Re: Re: Wanted - two pair cable for 12VDC and RS485

["Alan Ainslie" <alan.ainslie@xxxxxxxxxxx>]

A pedantic point of detail: screen capacitance should not contribute to
cable losses.

The losses in a properly matched cable are due to the dielectric losses in
the main.

All cables have distributed capacitance and inductance: from that comes the
'characteristic impedance' in this case 100 ohms.

The capacitance of the screen is effectively 'common mode'. and of  no
consequence, but is allowed for in the construction of the cable to
maintain
the desired  impedance.

Transmissions lines cannot really be thought of in terms of 'having to
charge the capacitance' without looking at the corresponding effect of the
inductance: we are then back to the 'characteristic impedance'.

So: screened cables should not, all else being equal, be any more lossy
than
unscreened.

The main practical issue with screened cables is that unless there is a
really good ground in the socketry the screen acts as an antenna and messes
the EMC side of things, both susceptibility and radiated.

However, good practice is that screened cables properly terminated have a
level of isolation from external fields much better than a twisted pair
without screening.

Well worth comparing some of the detail within the manufacturer  data
sheets: it is apparent that some screen technologies also perform much
better than others.  Very important in a hostile environment: like behind a
Plasma!

Alan Ainslie



>> When we use RS485 to shift data around buildings (which often have
high
>> amounts of EMI from VSDs etc), we always use STP. Why would a
screen be
>> detrimental to performance?
> It probably wont affect RS485 too much if the speed is quite low but
> screened cable has a much higher capacitance so therefore greater
losses.
>