Fieldbus Cable

[E287122]

Please advice on the importance of the following characteristics on FF cables and the values mentioned

1. Mutual Capacitance - max.60nF/km
2. Capacitance unbalance to Earth - max 2nF/km
3. Propogation Delay - max 1.7µs/km
4. Inductance - 0.65mH/km

Also please advice which are all the normal electrical characteristics to be followed strictly for a fieldbus cable.

Regards
N.Ramkumar, Honeywell.

[marisg]

The cable characteristics you have listed are certainly things that can be measured on a cable but they are not very relevant. The relevant characteristics are:

The wire pairs in the cable should be twisted and have individual shields.

The DC resistance of the wire determines how many devices and at what distance the cable will support. Most fieldbus cable has 18 GA wires.

The cable's should have a characteristic impedance of 100 +/-20 Ohms.

There is still some debate about the importance of the unbalance between each of the wires in a pair and the shield. I think you will find that cable manufacturers have a hard time in making reasonably priced cable with 2 nF/km unbalance. My recommendation is to look for cable with less than 10 nF/km. This requirement will weed out pure junk. Our tests and simulations here show very little effect of capacitative unbalance on fieldbus signals.