This is an excerpt from the May/June 2014 issue of InTech magazine by Debashis Sadhukhan, process controls system manager at NASA, and John Mihevic, retired DCS control system manager.

At the NASA Glenn Research Center (GRC) in Cleveland, Ohio, existing programmable logic controller (PLC) input/output (I/O) was replaced with distributed control system (DCS) I/O, while keeping the existing PLC sequence logic.Integrating DCS I/O to an Existing PLC

Until about the year 2000, the GRC’s central process system (CPS) distributed control system used PLC I/O to remotely control and monitor process equipment. For many years, these devices were the core of process I/O at the facility. As new industrial control and communication technologies emerged, the benefits of improving traditional I/O schemes for newer approaches became attractive. For this reason, NASA recently replaced several PLC I/O with DCS I/O hardware, while keeping the PLC processor in order to evaluate the interoperability of this new technology.

The new technology has improved throughput speed from the I/O to the operator screens. There are now many attributes to a data point, including diagnostic capability and calibration.

Plant description
The CPS at NASA Glenn provides combustion air to 1,250 pounds per square inch gauge (psig), altitude exhaust (to 90,000 ft), atmospheric exhaust, refrigerated air (to –90°F), cooling tower water, and service air for the major wind tunnels and propulsion test facilities at Glenn. The CPS consists of 20 miles of process piping and 600 valves to connect the above systems to the various test facilities. A DCS/PLC/pressure and surge controller system consisting of nearly 100 proportional, integral, derivative (PID) control loops and more than 12,000 I/O points monitors and controls the vast amount of equipment across the facility. More than 12 miles of dual-redundant data highway cable is installed to interface with the control/data system for these essential services.

Historically, the equipment for controlling and monitoring the process consisted of a PLC processor and its associated I/O distributed near process equipment. The PLC communicated to the DCS controller via the Modbus RTU protocol. The DCS controller then communicated to the operator console via a proprietary data highway network.

Reasons for conversion to DCS I/O
Although the existing I/O was adequate at one time, the need for its replacement became more apparent as the technology changed. While the existing I/O was readily available in the 1990s, more recently parts for repair and replacement were difficult to find. An upgrade to the new replacement PLC I/O was available, but it provided none of the benefits of the DCS I/O. The DCS I/O had much faster speed and a “quality status,” which was not available on a Modbus RTU serial link with a transmission rate of 19,200 baud.

The choice was to either replace the entire PLC system with the DCS system at one time, which required massive logic conversion and testing, or perform a two-phase implementation approach. The first phase would replace the PLC I/O with the DCS I/O. Then the second phase would involve converting PLC logic to DCS logic. The split approach was chosen to minimize downtime and prevent a complicated check-out process.

To read the full article on integrating DCS I/O to an existing PLC, click here.

About the Author
Debashis Sadhukhan, process controls system manager at NASA Glenn Research Center (GRC), has been employed at GRC since 1991 and is experienced in integration of DCS and PLC systems. He is currently president of the ISA Cleveland Section and bulletin editor. John Mihevic was DCS control system manager at GRC until his retirement in 2007.

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