Re: weird cables question

From: Groepaz (
Date: 2004-01-15 16:24:40

On Thursday 15 January 2004 15:21, Spiro Trikaliotis wrote:
> On Thu, Jan 15, 2004 at 01:05:56PM +0100, Groepaz wrote:
> > but a missing rts/cts connection is pretty much ridiculous - this wont
> > ever work right except when both end are faster than the actual
> > transfer is.
> There is also a software-handshake method (XON/XOFF, for example).

uhmz yes. you can also send snail mail or morse code :=P

seriously, software handshake is no option these days. it just plainly
sux, you dont want it. never.

> As I told, I'm not the expert. Anyway, from my understanding, this
> problem arises because the power lines on both sides can get different
> potential ("Potenzial" in german) from the PSU, so there is some current
> running through, trying to put balance this difference.

yes, but the diodes will avoid that (they are a "one way" for current)

ever seen one of these cheap centronics interfaces for c64 that have a
lot of diodes at the userport connector? guess what they are good for :)

> From what I remember, this happened on the serial lines (RS232C) in the
> company I told, and there were some Ampere running (but not damaging
> anything seriously). Flipping the power cord in the outlet solved the
> problem before they used the permanent solution: Using optocoupler to
> galvanically isolate both sides.

very common mistake, and done in 99% of all installations that use rs232

1) you are ***not*** supposed to connect protective and signal ground, this
is a violation of the rs232 standard (only protective ground is directly
connected to PE, signal ground -just like VCC- _is_ already galvanically 
isolated by the transformer in the PSU!) [note: the optocopplers would
still make sence for protecting the interface chips themselves by other
means, but thats not the point here]
2) you are ***not*** supposed to connect protective ground on both ends of
the rs232 cable, thats again a violation of the standard. (this is what
makes those high unwanted currents possible)

now guess what, those two pretty effectivly eliminate the problem you
are describing :) its btw also the main reason for any failing rs232
transfers i have seen so far (besides broken/wrong cables)

btw, using rs232 cables >10m are also a violation of the standard (and if
i had the time to waste i could probably prove by calculation that within
this range the resistance of the wire isnt high enough to provoke any
kind of harmful voltage differences)

solution: use ethernet damnit :=P 

> Have a look here (sorry, german only):
> According to this text, it makes a difference.

it makes a (subtle) difference when using that kind of bulk equipment you
get at walmart, yes indeed. (same reason for starcommander working here
and not there). ***decent*** rs232 solutions do not have this problem
at all. (that includes not violating the standard by any means, including
removing those common 1000m cables :))

oh btw that page has a slight mistake....according to VDE *any* kind of 
"new" installation (that includes installing a new power outlet or so)
requires you to install proper seperated pe/n/l atleast within the next
main "circuit". this is rarely done (because its not quite cheap) but
it would solve the problem nevertheless (atleast make it less bad) :) 
(and if you ever see such a "wrong" installation and want to be mean 
- a simple call to VDI or simelar institution will do the trick)


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