Macro Cycle Time On Ff H1 Segment

[chauhanrasikend]

My query is regarding the Macro cycle time on Foundation Fieldbus H1 segment
Suppose I have 10 pressure transmitter on a single FF H1 segment and configured one AI block in each transmitter whose execution time is 30 msec. My question is what will be the Macro cyle of that FF H1 segment . Is it 10 x30 msec= 300 msec?? Am I correct or not. Let me know your feedback at the earliest.
Or is there any other method to calculate the Macro cycle time?
Thanks

[Stephen Mitschke]

When you consider that the devices are running in parallel, calculating the macrocycle can be a little more involved. Many systems have various algorityms to efficiently calculate the macrocycle.

In your above example, with 10 pressure transmitters, you need to consider execution time and transmit time. If you assume 30ms execution time, then I would claim that all of those blocks can execute simultaneously.

However, we need to also transmit data on the bus. In this case, only one device can use the network at a given time. The typical time for transmitting an analog piece of data on the bus is about 10ms, but schedulers must also allocated time for "worst case delays" which can be as much as 48ms, depending on link settings. Again, the tools will provide you with the appropriate calculations.

So, calculating macrocycle involves managing the function block schedule (30ms in you case) + published data (48 x 10ms) + asyncronous activity.

The last bit of information may also be system dependant. At a minimum, one should allocate 50ms. If running that small, through, you risk a slower refresh rate for operator displays (non-publisher subscriber) as well as higher latency for alert broadcasts by the device. 100-200ms may be a bit more reasonable.

I might recommend that you download a demo version of a configuration tool. I believe companies like SMAR and National Instruments provide demos of their configuration tools. It can be very helpful to visualize how schedules are created.

-Stephen