Help choosing Fieldbus, Modbus, or HART?

[Ed Henrie]

I'm new to industrial comm. protocols but need to make some decisions regarding smart transmitters, controllers, and the future of our test area's data acquisition equipment. 

I have LV 7.1, but may upgrade to 8.2.  We are in an expansion phase which allows me to re-evaluate our methods of acquiring data.  I am moving towards the use of multivariable smart transmitters where applicable (venturi flowmeters) and digital comms enabled temperature and pressure controllers as well as data collection, command, and control via LV.  There are three industrial communications protocols which tend to gain my attention.   Foundation Fieldbus, Modbus, and HART.  It appears that Fieldbus and modbus are RS232/485 based and HART is a digital signal riding on top of the 4-20mA analog signal, but which must be accessed via a modem type device.  Does any HART modem work with any HART transmitter?  Is the resulting information in digital or analog format?  To what interface does the output of the HART Modem connect.  What price range am I looking at for HART comm devices?

I am already locked in with modbus for some temperature controllers, because that's the only protocol they have available besides just plain ascii, however it's my perception that the Fieldbus and Modbus protocols are not mutually exclusive.  Hart is somewhat exclusive due to the modem routing, but any or all of these "systems" can co-exist on the same pc.  Correct?  Comments?

What is the easiest and most reliable to interface to LV?  What are the advantages/disadvantages of the various protocols?  Do they all offer the same comm capabilities?  Is there a standard command set available?  Are there important questions I'm omitting?

I'd prefer to not have to wire an rs485 loop around my test areas.  Can these signals be easily/inexpensively converted to ethernet and transmitted to LV without much fuss? 

Any comments on ths subject would be greatly appreciated.

Regards,

Ed Henrie

[Krista H]

Hi Ed,

I think that rather then me suggesting the best communication type it would be better for you to know more about the two types of communication and decide what is best for your system. There should be no issues with having Fieldbus and Modbus on the same machine. The best way to explain Modbus is to lead you to this link that tells you what versions of LabVIEW support Modbus and leads you to other valuable information. There are several other usefull knowledge bases online at NI.com that could help you.

For Fieldbus.

1. About the serial communication

FOUNDATION Fieldbus is NOT RS232/485 based. We do need several serial interfaces for some special devices (such as our PCI-FBUS interface card); however, the individual shield twisted-pairs and the multiple pairs with overall shield are the most popular. Please contact Fieldbus FOUNDATION for the cable details. You can also get more information on the website or on the forms.

2. About the advantages and disadvantages of FOUNDATION Fieldbus

Please refer to the attached FF_and_NI-FBUS_Products.pdf for the benefits of FF and NI-FBUS products.

3. About LabVIEW

NI-FBUS provides 13 add-ons for LabVIEW. But please be aware that these add-ons only provide the minimum set of operations on FF device such as get the device/block list, read/write the parameters, set device address/tag, etc. If the you need the full control, I would like to suggest that you use NI-FBUS Configurator. For your information, the latest NI-FBUS software is available here. If the NI-FBUS installer detects LabVIEW in the same computer, it will install the add-ons automatically and these add-ons are free.

4. About the Ethernet

FF has 2 ways and means to connect the FF devices to the host system. One is use our PCI-FBUS or PCMCIA-FBUS interface card. Another way is through the High Speed Ethernet (HSE). For the HSE, we need a HSE Linking Device to act as the bridge between the devices and the host.

I hope that this information will help you. If you have any further questions please let me know. If  you have Fieldbus specific questions please contact Fieldbus support by emailing fieldbus.support@ni.com.