capacitance

[Mario.Castrillo]

Hi FF Guys,

Can anybody give the Capacitance range of the trunk cable for a segment circuit, probably 16 devices max. I got 1-4pair, 16AWG, twisted-shielded cable, 100pF/meter capacitance. BELDEN 3076A which is 18AWG is 78.9pF/m. I believe there should be a Capacitance range in which the FF circuit can operate...thanks...

[marisg]

Mario,

There are two AC and one DC cable characteristic that are of primary interest to fieldbus:

+ The Characteristic Impedance of the twisted pair should be 100 +/-20%.
+ The maximum attenuation in the fieldbus frequency range should be less than 3 dB/km.
+ The DC resistance of the cable affects how much current you can put through the cable, between the fieldbus power supply and the fieldbus devices at the other end, before you run run out of voltage for the devices.

These three parameters should be specified by the cable vendor.

The capacitance between the two signal wires is irrelevant. It is the characteristic impedance and attenuation that are important. The capacitance between each fieldbus wire and the shield should be reasonably equal. In my estimation, a 10 nF/km difference is acceptable. A greater value would suggests that the cable quality may be questionable.

For more information, please see the Fieldbus Wiring Guide in WWW.relcominc.com (http://www.relcominc.com).

[Mario.Castrillo]

Marisg, thanks very much, i will let you know the capacitance between wires when installed. What causes attenuation then in fieldbus...just one more? Regards, mac

[marisg]

Mario,

There are two mechanisms that cause attenuation:

+ The cable itself has resistance so the signal gets smaller as it travels from one end of the trunk cable to the other.

+ The spur cables at the end of the trunk cable look like capacitors because their length is much shorter than the wavelength of the fieldbus signals. These "capacitors" have to charge up and discharge with every voltage change of the fieldbus signal. This appears as attenuation.

We have tested a 1 km long trunk cable with 16 spur cables each 120 m long. The signals still looked good. I would not worry about attenuation. The issues to worry about is the proper instllation of the cables. Make sure they are not water logged, bent around sharp corners and the shield is not shorted to one of the wires. Check to see that the wires are stripped correctly, use ferrules on the wire ends if possible and use a torque limiting screwdriver to tighten them to wiring blocks.

If you send me your e-mail addrss, I will send you a list of good tools to use.

Regards
Maris