RS485 in Unshielded Cable 1

[jfpe]

I am working on a job where the electrical contractor pulled unshielded, twisted pair Cat 5 cable for RS485 communications. I know RS485 is usually run in shielded twisted pair, and I am wondering if this will work. The runs are all in dedicated metal conduit, which I think would function as a shield. Some of the runs are long, ~400'
Thanks,
John

 

[davidbeach]

Water ingress into wet location listed cable doesn't happen without a failure of the mechanical integrity of the insulation. For signal cable the results of water ingress can be minimal, but for power cables the results of water ingress can be spectacular; doesn't happen all that often considering the amount of cable in place.

 

[slavag]

Hi.
Of course it's depend on the application, in lot of cases, today, for avoid all such " problems" we used RS485 to FO convertrs. FO and convertors are so cheapper.
Regards.
Slava

 

[7anoter4]

I am very sorry and I have to apologize for my bad behavior against ScottyUK.
My hate in front of PVC and mainly against Cat 5 has a long history. A long time ago I designed a web of 400-500 ductbanks c/w manholes for a fossil fired power station.
Through this web I routed about 30 pair cables –I am not sure if they were really cat5 but very similar-for a distance of 2000 ft[or more].I don't remember what frequency was the carrier [may be 100kHz].At the beginning –let say 2-5 years – all the system worked excellently. And suddenly all it was changed. The signal was jammed and the system lost its stability.
So I checked the routing. First of all the manholes were full of water. The dedicated steel duct for instrument cables was partial used for power cables –on a fragment was even a medium voltage shielded cable. The end patch was deployed from the original place and so on. We decided to substitute all these cable for a screened [shielded] pairs cables. After then no trouble of this kind was noted.
But as ScottyUK said may be was a change in carrier frequency and the 100Khz was not from the beginning. Never the less the shielded pair mended the outage.

 

[ScottyUK]

No offence taken, so no need for any apologies.


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

So 7anoter4, were you sending just 485 down those 2000 footers cables?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MacGyverS2000]

600+ meters of unshielded data running parallel to power? Yikes! No wonder you have a bad taste in your mouth about it.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[GonzaloEE]

I worked once with RS422 in a chronometer system for those 1/4-mile car races. We used a cheap Ethernet UTP cable. RS422 was chosen to meet the full-duplex mode requirement. The system worked successfully at 9.6kbps for months, until a bulldozer cutted the cable, and it was replaced by a wireless system <

We left the 4 unused wires tied to GND to get a partial shielding. As stated before, twisted pair has a better common-mode noise balance on both wires, so we just left 100ohm shunt resistors at both ends for cable impedance matching, along with zeners and ceramic caps in parallel, to sink some line noise. All data was Manchester encoded to prevent loading lines (like Ethernet), and the bitrate was chosen low to stay away from any resonance.

Good Luck!

 

[itsmoked]

The shielding wouldn't have helped against a dozer... LOL

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[GonzaloEE]

haha, agreed