Power over Ethernet Vs RS422

[martin griffith]

Any suggestions on protecting RS422/485 when using CAT5 cable?
explanation here
http://www.poweroverethernet.com/articles.php?article_id=52

Most 422 line drivers seem to have a max common mode Voltage of +-12V.

These days everbody seems to be using CAT5 for almost every
conceivable signal.

Just wondering what you do for protection (insert condom jokes if
reqd.)?


martin

 

[Rene Tschaggelar]

martin griffith wrote:

Any suggestions on protecting RS422/485 when using CAT5 cable?
explanation here
http://www.poweroverethernet.com/articles.php?article_id=52

Most 422 line drivers seem to have a max common mode Voltage of +-12V.

These days everbody seems to be using CAT5 for almost every
conceivable signal.

Just wondering what you do for protection (insert condom jokes if
reqd.)?

Void the warranty of your device when connected to an improper
cable or source.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

 

[martin griffith]

On Wed, 17 Aug 2005 21:03:19 +0200, in sci.electronics.design Rene
Tschaggelar <none@none.net> wrote:

martin griffith wrote:

Any suggestions on protecting RS422/485 when using CAT5 cable?
explanation here
http://www.poweroverethernet.com/articles.php?article_id=52

Most 422 line drivers seem to have a max common mode Voltage of +-12V.

These days everbody seems to be using CAT5 for almost every
conceivable signal.

Just wondering what you do for protection (insert condom jokes if
reqd.)?

Void the warranty of your device when connected to an improper
cable or source.

Rene
LOL, I agree

Thanks


martin

 

[Richard H.]

martin griffith wrote:

Any suggestions on protecting RS422/485 when using CAT5 cable?
explanation here
http://www.poweroverethernet.com/articles.php?article_id=52

Most 422 line drivers seem to have a max common mode Voltage of +-12V.

Does RS-422 have the signature 25K resistance? (IIRC there is also a
capacitance check.) i.e., the -48v is not applied to the cable unless
the signature is present, to prevent damaging non-PD devices. Early
non-standard implementations were hard-wired on, and they quickly
learned of the incompatibilities.

The discovery process applies a low current-controlled voltage to probe
for the "signature" before the power is enabled. I don't know the
characteristics of this signal, but I'd expect it to be closer to 5v
than 48v for exactly this reason.

Richard

 

[Richard H.]

Rene Tschaggelar wrote:

Void the warranty of your device when connected to an improper
cable or source.

Practical, but I doubt customers will see it that way.

Corporate cable plant is RJ-45 jacks for everything, and the telephony
gear is less friendly than PoE. A very real hazard is a digital phone
jack or a modem line, which will have up to 48v on the center pair and
possibly the others, depending on the system.

Richard