Signaling considerations for control system migrations

Not every analog field instrument can be wired to any given analog input (AI) module, nor can any discrete output (DO) module be wired to any given discrete final control element.

Shane Hudson

Control system migrations inherently require a large variety of technical details to be analyzed and addressed in preparation for installation and start-up. In addition to software configuration and development, this also applies when connecting the software to the process.

At a high level, and especially during early project scoping stages, it is useful and appropriate to base control system migration analyses largely on input/output (I/O) counts by general types: analog inputs (AI), discrete inputs (DI), analog outputs (AO), discrete outputs (DO), and soft tags. After the early scoping stages—but before budgets are finalized and detailed design begins—the project team should consider signaling details that can impact project costs and schedule.

Control system criteria

There are many signaling details that need to be considered during a migration project. This is especially true if the project is based on retaining existing field instrumentation.

The control system could require a mix of single-ended and isolated DO modules to be compatible with existing field signals that include both simple solenoid valves and start/stop signals that are wired directly into and powered by existing motor control circuit transformers. The control system could require a mix of single-ended, differential, HART-capable, and isolated AI module types to be compatible with a mix of existing loop powered transmitters, self-powered transmitters, HART transmitters, and known ground loop issues.

The new control system could require several different DI and DO module types to be compatible with a mix of existing 24 Vdc and 120 Vac discrete signals unless the project designs and installs interposing relays. The variations in I/O module types referenced above can result in significant variations in I/O densities (channels per module), which then affect the physical cabinet space and I/O rack infrastructure required for a given I/O count.

Signal isolators or conditioners may be required to make existing field signals compatible with the I/O modules offered on the new system or to prevent purchasing different I/O module types for only a small quantity of unique field signals. DIs on the new control system could have higher input impedances than the existing system, which makes them more vulnerable to induced voltage on long cable runs than DIs on the existing control system.

Existing solid-state, output discrete, field device signals, unlike dry contacts, normally require a minimum current to fully conduct and switch on such that a dropping resistor may be needed if the input impedance at the new control system's DI modules is too high. In addition to field instruments, solid-state output signals are encountered when hard-wiring DOs from one control system, such as a small, packaged unit equipment programmable logic controller (PLC), to a supervisory control system, such as a plant distributed control system (DCS).

As a part of the migration project, the plant also could be installing discrete field instruments with Namur sensors that may not be compatible with all of the DI modules on the new control system. Also, when Namur sensors are newly introduced to an existing plant, maintenance personnel should be trained on the operation of Namur signals that, unlike simple dry contact position switches, require excitation voltage to function. Namur sensors also may have the opposite contact action of existing simple dry contact position switches and require inverted control system logic. 

Additional requirements

As part of the migration, the plant may want to change motor control circuits (MCCs) from two-wire control (single maintained signal) to three-wire control (momentarily requiring an additional DO) to enhance safety or for other process reasons associated with recovery from power interruptions.

Existing MCCs in older facilities could use 480 Vac signaling (tapped from two power phases) for motor start, stop, and run confirmation signals that would almost certainly require interposing relays on the new system unless the existing motor starters are replaced.

Existing AOs that currently drive a pair of split-range control valves may need to be implemented on the new system as two separate AOs to ensure sufficient voltage drop or to provide an intuitive operator interface on human-machine interface (HMI) graphics. The plant could use obsolete thermocouple-sensing temperature transmitters whose 4-20 mA outputs do not vary linearly with temperature and are supported by the existing, legacy control system but are not compatible with the new control system.

The plant could have field transmitters with an older, primitive fieldbus technology that was never widely adopted and hence is not supported on the new control system but is providing critical process data to operations personnel on the existing, legacy control system.

Although many newer control system DO modules have internalized this functionality, some 24 Vdc DO modules require reverse bias diodes across their outputs to prevent the energy from the collapsing magnetic fields on inductive loads from generating excessive voltages that then damage electronics. The field excitation voltage for AIs on the new control system can be significantly different than the existing control system, which can lead maintenance personnel troubleshooting efforts astray. 

Shane Hudson is a senior engineer at Maverick Technologies. Maverick Technologies is a CFE Media content partner, a CSIA Level 1 member, the Control Engineering System Integrator of the Year in 2011, and was inducted into the Control Engineering System Integrator Hall of Fame in 2012. Edited by Jack Smith, content manager, CFE Media, Control Engineering, jsmith(at)


Key concepts

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