Path: csiph.com!xmission!news.snarked.org!news.linkpendium.com!news.linkpendium.com!panix!not-for-mail From: Grant Edwards Newsgroups: comp.arch.embedded Subject: Re: RS485 over optical fiber cable Date: Wed, 31 Oct 2018 16:48:17 +0000 (UTC) Organization: PANIX Public Access Internet and UNIX, NYC Lines: 36 Message-ID: References: <7686c6c0-7f2a-49b9-94d2-da49b7ab32ef@googlegroups.com> NNTP-Posting-Host: 67-130-15-94.dia.static.qwest.net X-Trace: reader1.panix.com 1541004497 3027 67.130.15.94 (31 Oct 2018 16:48:17 GMT) X-Complaints-To: abuse@panix.com NNTP-Posting-Date: Wed, 31 Oct 2018 16:48:17 +0000 (UTC) User-Agent: slrn/1.0.3 (Linux) Xref: csiph.com comp.arch.embedded:28302 On 2018-10-31, John Speth wrote: > On 10/31/2018 7:03 AM, Grant Edwards wrote: >> On 2018-10-31, pozz wrote: >> >>> I think I need it for 500m link. Is the usual 120R ok? >> >> The terminating resistor value is chosen to match the cable's >> impedance. > > I have no experience with long wire RS485 applications. I find it hard > to believe that an appropriately sized resistor is not on the top of the > list of RS485 best practices. I have solved so many "long" wire > problems by just adding termination resistors (in my case long wires are > actually short but appear long due to edge speeds). And don't forget the _bias_ resistors to make sure that the line idles in the proper state when no drivers are enabled. And yes, a signal ground connection is also _required_. Even though the signaling is differential, a signal ground is required to make sure you don't violate the common-mode voltage specs on the transceivers. I've been dealing with customer RS485/RS422 issues for almost 30 years, and they're almost due to missing bias/terminating resistors or missing signal ground. You can connect signal ground to Earth ground if you want, but _only_at_one_point_. Breaking that rule can start fires (yes, I've seen the melted, charred results of connecting signal ground or cable shield to Earth at to different points). -- Grant Edwards grant.b.edwards Yow! Sometime in 1993 at NANCY SINATRA will lead a gmail.com BLOODLESS COUP on GUAM!!