Modbus modules still connect

Every day, nearly 750,000 barrels of crude oil and refined products flow through 16 U.S. pipelines owned by Conoco Oil (Houston, Tex.). In 1994 Conoco Oil evaluated its legacy system and didn't like what was there. The circa-1978 centralized control system used multidrop, leased telephone lines.


Every day, nearly 750,000 barrels of crude oil and refined products flow through 16 U.S. pipelines owned by Conoco Oil (Houston, Tex.). In 1994 Conoco Oil evaluated its legacy system and didn't like what was there. The circa-1978 centralized control system used multidrop, leased telephone lines. They were concerned about the availability of replacement parts for the old RTUs, loop controllers, and alarm annunciators. Their data entry system was cumbersome, in some cases requiring that data be entered two or three times.

'The company reviewed its control system strategy and concluded that it had a unique and hard-to-maintain system. It consisted of dedicated, custom built and programmed RTUs along with a lot of odds and ends,' said Richard Parker, lead project engineer for Conoco Oil.

Conoco replaced leased telephone lines with a VSAT satellite system. The VSAT Ku Band Network system supports 170 sites where Conoco has 200 PLCs and RTU addresses handling about 20,000 I/O points. In addition to the VSAT, Conoco installed a dial backup using analog lines communicating via Modbus to 140 critical sites.

'To make a long story short,' said Ken Hopwood, development engineer for ProSoft Technology, 'when Conoco installed the new equipment, the old equipment couldn't communicate with it. So, they used the ProSoft 3150-MCM module as an interface.'

ProSoft then went into development for Conoco to create a new module especially for them: the 3150-CMS module.

'Conoco needed to eliminate continuous polling that was necessary on their old system,' says Mr. Hopwood. 'It created communication delays when using the expensive satellite system. The 3150-CMS is a Modbus Slave with a few modifications. It has Report by Exception, giving it the ability to send timed, unsolicited data reports to the Master Station in Houston. This eliminated the need for continuous polling.'

A Modbus communication module from ProSoft Technology enabled Allen-Bradley
SLC controllers to talk to compressors.

From pipelines to air pressure

A new venture between the U.S. and Chinese governments sent system integrators to Rockwell Automation-Denmark in search of state-of-the-art technology for the new Shanghai General Motors (SGM) plant. An essential part of this technology includes air compressors. Because workers use pneumatic tools to attach components like tires and seats, it was essential that SGM have the most up-to-date compressors available and that they be equipped with central control.

Atlas Copco, a world market leader in the field of air compressors, was contacted to supply the air compressors for the 160,000-m2SGM plant.

'Two different compressor models were chosen for the plant,' says Lenus Hong, Asian regional sales manager for ProSoft Technology. 'Six were the ZH Model centrifugal type with an Allen-Bradley SLC embedded in their control system and two were the Z-pack Model, without an SLC. This created a problem when it came time to network all of the compressors together via an HMI package. It's a good thing the system integrators found out that the Z-pack compressor had built-in Modbus communication.'

In order to create a central control for the eight Atlas Copco compressors, system integrators used ProSoft's 3150 Modbus Communication Module to link the compressors to the SLC DH-485 network and back to the HMI Host Station.

'By using ProSoft's module, I was able to directly connect Allen-Bradley's SLC with the compressors, using the Modbus protocol,' said Chen Zong Liang, general manager of system integrator Shanghai Yuandong. 'With this central control, it was possible to stagger the actions (start, load, unload, or stop) of every compressor according to the charge situation.'

When asked how the ProSoft module improved the plant processes, i.e. functionality, speed, convenience, or financial benefits, Liang simply replied, 'It just can't work without it.'

For more information visit

Author Information

Danetta York, marketing specialist, ProSoft Technology Inc. (Bakersfield, Calif.) and Gary A. Mintchell, senior editor ( )

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