RS 485 and HART Protocol

Hart runs over the top of a 2-wire 4-20mA analog loop and is intended for communicating with individual instruments come re-configuration time.

RS485 is a networking standard.

<Also kindly indicate which one is better to use with field instruments or with PLC??>

"Horses for Courses". Please review the articles in Wikipedia and go from there.

Short answer is like PW says, RS485 is an apple and HART is an orange. I imagine that you are asking more of the difference between MODBUS (the protocol typically used over 485 networks) and HART.

RS485 is a networking standard that I have typically seen in MODBUS communication. In my experience, it is used to communicate mass data from one thing to another every few seconds or so. For instance, I have created an RS485 network to talk to different gas chromatographs that were each sending 60 points of data back to the control system. Individual 4-20mA signals for this would have been nutty. That being said, I would not use a 485 communication for control purposes.

There are different types of 485 networks, two-wire and four-wire and different ways to set it up. Sometimes you need terminating resistors, sometimes you don't, there are transmitters and receivers, etc. Network topology is a big deal in that you have point to point communication, you can get things screwed up easily that way by creating rings and/or stars. RS485 is a mature technology and very reliable in my experience when correctly set up. But it is a wiring and hardware standard, not a communication protocol. I usually use the MODBUS protocol on a 485 network and this seems to be the industry standard.

HART is a protocol, and the only experience I have with that is with field instruments being HART capable meant that you could use certain instruments to talk to them for things like calibration. I have used HART protocol to send feedback from valves (position) into the DCS system over their 4-20mA analog output wires.

To elaborate, the HART protocol (which is an acronym for "highway addressable remote transducer" protocol) is a communication protocol delivered using the Bell 202, two tone modulation standard over two wires. It is generally delivered over the two-wire current loop that is used to deliver the 4-20 ma analog signal from instruments and other devices. The HART protocol also allows up to 15 instruments to share a loop if the analog signal isn't being used (called "multidrop operation").

The RS485 standard, as the other replies have stated, is used primarily to deliver communications on a modbus network or some other proprietary protocols. The main difference between modbus and HART is that modbus will support up to 255 devices, but they cannot be powered by the communications loop.

-Bob

Hi

The main diference is that the HART protocol allows digital values and analog values of current, while the RS-485 is a communication stantard. GIE, my Company has developed a HART to Modbus RTU protocol converter that allows connect a HART instrument using the RS-485 protocol. Also the device can be used as a HART modem.

The website of the product is:

http://hartmodbus.giemdp.com.ar

for any consult my email is uriz@giemdp.com.ar

Best regards

Eng. Alejandro URIZ

GIE

Argentina