Is Hart the same as 4-20mA?
HART is an acronym for Highway Addressable Remote Transducer. It involves a digital signal superimposed onto a 4-20mA analog signal in order to encrypt instrument information beyond the standard analog signal. The actual analog signal from the device is represented by the blue curve.
Is HART protocol still used?
Many years ago, HART became and now remains the most pervasive digital communications protocol in process control—if you have analog 4-20 mA, you probably have HART.
What is FSK in HART?
“HART” is an acronym for Highway Addressable Remote Transducer. The HART Protocol makes uses Frequency Shift Keying (FSK) standard to superimpose digital communication signals at a low level on top of the 4-20mA. As the digital FSK signal is phase continuous, there is no interference with the 4-20mA signal.
Why do we use 4-20 mA and not 0 20ma?
The 4 mA is used as “range zero”. If the range zero were truly zero mA, then there would be no current, and therefore no power to drive the field instrument. Field instrument would require 3 or 4 wire devices. Additional wiring costs come into account.
Is the 4 ~ 20mA signal the same as the Hart signal?
They are basically the same signal but the 4~20mA has HART protocol signal superimposed to the analog 4~20mA. information, temperature , pressure, status, tag, depending on what the Hart Instrument can make available for the user… Good Luck!
What’s the difference between Hart and FSK Bell 202?
If your device doesnt’ understand HART, then it just looks like plain old 4-20. The HART standard imposes a digital signal at 1200 bps on a standard 4-20mA system using frequency shift keying (FSK – Bell 202 Standard) – with two sine waves (1.2 kHz for “1” and 2.2 kHz for “0”) superimposed on the DC analogue (4-20mA) signal.
Can you use multi drop with a Hart signal?
If you want to use HART with multi-drop, then the 4-20 mA component is ignored and you just communicate “digitally” with HART (the trunk is only two wires, so with all the devices chattering away the 4-20 mA signal is useless). Alternatively, you can use single drop and get both HART and 4-20 mA data.
How many masters can a Hart signal support?
In effect, HART supports two communications channels separated in frequency over a single pair of wires: the 4-20ma (0-100Hz) and modualted (ie. analog) half-duplex communication (1000Hz – 2500Hz). HART actually supports 2 masters (not one) using an implied token passing algorithm.
