Microprocessor Control of Remote Pipeline Sites

Nov. 12, 2010
Looking for Information on RTU, MTU and compressor automation technologies? Ask the Experts

"Ask the Experts" is moderated by Béla Lipták (, process control consultant and editor of the 3-volume Instrument Engineer's Handbook (IEH). He is recruiting co-authors for the 5th edition now and welcomes contribution offers from qualified colleagues. Also, if you have questions for our team of experts, please send them to [email protected]

Q: I am working on a fun historical project and am trying to find information on the SCADA systems used on gas pipelines in the late 1970s and early 1980s. I have found a few articles, but because it is pre-Internet, information is sparse. In particular, I am looking for information on the RTU, MTU and compressor automation technologies used on projects that were installed on projects such as the Alaskan Pipeline:

  • Who manufactured these systems?  In those days, the players seem to be companies like AGTL/NOVA, Baker and DATAP, not the ones we see today.
  • How was SCADA deployed and what were the design strategies? How active was the control at the MTU? How were critical systems like compressor control managed on the SCADA system?
  • What were the underlying processors and operating systems? This was pre-PC, so I guess VAX and 8080, but there were also HP projects from around then. Were most of the RTUs "home-grown" or did companies like Siemens have their early S-Series processors in this market?
  • What was considered leading-edge technology and who was working on it? Calgary had a very vibrant community of SCADA inventors. Does anyone know any of the people involved in those early projects?
  • Does anyone have ideas where else to ask?

I was a very young engineer in those days and never really got involved in the SCADA technology from that period. After 1986, I have enough information, but by then the systems had started evolving to the PC-based systems we see today.

Eric Byres P.Eng
[email protected]

A: In the late 1970s and early 1980s, Modcomp computer systems were widely used in SCADA and pipeline systems. They are still in business, but no longer make computers. They competed against the Data General and Digital Equipment Co. (DEC). (Two more names that have not lasted.) 

I did software support on the Modcomp II computers running the Max 2 operating systems on the Alaskan pipeline. The Modcomp systems were designed for real-time applications, including steel mills, nuclear power plants, refineries, chemical plants and pipelines. I had heard, but can't confirm, that Modcomp received the business because Digital did not want to take the liabilities for running in these risky applications.  

In the SCADA system, the MTUs were PLCs that performed local control. They used radio transmitters and receivers connected to modems to send periodic updates (I seem to remember about 5 minutes or more) to the Modcomp II SCADA system. The SCADA system was implemented in Fortran (remember that language?), and was custom-built for the pipeline. The system provided pipeline operators with real-time visibility into the operation of the pumping stations and material flows and pressures. They could send commands to the MTUs and pumping stations using the same method for control. The Modcomp SCADA system sent its data to a corporate system that provided overall (across the U.S.) visibility of the status. I think this system was also custom-built for the company. Here is a link to information about Modcomp:

Dennis Brandl
[email protected]

A: I worked for ModComp in the late 1970s. That was before the PLC era really started. I don't know what equipment ModComp used for the RTUs then, but later they developed the Modex, a small ModComp CPU with integrated I/O. Before that, most of the RTUs were built from Computer Products I/O boards.

One of the largest SCADA projects that I worked on was for the Tennessee Valley Authority (TVA). They had RTUs with PDP-11 computers at every dam site. Each was equipped with a local control center, but connected by leased telephone lines to SCADA remote master stations. As I remember, this system with more than 100 remote and master nodes was supplied by a Swiss or German supplier with a proprietary communications protocol.

The other SCADA system I remember was made by EMC, and was based on the DEC PDP-11 at first and the VAX later. Eventually, the RTU for this SCADA system was built with the TI-550 PLC, but later was switched to use one of the Siemens Simatic series after Siemens purchased the PLC division of Texas Instruments.

Richard H. Caro, CAP
[email protected]

Figure 1. The pre-industrial, the industrial and the post-industrial cycle of energy generation and consumption shown as batch and continuous processes. (Click here to enlarge image)Q: You wrote about the suitability of the process control principles to control the economy, the weather, the protection of our rivers, the stabilizing of global warming, etc. You also mentioned that the delayed conversion from exhaustible to inexhaustible energy sources can change the process of human evolution from a "continuous to a batch" one. What did you mean by that?

Nicholas Wagoner
[email protected]

A: As you know, a batch process is one that has a beginning and an end. The figure on the right illustrates the "process of life" as it operated prior to the industrial age, during it, and how it should operate during the coming post-industrial age.

During the pre-industrial age, the food energy from the vegetation plus water and oxygen were sufficient to maintain the continuous process of life on the planet indefinitely.

In the present industrial age, more and more exhaustible energy is obtained from the diminishing fossil and uranium deposits (shown in red). This cannot continue forever, and therefore I call the present operation a batch process.

In the future, when the exhaustible deposits are gone, hopefully we will have converted to a solar-hydrogen based energy economy that depends on the free, clean and inexhaustible energy supply from the sun. You can find a detailed description of such a power plant and reversible fuel cell (RFC) at

Béla Lipták
[email protected]