RS485 - MODBUS or PROFIBUS or ....

Meindert Sprang wrote:
> "Not Really Me" <scott@validatedQWERTYsoftware.XYZZY.com> wrote in
> message news:7cral9F28uaiiU1@mid.individual.net...
>> Don't try this with SPI, the distance is too great.  I think it is
>> unlikely to work over I2C for the same reason, but I have less
>> direct experience with that.
>
> As Vladimir already pointed out: IDE, GPIB and Centronics were also
> single-ended at TTL level and all worked fine over at least 2 ft of
> flatcable. The SPI cable from my programmer is also 1 ft and I have
> never experienced any problems. Looking at the signals with my
> oscilloscope does not show any ringing, overshoot or reflection that
> matters.
>
> Meindert

Well, he said 4 ft, so that would be more trouble than 1ft.  I did have 
either a 12 or 20 ft Centronics cable, but that is a slow interface.

What speed SPI?  We used ADCs ADuC8xx series and had horrible results at 1 
ft until we used twisted pairs.  It's been a few years but I think it was 
set for 10Mbps but was still flaky at 250Kb, the lowest speed it had.  Only 
choices were 1M, 10M and 250K.

I would still not recommend SPI or I2C for this.  It is always better to 
used something intended for the use that you have and neither of these does.

Scott 

"Not Really Me" <scott@validatedQWERTYsoftware.XYZZY.com> wrote in message
news:7d5t0gF2amsl4U1@mid.individual.net...
> Well, he said 4 ft, so that would be more trouble than 1ft.  I did have
> either a 12 or 20 ft Centronics cable, but that is a slow interface.
>
> What speed SPI?

1.8 MHz.

> We used ADCs ADuC8xx series and had horrible results at 1
> ft until we used twisted pairs.  It's been a few years but I think it was
> set for 10Mbps but was still flaky at 250Kb, the lowest speed it had.

I think something else must have been wrong. I just ran a 5MHz square wave
through 3ft of speaker cable, driven from a 50 Ohm generator. The signal
comes out good. Overshoot is 17% and no degradation of risetime (better than
5ns). when terminated with 100 Ohm, the overshoot disappears. The same
signal driven from a TTL gate shows 25% overshoot.

So with a bit of a decent driver and termination, the OP can get good
results with simple single ended terminated wire and a 50 Ohm driver. RS-485
or CAN would be overkill.

> I would still not recommend SPI or I2C for this.  It is always better to
> used something intended for the use that you have and neither of these
does.

And when is something intended for the use? IMO anything that gives a good
reliable result.

Meindert

On Jul 27, 4:51=A0pm, "Not Really Me"
<sc...@validatedQWERTYsoftware.XYZZY.com> wrote:
> Meindert Sprang wrote:
> > "Not Really Me" <sc...@validatedQWERTYsoftware.XYZZY.com> wrote in
> > messagenews:7cral9F28uaiiU1@mid.individual.net...
> >> Don't try this with SPI, the distance is too great. =A0I think it is
> >> unlikely to work over I2C for the same reason, but I have less
> >> direct experience with that.
>
> > As Vladimir already pointed out: IDE, GPIB and Centronics were also
> > single-ended at TTL level and all worked fine over at least 2 ft of
> > flatcable. The SPI cable from my programmer is also 1 ft and I have
> > never experienced any problems. Looking at the signals with my
> > oscilloscope does not show any ringing, overshoot or reflection that
> > matters.
>
> > Meindert
>
> Well, he said 4 ft, so that would be more trouble than 1ft. =A0I did have
> either a 12 or 20 ft Centronics cable, but that is a slow interface.
>
> What speed SPI? =A0We used ADCs ADuC8xx series and had horrible results a=
t 1
> ft until we used twisted pairs. =A0It's been a few years but I think it w=
as
> set for 10Mbps but was still flaky at 250Kb, the lowest speed it had. =A0=
Only
> choices were 1M, 10M and 250K.
>
> I would still not recommend SPI or I2C for this. =A0It is always better t=
o
> used something intended for the use that you have and neither of these do=
es.

The problem is the rate of rise and fall, not the actual clock
frequency. If the rise time on the 250kb clock (or data line) is
5nanosec then one is actually looking at 100Mhz timing.

Meindert Sprang wrote:

> "Not Really Me" <scott@validatedQWERTYsoftware.XYZZY.com> wrote in message
> news:7d5t0gF2amsl4U1@mid.individual.net...
> 
>>Well, he said 4 ft, so that would be more trouble than 1ft.  I did have
>>either a 12 or 20 ft Centronics cable, but that is a slow interface.
>>
>>What speed SPI?
> 
> 
> 1.8 MHz.
> 
> 
>>We used ADCs ADuC8xx series and had horrible results at 1
>>ft until we used twisted pairs.  It's been a few years but I think it was
>>set for 10Mbps but was still flaky at 250Kb, the lowest speed it had.
> 
> 
> I think something else must have been wrong. I just ran a 5MHz square wave
> through 3ft of speaker cable, driven from a 50 Ohm generator. The signal
> comes out good. Overshoot is 17% and no degradation of risetime (better than
> 5ns). when terminated with 100 Ohm, the overshoot disappears. The same
> signal driven from a TTL gate shows 25% overshoot.
> 
> So with a bit of a decent driver and termination, the OP can get good
> results with simple single ended terminated wire and a 50 Ohm driver. RS-485
> or CAN would be overkill.

You are correct.
I have the Altera Byteblaster on the 10 ft cable. Doing the SPI both 
directions by bit banging at ~1.5 MHz.  No problem.

> 
>>I would still not recommend SPI or I2C for this.  It is always better to
>>used something intended for the use that you have and neither of these 
> does.
> 
> And when is something intended for the use? IMO anything that gives a good
> reliable result.

It depends. Once I was surprised to see the remote sensor with the 
Dallas 1-wire interface operating OK over ~15m of the twisted pair; 
however I've also seen the I2C not working on the same PCB with the SMPS 
because of the problematic layout.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

On Mon, 27 Jul 2009 08:51:56 -0600, "Not Really Me"
<scott@validatedQWERTYsoftware.XYZZY.com> wrote:

>Meindert Sprang wrote:
>> "Not Really Me" <scott@validatedQWERTYsoftware.XYZZY.com> wrote in
>> message news:7cral9F28uaiiU1@mid.individual.net...
>>> Don't try this with SPI, the distance is too great.  I think it is
>>> unlikely to work over I2C for the same reason, but I have less
>>> direct experience with that.
>>
>> As Vladimir already pointed out: IDE, GPIB and Centronics were also
>> single-ended at TTL level and all worked fine over at least 2 ft of
>> flatcable. The SPI cable from my programmer is also 1 ft and I have
>> never experienced any problems. Looking at the signals with my
>> oscilloscope does not show any ringing, overshoot or reflection that
>> matters.

Typically in flat cable busses every other connector carries some
signal and every other conductor is connected to a signal ground. With
a signal ground on both sides of the signal conductor, there is a well
defined characteristic impedance on the line (unless there are
metallic surfaces close to the cable), so it is possible to build a
matched system with no over/undershots. 

However, any external magnetic or electric field can induce noise to
the connection, since the signal and ground wires are not twisted
around each other that would cancel some of the induced noise.

The construction also requires  that no other DC or mains frequency
currents are flowing in the signal ground conductors. In some cases
this has required strapping together the frames of the CPU and the
disk cabinet with 5-10 cm wide copper braids :-) in order to avoid
problem caused by potential differences in the mains neutral wiring.
 

>What speed SPI?  We used ADCs ADuC8xx series and had horrible results at 1 
>ft until we used twisted pairs.  It's been a few years but I think it was 
>set for 10Mbps but was still flaky at 250Kb, the lowest speed it had.  Only 
>choices were 1M, 10M and 250K.

While the input and output pins are labeled as "TTL compatible" on
most modern digital chips, have you actually looked at the input and
output pin characteristics for input and output impedance and how well
it matches the characteristic impedance of the flat cable (typically
slightly below 100 ohms) ?

Paul