For wiring in a HART system, a twisted pair cable, either a shielded pair or a common shield over a cable containing many twisted pairs, should normally be used. In the latter case, it is important not to use other signal pairs that could interfere with HART communication. (They can be used for other HART communication lines, or for pure analog lines, provided the HART protocol limits on analog signal rate of change are met – see the Analog Signal Bandwidth chapter.)
If the cable is longer than a few meters, its resistance and capacitance can be significant to limit the RC time constant (see the “Signal Attenuation and Distortion” chapter). In addition, its resistance may need to be considered when calculating the loop voltage drop required for each two-wire system.
Cable parameters depend on the diameter of the conductor, type of insulation and thickness. Capacitance is measured from one conductor to all others to the screen; resistance is measured for both conductors connected in series. Apply the measured values or specifications for the specific cable used in the installation.
Characteristics of communication cables
|Shielded twisted pair for computers||65 pF/m||120 Ohm/km|
|Industrial shielded twisted pair||150 pF/m||120 Ohm/km|
|Shielded multicore cable||200 pF/m||120 Ohm/km|
When using one primary device and a main device with a load resistance of 250 ohms, in the absence of another any significant resistance, a time constant of 65 μs allows a total capacitance of 0.26 μF. For the capacitance to be 0.01 μF (10,000 pF) for the master and primary device, the cable capacitance must be up to 0.25 μF. However, if the cable resistance is 110 ohms, R becomes 360 ohms, which results in an allowable total capacitance of only 0.18 μF. This would correspond to a cable length of 900 meters at 200 pF per meter. If you want to extend the cable length to a maximum HART length of 1500 m, you must carefully select the cable so that it has either lower capacitance or lower resistance (thicker cores).
In a system with ten single-channel sensors (CN = 5), all connected devices with a total capacitance of 0.255 μF (10 x 25000 pF + 5000 pF) allow a cable capacitance of only 0.005 μF. In this case, the permissible cable length should be between 25 and 75 meters, depending on the type of cable. (The cable resistance is negligible in this case.)
Properly ground the system to prevent interference from external signals. The signal loop and the cable shield must be grounded at the same point. The shield of the cable must not be connected to the body of the instrument unless it is isolated from earth. The common ground point should usually be at or close to the primary master (eg control system).