Bus systems and measuring 4-20ma

Can anyone explain the relationship between Profibus DP/PA, Hart and ASI bus systems.

Also need to understand instead of using profibus PA. what is recommended so that I can measure 4-20ma output signal on the field?

There are some differences in the communication protocol capabilities, and therefore

- PROFIBUS-DP primarily used with variable speed drives and motor starters

- PROFIBUS-PA, HART, and FOUNDATION fieldbus primarily used with transmitters, analyzers, and valve positioners

- ASI primarily used with on/off switches and actuators

- HART digital communication is slow and is not used for closed loop control. Closed loop control is done using analog 4-20 mA

- FOUNDATION fieldbus is suitable for closed loop digital control using cyclic digital real-time data on a time-synchronized and scheduled, precisely periodic macrocycle

- ASI only communicates cyclic I/O, no acyclic diagnostics or setup

- PROFIBUS-DP/PA, HART, and FOUNDATION fieldbus support acyclic data for intelligent device setup and diagnostics

- PROFIBUS-DP/PA, HART, and FOUNDATION fieldbus support EDDL (www.eddl.org) for intelligent device management

- PROFIBUS-PA, HART, and FOUNDATION fieldbus are intrinsically safe and long distance, suitable for use in the process industries

- ASI is not intrinsically safe, and not long distance, but is very fast

See the guide on Institutionalizing Device Diagnostics here for a cross-reference table of protocols and devices:

http://www.eddl.org/DeviceManagement/Pages/DeviceDiagnostics.aspx

The principal beauty of PROFIBUS-PA and FOUNDATION fieldbus H1 is that you can do away with analog 4-20 mA and the associated drawbacks. Use either native PA or H1 devices, this way you eliminate 4-20 mA completely.

Or are you saying that you need some device not available in a PA or H1 version? What device is that? For Fieldbus you can see available devices here:

http://www.fieldbus.org/index.php?option=com_mtree&Itemid=324

If a native device is really not available, then use a converter device which has a PA or H1 connection, and 4-20 mA input/output as needed. See the Fieldbus Foundation website:

http://www.fieldbus.org/index.php?o...ed&ffbmfg=&ffbcategory=Converter+(Transducer)

Cheers,
Jonas

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