[TriEmbed] OT-ish: TVS best practices?

[Scott Hall]

I'm still at work and don't have the time to go into a long answer, but here are
a couple of pointers to help:

Similar question as yours, posted on a forum:
"RS485 and power on the same cable":
https://www.electronicspoint.com/threads/rs485-and-power-on-the-same-cable.37397/

A couple of great sites for RS485 are
http://www.rs485.com/
http://www.bb-elec.com/

(warning: most of these are hawking products...)

Bourns Application Note: Handling Transient Threats in RS-485 Systems
http://www.bourns.com/docs/Products-General/Bourns_RS-485_AppNote.pdf

Using Isolated RS-485 in DMX512 Lighting Applications
http://www.analog.com/en/analog-dialogue/articles/isolated-rs-485-in-dmx512-lighting.html
http://www.analog.com/media/en/analog-dialogue/volume-43/number-4/articles/isolated-rs-485-in-dmx512-lighting.pdf

Transmit High Data Rates Over Long Distances with Murata’s NM485D6S5MC Isolated
RS485 Driver, Now at Mouser
https://www.electronicsmedia.info/2017/03/16/transmit-high-data-rates-long-distances-muratas-nm485d6s5mc-isolated-rs485-driver-now-mouser/

Building a Reliable RS485 / RS422 Network
https://www.commfront.com/pages/building-a-reliable-rs485-rs422-network

SCADALink Industrial RS485 Solutions
https://www.scadalink.com/products/data-communications/industrial-rs485/

----
When I did work in Radio & Television engineering, we used the center-taps of
high-speed transformers to carry power over the signal pairs, making sure to use
transformers designed for the extra DC bias to avoid saturation:

Full Duplex RS-485 (2 pair)

TX + --------}|{--------------/ /------------}|{------- + TX
             }|{                             }|{
             }|{--- CT +24V      ~+22V CT ---}|{
             }|{                             }|{
TX - --------}|{--------------/ /------------}|{------- - TX

RX + --------}|{--------------/ /------------}|{------- + RX
             }|{                             }|{
             }|{--- CT SGnd       SGnd CT ---}|{
             }|{                             }|{
RX - --------}|{--------------/ /------------}|{------- - RX


You do know that my weekend "business" is Smart-Home / Smart-Energy, don't you ?

On 10/30/2017 01:57 PM, Brian via TriEmbed wrote:
> Hi Group,
>
> I've been working on a design for zoned HVAC which involves having electronics
> at the ends of long cables.  As such, good transient voltage suppression
> becomes a more important concern.
>
> My device will be connected via Cat-5 UTP across which will be supplied +6V,
> GND, and an RS-485 differential pair.
>
> I'm wondering if anyone in the group has suggestions on the ideal application
> of TVS elements.  I've seen reference designs for RS-485 that have paths to
> ground for each of the signal lines, but I've also got the +6V line to worry
> about.
>
> Can I just provide paths from each signal to GND, or do I need to provide
> paths from every signal to every other signal?  It seems to me that
> theoretically, all four conductors could be floating at different potentials
> relative to each other, and clamping to GND wouldn't necessarily protect
> against a damaging potential between a 485 wire and +6V.
>
> All input greatly appreciated!
> Thanks,
> -Brian
>

-- 
Scott G. Hall
Raleigh, NC, USA
ScottGHall1 at GMail.Com

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