Tamron MST2000 Cf +, 13.4HART Communication Distance, Power Supply Requirements, 65 ×, R × C

OPERATION MANUAL MST2000 HART® Communications Information

Typical Point to Point Connection with analog signaling.

If the ca ble is lon ger than sev eral me ters, it’s re sis tance and ca pac i tance may be come significantinthe HART® RC time-constant lim i ta tion ® x C[65 mi cro sec onds). When us ing a sin gle field de vice and a host with a 250 ohm load and no other sig nif i cant re sis tance, the 65 mi cro sec ond lim i ta tion would al low

0.26uF of ca pac i tance for the sys tem. Al lowing 0.01uF (10,000 pF) for the host and field de vice (each having a CN=1), the to tal ca ble ca pac i tance could be up to 0.25 uF. However, if the cable resistance was 110 ohms, the sys tem re sis tance be comes 360 ohms, which then al lows for a to tal per mit tedcable

ca pac i tance of 0.18uF. This cor re sponds to a nom i nal ca ble length of 900 me ters for a ca ble with a rat- ing of 200 pF/meter. If the cable needs to be extended upward towards the maximum HART® cable length of 1500 me ters, a ca ble with a lower ca pac i tance rat ing must be se lected.

•In a multi-dropped system, the additionalcapacitance from each net worked trans mit ter must also be considered. Each transmitter has an es tab lished CN value. A CN value of 1 in di cates that the transmitterrepresents 5000 pF of load capacitance.

•The MST2000 trans mit ter has a CN Value of 1 ( 5000pF).

•The internalresistance of the MST2000 trans mit ter is in ex cess of 100,000 ohms and can be ig- nored in the cable length calculations.

13.4HART® Communication Distance

Up to 1.5 km (1 mile) when using multiple twisted pair cables. Communication distance varies depending on type of cable used.

Use the following formula to determine cable length for specific applications:

L =	65 × 106	−	(Cf + 10000)
	(R × C)		C

Where:

L = Length in Meters or Feet

R = Resistance in Ohms (Ω ) including barrier resistance.

C = Cable capacitance in pF/m or pF/ft

Cf = Maximum shunt capacitance of receiving devices in pF/m or pF/ft.

13.5 Power Supply Requirements

To minimize signal degradation of the HART® communication signals, the following power supply specifications are required.

•Voltage: . . . . . . . . . . . . . . . . . . . . . . . 24 V DC (typical)

•Max i mum rip ple (47 to 125 Hz): . . . . . . . . . . 0.2 V (peak to peak)

•Max i mum noise (500 Hz to 10 kHz): . . . . . . . . 1.2 mV

•Max i mum se ries im ped ance (500 Hz to 10 kHz):. . 10 ohms

Brandt Instruments, Inc. MST2000 Multivariable SMARTFLOW ® Transmitter Page 27