Is termination needed for short and slow RS485?

Paul Keinanen wrote:
> On Sun, 19 Sep 2004 20:45:28 +0200, Rene Tschaggelar <none@none.net>
> wrote:
> 
> 
>>mrsoft99 wrote:
>>
>>>We are running some led billboard displays using a multi-drop 2-wire
>>>RS485 network.
>>>The cable length is 50 feet; the baud rate is 2400 baud.
> 
> 
> At least you should have somewhere a pull-up and a pull-down resistor
> to drive the bus into the idle mark ("1") state when there is no
> active transmitters.
>  
> 
>>It could work without teminating resistors.
>>What is essential is the common more range of some
>>7 V or so. 
> 
> 
> This is essential.
> 
> 
>>So you should always have the GND as
>>reference together with the differential signal.
> 
> 
> This might generate more problems than it solves.
> 
> If the equipment are ground referenced to the electric system neutral
> (directly or through PE connected to neutral), the reason for the
> equipment ground potential differences is the different voltage drops
> in the mains system neutral wiring carrying the whole mains neutral
> current. The drops can be quite with large single phase loads or even
> in a three phase system there can be quite considerable 3rd harmonic
> currents at 150 or 180 Hz if there are a lot of switching mode power
> supplies in each phase.
 >
 > [snip]

You're perfectly right Paul.
I was somewhat unclearly indicating that the GND
should be designed into the system, into the connector
and into the cable. Current flow through it should
be avoided. Should at some point be decided that isolators
were required, the GND could be used. The 100 Ohms in
series with the GND could als be employed to detect
current flow. Depending on the location, measuring
GND differences are unpractical.
I actually run +5V together with the GND too. This was
isolators can be fully powered through the cable.

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

Tony Williams wrote:
> mrsoft99 <mrsoft99@yahoo.com> wrote:
>
>> We are running some led billboard displays using a multi-drop
>> 2-wire RS485 network. The cable length is 50 feet; the baud rate
>> is 2400 baud.
> 
> It is better to use 3-wire for differential comms so that the
> receivers have at least some idea of the common mode voltage.
> Also a low-pass filter on the input of each receiver can be
> useful in avoiding difficulties with line-ringings, (from a
> line that is being driven too hard), or stray interference.
> 
>  RS485 lines.
>  A  B  0V
>  |  |  |
>  |  |  |    2k           _
>  +--|--|----/\/\---+----| \
>  |  |  |           |    |  \
>  |  |  |    2.2nF ===   |Rx >--
>  |  |  |           |    |  /
>  |  +--|----/\/\---+----|_/|
>  |  |  |    2k             |
>  |  |  |                   |
>  |  |  +----/\/\-----------+---0v
>  |  |  |    100R
> \|/ | \|/
>  More receivers, as above.
> 
>  2400 bits/sec is 417uS per bit.
> 
> 2+2k*2.2nF is an 8.8uS RC time constant, which will get to 99.9%
> of where it is supposed to go in about 44uS, or about 10% of
> each bit-width.
> 
> The 100R resistor connecting each receiver 0v to the line 0V is
> there to limit the current in case of earth loops.  In fact if
> there is any voltage across any 100R (when not transmitting)
> then you know that you probably have an earth loop problem.

That 100 ohms could function as a fuse.  Grounds can have massive
differences, think start-up currents into elevator motors and the
ilk.  The fault there is the connection to 0v, assuming that is
connected to local ground.  Any long wire can have large voltages
inductively induced by nearby equipment.

-- 
Chuck F (cbfalconer@yahoo.com) (cbfalconer@worldnet.att.net)
   Available for consulting/temporary embedded and systems.
   <http://cbfalconer.home.att.net>  USE worldnet address!