Reply by Christopher Leddy●March 16, 20042004-03-16
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
Reply by Dennis Clark●March 13, 20042004-03-13
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 *
============================================================================
Reply by Paul Keinanen●March 12, 20042004-03-12
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
Reply by Spehro Pefhany●March 12, 20042004-03-12
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
Reply by Dennis Clark●March 12, 20042004-03-12
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 *
============================================================================
Reply by Jim Stewart●March 12, 20042004-03-12
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.
> 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'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:
>
> Speed vs. termination resistors vs. line length.
> Realistic upper limits of number of drivers on a bus (I've heard 8)
> Comparisons of the driver chips available.
>
> I'd appreciate any and all aid I get.
>
> regards,
> DLC
Dennis,
I'm in the process of typing up my notes on RS-485. I'm using it
on a project and needed to get to know it better. I see you have been
given a number of references, but I have a few more to add. Go to TI,
National Semi., and Maxim, and B & B Electronics. I don't have the
URL's, but it won't take you long to find them. In addition, Jan
Axelson had a good article in Circuit Cellar ("Designing RS-485
Circuits") that is available on the internet.
Now to answer some of your questions. Think of differential as
being the digital signal and the inverse of it on the two lines. At
least that is what you start with, depending on biasing. As you may
suspect, the signal will attenuate over distance. As long as there is
more than a 200 mV difference between A (the true or '+' line) and the B
(or negative or '-' line), the receiver will be able to determine the
state. If the difference is less than 200 mV, then the state of the bus
is unknown.
How does one insure that the bus state is known? You bias it by
tying the A line to 5 volts through a resistor. Typical values range
from 560 to 750 ohms. The B line is tied to ground with the same value
resistor.
You may have noticed that I specified a range of bias values. The
reason has to do with termination. First, is termination needed? The
answer is that depends on the signaling speed (bps) and the length of
the transmission line. If the line length is much shorter than the
electrical length of the bit window, then termination is not needed.
When you get the documents mentioned you will see calculations to
determine this. The idea is that the wave front, or Trise, needs time
to dissipate before you are into the center of the bit. It is assumed
that 4 or 5 round trips around the transmission path will attenuate
Trise well enough. Remember the purpose of termination is to match the
cable Zo to the load. 24 AWG wire has a typical Zo of about 120 ohms,
so if termination is needed, there would be a 120 ohm, 1/2 watt resistor
at each end.
The problem with termination is that it makes the system less
flexible if you plan to be adding and removing nodes at will. So you
may want to do a trade-off where the speeds are slow enough that
termination is not needed. The other problem with termination is that
it is a load on the driver, and increases the current required.
The number of drivers is not limited. Of course you can't have
more than one driver on at a time because that will play havoc with the
data integrity. The number of receivers is what is limited. A receiver
can have 1, 1/2, 1/4, or 1/8 unit load (check with the receiver
specifications). A unit load is about 12K ohms. So,if all of your
receivers are one unit load, the spec. says that you can have up to 32
of them. More if you use receivers of a fractional unit load.
Comparing chips is something I have to let up to you because they
all have pros and cons.
I'm out of time, but I hope this helps somewhat. Let me know if
you have more questions.
Sincerely,
Dennis Zimmerman
--
If sending a reply you will need to remove "7UP".
Reply by Bryan Hackney●March 12, 20042004-03-12
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.
>
3.3V parts interface nicely with 5V parts. Maybe out of spec.
Reply by Grant Edwards●March 12, 20042004-03-12
On 2004-03-12, Meindert Sprang <mhsprang@NOcustomSPAMware.nl> wrote:
>>> [...]
>>> A receiver only looks at the voltage DIFFERENCE between A and
>>> B, the absolute level against ground does not matter. So even
>>> when A and B swing between 12 and 13V, it would be a
>>> differential system, albeit with a huge DC offset. But this is
>>> of no importance for the receiver.
>>
>> True within limits. The recievers do have limits on the
>> allowed common-mode voltage (the DC offset). A typical spec
>> would be something like -7V to +12V. For such receivers, 13V
>> would be outside the recievers specs, as would -8V.
>
> I know. It was merely meant as an example to indicate that the
> absolute (=common mode) voltage on both lines does not matter
> for the differential signal.
I was pretty sure you knew, but I wanted to make sure the OP
didn't get bit by common-mode voltage problems (been there).
--
Grant Edwards grante Yow! I selected E5... but
at I didn't hear "Sam the Sham
visi.com and the Pharoahs"!
Reply by Meindert Sprang●March 12, 20042004-03-12
"Grant Edwards" <grante@visi.com> wrote in message
news:4051cd7c$0$41291$a1866201@newsreader.visi.com...
> On 2004-03-12, Meindert Sprang <mhsprang@NOcustomSPAMware.nl> wrote:
>
> > I think you're misunderstanding the term differential. These
> > drivers are powered from 5V single supply. By setting the A
> > line to 5V and the B line to 0V and vice-versa, you get a
> > differential signal between A and B. A receiver only looks at
> > the voltage DIFFERENCE between A and B, the absolute level
> > against ground does not matter. So even when A and B swing
> > between 12 and 13V, it would be a differential system, albeit
> > with a huge DC offset. But this is of no importance for the
> > receiver.
>
> True within limits. The recievers do have limits on the
> allowed common-mode voltage (the DC offset). A typical spec
> would be something like -7V to +12V. For such receivers, 13V
> would be outside the recievers specs, as would -8V.
I know. It was merely meant as an example to indicate that the absolute
(=common mode) voltage on both lines does not matter for the differential
signal.
Meindert