October 2008

Assured wireless integration success

By Louis De Silvio

Most people do not want to embark on the costly upgrading of a SCADA system, especially when adding radios, without assurance of success.

That is why multipoint enhanced signal handling (MESH) enabled systems can be deployed stand alone, in parallel with SCADA systems, or integrated directly into the legacy SCADA. Further, most systems can be migrated seamlessly through these three scenarios over time as end user confidence builds.

For example, a stand-alone system is possible where all sources and all targets are on the same wireless system, with the radios communicating in either MESH or V configuration (name derived from the shape of the letter V indicating signal path) protocols.

In the parallel scenario, the base station computer can connect to the SCADA system where its attached computer can accept polling, as with MODBUS, and the SCADA system can record a copy of all its activities, or can be involved in source or target activities using its previously attached devices.

Finally, the base station radio can fully integrate with the SCADA system, which can then monitor the process, control it, and allow seamless integration with the existing system.

Indeed, wireless is considerably more cost effective and flexible than wiring devices when the environment is suitable.

When designing a SCADA system, one must consider a variety of factors, not the least of which is the number of I/O points or "tags," the number of interrelationships among them, and the processing power and memory required to run the entire system efficiently.

As we move forward in pushing the technology envelope, it seems reasonable that one could take the proportional processing power and the proportional memory of the centralized SCADA computer required to support each I/O point, and distribute it to the I/O points via a processor and memory on a board.

By distributing a processor and memory to the I/O point, we achieve localized decision functionality, which is comparable to that of the SCADA, albeit on a smaller scale. In fact, each distributed point would have a great excess of capacity just based on processor technology.

The ability to add memory would also add functionality beyond that required to support just the I/O, decisions, and interface requirements to the radio.

At this point, a PLC can add to the mix along with a MESH radio. Another option would be to simply add the processor and memory to the MESH radio function, and program the processor using standard programming languages like C++ or others. In this case, a PLC is not necessary, although a higher level of knowledge of programming language is.

As time goes by, more and more people will become sophisticated in programming languages to the point where the PLC may actually become obsolete.

A hybrid version of that is the PLC-on-a-chip, by Divelbiss. They have taken a PLC processor and memory and scaled it down to the proportions necessary to support a reasonable number of I/O.

Since it is in chip form, the chip can easily integrate with a board of any design. The PLC-on-a-chip is a fully functional PLC and uses standard ladder logic programming. It is the step just before eliminating PLCs altogether and going with the processor and memory scenario mentioned previously.

Now, we can build a single board capable of supporting a MESH radio, a PLC-on-a-chip, a processor and memory, and I/O connectivity. If we delete the PLC from the board, we have a MESH radio, a processor and memory, and I/O connectivity left.

This approach, when combined with appropriate HMI (human-machine interface-computer screens) software, allows us to interface the MESH radio and its board directly with external PLCs from various manufacturers, or eliminate the PLCs altogether and program the processor to do what we want.

There is one additional feature, which is valuable in either case. It involves storing the source code associated with the compiled code in use at any time either with or without the PLCs.

In the case of either the PLC-on-a-chip, or an external PLC, the associated processor and memory first receives and stores the packets containing the new object code for reflashing the PLCs.

Once the PLC reflashes, the processor sends a message to the base station requesting a copy of the source code used to compile the newly flashed object code. The processor then stores the currently used source code in its local memory, ancillary to, and outside of, the PLC.

Once this is done, any user from anywhere in the world with a secure connection to the base station exists, can "ping" the processor through the MESH, and receive a copy of the then-current source code in use with the object code being executed in the PLC. Since this source code resides outside the PLC, pinging the processor does not interfere with PLC functions, nor does it take PLC cycle time to reply.


Louis De Silvio (lou@industrialt.com) is an ISA member and president of Industrial Telemetry in Collinsville, Okla. His paper from the 53rd International Instrumentation Symposium is SCADAless SCADA using wireless MESH radio telemetry. Read it at http://www.isa.org/filestore/MeshFeature.pdf.