Fieldbus Network Timing Parameters – Compel Data

One would think that because the Fieldbus Foundation defines so many other parameters in its standards that the time for the Compel Data message used for the Publish/Subscribe messages on which control in the field is based would also be standardized. Not so. This sometimes makes it difficult for designers to properly estimate the macrocycle requirements, which is unfortunate since in many cases the bandwidth of the Fieldbus system is now the design constraint.

The following are what I have been told are the times for the Compel Data message in various hosts:
•    Emerson DeltaV – 30 milliseconds
•    ABB – 35 milliseconds
•    National Instruments – 30 milliseconds
•    Honeywell Experion 15-17 milliseconds

If you have information on the Compel Data time for any other DCS host systems (Yokogawa, Ovation, GE, Invensys, Siemens, Yamatake/Azbil, etc) please reply with comments to this post so other readers can learn this information as well. Thank you for your assistance in expanding the knowledge of our community.

More news about this week's (June 21-23) ISA Communications Division Symposium on Thursday June 23.

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  • <p> “Compel Data” is just the message that initiates the data transfer from the publisher to the subscriber for the link. The CD only takes a few ms. The subscriber will then make the data transfer. It is this whole publisher-subscriber function block link transaction which takes about 30 ms, not the CD. Think of it as a slice in the schedule. </p> <p> 30 ms is the DEFAULT function block time in DeltaV. In DeltaV Explorer this setting is called “minimum schedule spacing”. 30 ms works great with old and new devices. Devices from 1996 used 8-bit microprocessors and were very slow. They needed 30 ms. </p> <p> Modern FF devices are very fast. On some projects that require faster we set it to 15 ms. This is good if you want to cram many loops into 250 ms macrocycle </p>


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