The HART specification requires that a HART Master (the device issuing HART commands) be connected to a nominal load of 250R. If your load is too far from this, communications will either not work at all or be intermitent.
Steve
PS You can check the spec for the actual limits
If you fail to connect, that is the outcome, isn't it?
I think you meant the question to be, "What is the outcome if we fail to use a 250 ohm resistor?"
If the loop already has somewhere a minimum 250 ohms (some long ago memory says that the real minimum level is 238 or 242 ohms or some such odd number) then the HART communicator or HART modem will work OK.
If there is insufficient resistance in the loop, then the HART comm link will not run.
The 4-20mA can still function (assuming it is functional) with lower loop loads, but HART won't.
The earlier 275 communicator had banana jacks up onthe top where a dual banana plug with a resistor could be inserted to make this easy.
I'm not sure if the 375 does the same thing or not.
I carry a 250 ohm resistor with alligator clips on both ends to insert in the loop, if needed, when using a HART modem. I rarely need it. Many PLCs use 250 ohms on their 4-20mA Analog Inputs. Not all, but many do.
Yes. You need 250 ohm in the loop, from somewhere. If the cable resistance and other resistors in the loops such as input shunts are less than 250 ohm then you need to add resistance so the total exceeds 230 ohms. Many times the DCS input card has a 250 ohm resistor so no extra has to be added. However, during bench test with DC power you need to put in a 250 ohm resistor. It need not be accurate.
HART transmits using current +/- 0.5 mA and receives a voltage. One purpose of the 250 ohm resistor is as a shunt to convert transmitted current to received voltage. The other purpose is to ensure that the DC power supply does not short circuit this AC HART signal (1200/2200 Hz).
Therefore, if you do not have the 250 ohm resistor communication cannot work because you don't get the voltage and the current is shorted.
Note that the 250 ohm HART resistor serves the same purpose as the Fieldbus shunt and power supply impedance "conditioner".
To learn more about the HART signal take a look at chapter 11 of the yellow book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance" buy online: http://www.isa.org/fieldbuses
Pradeep
The resistor doesn't have to be 250 Ohms, anything from 100-600 will work equally well. The power supply is a low impedance to the hart signal shorting it out. Adding the resistor raises the impedance so that the signal can be detected by the calibrator. A small reactor would also work. If you are working in a Hazardous location with barriers you have to keep that in mind also, some barriers won't allow the Hart signal through.
We have just experienced that during loop checks using the DeltaV AMS that the P+F AI Splitters (IS barriers KFD2-STC4-Ex12O) "sometimes" lock the HART field transmitter into the last forced output and we are currently trying to figure out why this is the case... Any ideas?
It’s a while since I've used (AMS) but try rebuilding the hierarchy when a lock occurs. I'm not fully sure what you mean by “lock the HART field transmitter into the last forced output,“ could you explain further?
The HART Protocol demands minimum loop resistance of 250 ohms. Generally if you connect HART to transmitter in the loop itself you need not to connect resistor as the loop resistance (wires + cards and so on) is more than 250 ohms. Problem arises only when you try to connect HART in workshop. There you connect HART directly to the transmitter, not through the loop. That time you need to add additional resistance.
That's interesting Roberto, I haven't seen that before, it confirms that the resistance range is quite high, not 250 as some believe. I'm sure if you had a reactor the resistance could be almost zero and still work the HART signal just needs to be able to pull the line up/down.
Nope. The resistor cannot be almost zero (for transmitter/analyzer loop current-controlling type devices) because sending is by current and receiving is by voltage. If the resistance is too low the TX current times resistance becomes a signal amplitude (voltage) so low that the RX circuit cannot receive it.
To learn more about the HART signal take a look at chapter 11 of the yellow book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance" buy online: http://www.isa.org/fieldbuses
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