DH+ Isolation


Thread Starter

Bradley Timm

Can any one suggest a supplier of an isolator for RS485.

The ones I have found do not support BAUD rates above 120k and I need 230k


Bradley Timm
Cape Town

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Eric M. Klintworth

B&B Electronics, catalog number 485OPDR (that's letter O, not number 0),
supports to 460K bps, $155, 10 to 30 VDC, even DIN rail mount!

But I'm confused--DH+ is not RS485, not even close I don't believe. The
only good way I know of to isolate DH+ is fiber transceivers, but forget
$155. They are certainly available, but I can't remember the
manufacturer just now.

Hope that helps,
Eric M. Klintworth, P.E.
Columbus, Ohio, USA
Hi guys,

The original posting was asking how to electrically isolate Allen-Bradley DH+
networks, and suggestions were traded about commercial RS-485 isolators (or
not), so I'd like to clarify a little bit.

DH+ is not equivalent to RS-485 or RS-422 or any other EIA standard. It's about
17 volts differential between the blue and clear wires. An RS-485 isolator may
work with DH-485 (similar protocol, different electrical standard), but not with

Fiber optic repeaters are very popular for running long distances, making "star"
networks, and running through high-noise areas. The Allen-Bradley 1771-AF is
obsolete but still available; instead I strongly recommend our partner company
Phoenix Digital in Arizona [ (602) 483-7393].

Another common task is to suppress spikes on the DH+ line; for that application
both Transtector and Leviton make simple products that can defend against
induced spikes.

As far as a DIN-rail mounted little DH+ to DH+ isolator to just electrically
isolate two trunk segments.... I haven't seen that sort of application done.
I'd be curious to hear if anyone has seen such a device? I do a lot of bridging
with 1756-DHRIO modules, but not with electrical-only isolators.


Ken Roach
Tech Specialist
A-B Seattle
[email protected]