RS485 chips and differential voltage

Jim Stewart wrote:

> Dennis Clark wrote:
> 
>>   I've been reading up on RS485 drivers like the LTC485, 75176B and 
>> others.
>> They look too simple in pin count to supply the + and - voltages needed
>> for differential drives.  Are there some chips that supply the +/- 
>> voltages
>> that other chips simple scavange and use?  Or do these chips I note 
>> actually
>> supply the +/- voltages for the differential outputs?  The chip specs 
>> don't
>> say.
> 
> 
> I'm not sure that I'm answering your question, but
> I'll give it a try anyway.  RS-485 does not use
> plus and minus supplies.  It uses +5 volts only,
> and sends it differentially on the pair of signal
> wires.
> 
> In other words, the equivalent of a "high" or
> "marking" RS-232 signal would have the + RS-485
> signal at +5 volts and the - signal at ground.
> A "low" or "spacing" signal would have the + RS-485
> signal at ground and the - signal at +5 volts.
> 
>   I've a project that I'm going to propose using an RS485 network but
> 
>> am not sure of all the pieces needed.  Can anyone supply me with a place
>> to look for the "ABC's" of RS485 networking to answer questions like:
> 
> 
> This will get you started:
> 
> http://www.maxim-ic.com/appnotes.cfm/appnote_number/736
> 
> The authoritative organization for the standard
> is the EIA.  You can find them at www.eia.org
> and do an incredible sillywalk to try to find
> out how to buy a copy.

Another good read is Bob Perrin's Circuit
Cellar article on RS-485


http://www.circuitcellar.com/library/ccofeature/perrin0799/index.asp

Guys,

  Thanks a bunch for all the help, and you've set me straight.  I don't 
know where I came to the idea that differential always means +/- voltage,
but this explains why I never found the specs saying so (even though all
of them mention + and - voltages in the spec limits of the chips), there
isn't any - voltage.  I've been going through the B&B appnotes papers, 
which are VERY good at explaining termination and line properties.  I
was obviously confused by the lack of any explanation of what voltage was
on the transmission lines - either because I couldn't find the reference
or because my tired eyes can't read the small graphics that showed wave
forms. :(

  You've all been a ton of help.  It has confirmed my conviction that I
want to use RS485 in this RFP I'm responding to, the originators were
thinking of I2C and the existance of a high volume vacuum pump, stepper
motors and other high voltage lines made their idea sound like an unwise 
design decision.

best regards,
DLC
-- 
============================================================================
* Dennis Clark         dlc@frii.com                www.techtoystoday.com   * 
* "Programming and Customizing the OOPic Microcontroller" Mcgraw-Hill 2003 *    
============================================================================

On 12 Mar 2004 17:30:33 GMT, the renowned Dennis Clark
<dlc@io.frii.com> wrote:


>  You've all been a ton of help.  It has confirmed my conviction that I
>want to use RS485 in this RFP I'm responding to, the originators were
>thinking of I2C and the existance of a high volume vacuum pump, stepper
>motors and other high voltage lines made their idea sound like an unwise 
>design decision.

Yes. If you really want to make it solid in a noisy environment, go
for opto-isolated RS-485. 

Best regards, 
Spehro Pefhany
-- 
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

On Fri, 12 Mar 2004 07:52:50 +0100, "Meindert Sprang"
<mhsprang@NOcustomSPAMware.nl> wrote:

>"Dennis Clark" <dlc@io.frii.com> wrote in message
>news:405152b2$0$198$75868355@news.frii.net...

>> Realistic upper limits of number of drivers on a bus (I've heard 8)
>
>That is in the datasheets. The 'standard' driver (75176) follows the RS-485
>spec, which says 32 drivers. But the more modern types from LT,AD and Maxim
>have a lower load and allow 128 driver on one bus.

Those numbers are based on the receiver DC bias currents only. 

When running at high speeds (>1 Mbit/s), the stubs connecting the main
line to each node become quite critical. Long stubs and stubs too
close to each other can cause a lot of harm at a lower number of
nodes. 

Ideally, the main bus should run _through_ the connector connecting
nodes. The transceiver inside the node should be close to the
connector. This will minimise the distributed capacitance due to the
stub and more than 30 nodes can be connected to a bus.

Paul
 

Spehro Pefhany <speffSNIP@interlogdotyou.knowwhat> wrote:
: On 12 Mar 2004 17:30:33 GMT, the renowned Dennis Clark
: <dlc@io.frii.com> wrote:


:>  You've all been a ton of help.  It has confirmed my conviction that I
:>want to use RS485 in this RFP I'm responding to, the originators were
:>thinking of I2C and the existance of a high volume vacuum pump, stepper
:>motors and other high voltage lines made their idea sound like an unwise 
:>design decision.

: Yes. If you really want to make it solid in a noisy environment, go
: for opto-isolated RS-485. 

That is plan B - Like all projects, it is cost sensitive.  To what degree?
I'll find out.

Again, thanks all.

DLC


: Best regards, 
: Spehro Pefhany
: -- 
: "it's the network..."                          "The Journey is the reward"
: speff@interlog.com             Info for manufacturers: http://www.trexon.com
: Embedded software/hardware/analog  Info for designers:  http://www.speff.com

-- 
============================================================================
* Dennis Clark         dlc@frii.com                www.techtoystoday.com   * 
* "Programming and Customizing the OOPic Microcontroller" Mcgraw-Hill 2003 *    
============================================================================

Spehro Pefhany wrote:
> On 12 Mar 2004 17:30:33 GMT, the renowned Dennis Clark
> <dlc@io.frii.com> wrote:
> 
> 
> 
>> You've all been a ton of help.  It has confirmed my conviction that I
>>want to use RS485 in this RFP I'm responding to, the originators were
>>thinking of I2C and the existance of a high volume vacuum pump, stepper
>>motors and other high voltage lines made their idea sound like an unwise 
>>design decision.
> 
> 
> Yes. If you really want to make it solid in a noisy environment, go
> for opto-isolated RS-485. 
> 
> Best regards, 
> Spehro Pefhany
Or at least use protected 485 parts.  Ground jumps around motors will 
zap the drivers and receivers.   Opto isolated protected parts would be 
the best choice in the environment listed above.


					Chris