Inevitably, I/O has to be added or changed late in a project. Maybe a pressure switch gets changed to a pressure transmitter, so a discrete input needs to be changed to an analog input. Or, maybe a skid has not been accounted for and suddenly the system has to accommodate for a number of new I/O points and a controller with spare I/O capacity has to be found.

With this scenario, delays can happen to increase the risk associated with a project.  Many I/O changes, or sometimes for other reasons, it is necessary to split up a controller, which can cause additional delays to the project. A controller may have to be split if more I/O is required, but no single controller can handle the new I/O. This means that configuration and I/O for process equipment has to be moved from one controller to a new one.

Control configuration is usually easy to reassign to a new controller, but physically moving I/O cards and cross marshalling can be more complex. First of all, if I/O that has already been cross-marshalled needs to be moved to another controller, the multi-core cables from the field have to be moved from one marshalling panel to another. Then, the cross-marshalling has to be redone, which causes costly delays. This may also increase the need for panel space.

Electronic marshalling
As in wired marshalling, the multi-core cables from the field still land on the right side of the terminal blocks in the marshalling cabinet, so there are no changes to the work done by technicians that wire up the multi-core cables from the field. However, there are no cross-marshalling wires; all of the work, design, and engineering associated with cross-marshalling simply go away, because each individual I/O channel will be Electronically Marshalled to any controller in the system. This means that the I/O can be bound to specific controllers at any stage in the project. If late changes are made to I/O types, or new I/O is added, no adjustments to existing wiring or the cabinets must be made. New I/O can be added to the marshalling cabinets and electronically marshaled to the controllers that need the I/O.

Components
CHARacterization Modules (CHARMs) are the components involved with Electronic Marshalling that allow the late characterization of any type (AI, AO, DI, DO, RTD) of I/O signal. A CHARM is simply a component that includes a passive A/D converter and signal characterizer. CHARMs that are inserted onto the terminal block where field wires are landed. In fact, landing of field wires is much more forgiving, because the wires can be anywhere on the strip‘, without regard to a particular order or type of I/O. So, if wires for the following signals are landed: AI, AI, DI, DO, AO, DI, DO down the terminal strip, that is fine. After wiring, the technician sees the first channel is an AI, reaches into the bag of AI CHARMs, clicks in the CHARM and is done.

Link to the full version of this white paper at  bit.ly/awtech004

This white paper was written by Emerson Process Management. Publication Date: February 2011