasks, less footprint, less wire and fewer wiring problems.
All told, the use of electronic marshalling in junction boxes has the potential to eliminate scores of hours of engineering, design and installation work while improving the ability of a project to accommodate late-stage engineering changes while minimizing rework and schedule impacts. The traditional marshalling cabinets and I/O cabinets are effectively gone, along with design and installation tasks related to such tedious details as fuses, spares, jumpers and terminations, wiring diagrams and cable layout—not to mention the design of the cabinets and junction boxes themselves.