Cables to use with Prosoft MVI56E-MCM RS-485 2/3 wire

[AREDA]

Hi all,

I wonder what's the best type of cable to use for RS-485 2 wire + GND
That is, Data+, Data- & GND

I know that Belden 9841 (not recommended for GND connection to use shield) / 9842 with AWG24

But my problem is, if we need to use intermediate terminals (e.g. for connection from one panel to another, and also to terminate unused core & screen), then AWG24 into screw terminals seems to be flimsy, and bigger conductor size would be better.
However, I'm reminded that the characteristic impedance of the cable for RS-485 is expected to be 120 ohm.
Frankly, I'm not expecting very long runs - maximum 30m or less.

Online it says the "GND" connection should not be ignored, especially if both systems don't have the same 0V reference, also the shield should not be used for the GND connection.

Thanks

 

[Ken Moore]

Does Automation Direct ship to the UK?
With such short runs, I would use AD RS-422/485 cable.

https://www.automationdirect.com/ad...ions&msclkid=b9dcbc45f60f18633abdd9e39f43a71a

 

[danw]

I once used Belden 8770, 3 conductor, STP, 70 Ohm impedance.



https://www.farnell.com/datasheets/2331478.pdf


I used two 120 Ohm resistor in parallel as terminating resistors, not a perfect match, but close enough. Worked OK at 38.4K baud, point-to-point.

 

[AREDA]

Thanks gents, appreciated

 

[Puddle]

I use this (Belden 8760). Got around 50 RS-485 instruments around site and the only issues I've had all turned out to be my fault.

https://uk.rs-online.com/web/p/audio-cable/0823099?gb=s

 

[AREDA]

Thanks Puddle
I do believe it works, it's just not ticking one box for me
in many sources and manuals, the 3rd connection "GND" is labelled optional, but it is said better to connect it where the 0V reference may not be the same at both ends. Many RS-485 ports would already offer the GND terminal.

Ideally, I'm looking for a 1.5 pair cable (also LSZH to compound the issue)

I think this cable is going to be ok
https://www.belden.com/products/Cable/Electronic-Wire-Cable/RS-485-Cable/9842LW

It's a trade-off, I don't like the small AWG24 when it comes to screw-terminals, but they do mention a wide range from AWG26 (or even AWG30, 0.2mm2) to AWG 12 (4mm2).

 

[AlfredoQuintero]

PROFIBUS DP physical layer is RS-485.
Maybe standard PROFIBUS cable would be suitable an easy to find amongst your suppliers.
Just an idea.

 

[AREDA]

Thanks Alfredo
that seems to be also 2 conductors not 3.
I'm sure this means the GND conductor is not really needed as reported in some articles and as made available by some manufacturers.

 

[AlfredoQuintero]

AREDA, the Profibus cable is shielded, so the shield is the third conductor.

 

[AREDA]

Yes I understand about the shield, but the shield can go to Earth on one of the two ends (like you'd do with any homerun cable with screen); the "GND" terminal on the RS-485 interface should not be used for that, imo.

 

[AREDA]

My friend found this, which explains the conundrum, however I think 1 pair + shield is what I've decided to go with.
https://library.e.abb.com/public/19..._RS-485designandinstallbestpractices_REVA.pdf