maximum ranges of serial, spi, and I2C?

Hi there - can anybody tell me the maximum ranges of serial, SPI, and
I2C? Specifically, I'm referring to I2C running at 400Kbps, serial at
1Mbps, and SPI at 1Mbps. I guess with serial the physical layer used
changes things around alot - so I'm referring to RS-232. Is there a
better name than serial? I mean, serial seems pretty vague, though I
suspect most know what I am referring to.

Also, how do serial, SPI, and I2C compare in tolerance of noise? I
would assume that RS-232 serial can handle it the best, while SPI and
I2C both probably don't tolerate it very well at all.

On a related note - anybody know how RS-232 and CAN compare in
handling noise? I suspect that CAN, even though the standard physical
layer is lower voltage than RS-232, handles it better due to being a
differential bus.

Thanks!

-Michael

Michael wrote:
> Hi there - can anybody tell me the maximum ranges of serial, SPI, and
> I2C? Specifically, I'm referring to I2C running at 400Kbps, serial at
> 1Mbps, and SPI at 1Mbps. I guess with serial the physical layer used
> changes things around alot - so I'm referring to RS-232. Is there a
> better name than serial? I mean, serial seems pretty vague, though I
> suspect most know what I am referring to.
> 
> Also, how do serial, SPI, and I2C compare in tolerance of noise? I
> would assume that RS-232 serial can handle it the best, while SPI and
> I2C both probably don't tolerate it very well at all.
> 
> On a related note - anybody know how RS-232 and CAN compare in
> handling noise? I suspect that CAN, even though the standard physical
> layer is lower voltage than RS-232, handles it better due to being a
> differential bus.

If you look at i2c, you will often find a capacitance spec and the
FM+ has higher current drivers, which allows lower pullups, and thus
tolerates more cable length.

SPI is a nominal 5V TTL/CMOS bus, so has no cable drive specs.

RS232/RS485/ProfiBUS etc are the stds for longer cable runs, and you can 
find the specs on the cable drivers.
CAN also adds a handshake delay, so has lower MAX bauds than RS485/422.

All systems have a speed/distance trade off : The longer the cable, the 
lower the speed.

If you do not need 'clip on' compatible operation, you can extend most
BUS lengths with better cable drivers and handling.

-jg

On Wed, 13 Jun 2007 01:11:29 -0000, Michael <nleahcim@gmail.com>
wrote:

>Hi there - can anybody tell me the maximum ranges of serial, SPI, and
>I2C? Specifically, I'm referring to I2C running at 400Kbps, serial at
>1Mbps, and SPI at 1Mbps. I guess with serial the physical layer used
>changes things around alot - so I'm referring to RS-232. 

The RS-232 specification only allows for 15 m and I very much doubt
that if all the requirements even in the newer versions of the RS-232
is observed that you even could run 1 Mbit/s through it.

A much better serial interface form is the RS-422/485 standard. It is
differential and the impedance levels match with the transmission line
(twisted pair) impedance. You should be able to run more than 100 m at
1 Mbit. Profibus-DP (which is essentially RS-485) can reach 100 m at
12 Mbit/s even in a multidrop configuration with some attention to
detail.
 
>Also, how do serial, SPI, and I2C compare in tolerance of noise? I
>would assume that RS-232 serial can handle it the best, while SPI and
>I2C both probably don't tolerate it very well at all.

Those are unbalanced systems, so they suffer from ground potential
differences (including ground bounce) even with quite short distances.
Balanced, differential mode systems, such as RS-422/485, CAN etc. will
tolerate ground potential problems within the common mode voltage
range (typically -5 .. +12 V and if large common voltages may exist,
use optoisolation) and cancels most common mode electrostatic and
magnetic interference.   

Paul

On Wednesday, in article
     <0u0v63h5cgfvmkmjt7vn63qop7qe5t0ljf@4ax.com> keinanen@sci.fi
     "Paul Keinanen" wrote:
>On Wed, 13 Jun 2007 01:11:29 -0000, Michael <nleahcim@gmail.com>
>wrote:
>
>>Hi there - can anybody tell me the maximum ranges of serial, SPI, and
>>I2C? Specifically, I'm referring to I2C running at 400Kbps, serial at
>>1Mbps, and SPI at 1Mbps. I guess with serial the physical layer used
>>changes things around alot - so I'm referring to RS-232. 
>
>The RS-232 specification only allows for 15 m and I very much doubt
>that if all the requirements even in the newer versions of the RS-232
>is observed that you even could run 1 Mbit/s through it.

That old myth. The RS232 spec (old versions) actually quoted that at
15m on a poor cable, a certain frequency, and lots of pairs used, for
modem signalling, that the noise level got to a certain level. There was
NO length spec, as it is very cable type dependent. There was no mention
of this being outside of the noise spec.

I have seen tests done at 38k baud that were done with front door bell
(admission bell) wire still on drums, where over distances greater than
1km there was minimal degradation. This was using RS232B class drivers.

In 70's and 80's I have worked in companies where there was RS232 cable
runs to lots of VTxxx terminals, where the *minimum* cable run was 20m to
get out of computer rooms and into ducting runs.

Choose your cable carefully. Yes, still observe what your distance/speed
tradeoffs are.

-- 
Paul Carpenter          | paul@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/>    PC Services
<http://www.gnuh8.org.uk/>              GNU H8 & mailing list info
<http://www.badweb.org.uk/>             For those web sites you hate

Paul Keinanen wrote:
> Michael <nleahcim@gmail.com> wrote:
> 
>> Hi there - can anybody tell me the maximum ranges of serial, SPI,
>> and I2C? Specifically, I'm referring to I2C running at 400Kbps,
>> serial at 1Mbps, and SPI at 1Mbps. I guess with serial the
>> physical layer used changes things around alot - so I'm referring
>> to RS-232.
> 
> The RS-232 specification only allows for 15 m and I very much
> doubt that if all the requirements even in the newer versions of
> the RS-232 is observed that you even could run 1 Mbit/s through it.
> 
> A much better serial interface form is the RS-422/485 standard. It
> is differential and the impedance levels match with the
> transmission line (twisted pair) impedance. You should be able to
> run more than 100 m at 1 Mbit. Profibus-DP (which is essentially
> RS-485) can reach 100 m at 12 Mbit/s even in a multidrop
> configuration with some attention to detail.
> 
>> Also, how do serial, SPI, and I2C compare in tolerance of noise?
>> I would assume that RS-232 serial can handle it the best, while
>> SPI and I2C both probably don't tolerate it very well at all.

Bear in mind that all of RS422, RS232, RS485 are hardware specs,
and that no error control or correction is embedded in any of
them.  That is something you add when designing your own protocol.

-- 
 <http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt>
 <http://www.securityfocus.com/columnists/423>
 <http://www.aaxnet.com/editor/edit043.html>
 <http://kadaitcha.cx/vista/dogsbreakfast/index.html>
                        cbfalconer at maineline dot net



-- 
Posted via a free Usenet account from http://www.teranews.com

Michael wrote:

> Hi there - can anybody tell me the maximum ranges of serial, SPI, and
> I2C?

SPI and I2C are not intended for the distant communication. They should 
be used for the connections within a module. Running I2C or SPI on the 
outside of a device is a bad idea.

> Specifically, I'm referring to I2C running at 400Kbps, serial at
> 1Mbps, and SPI at 1Mbps. I guess with serial the physical layer used
> changes things around alot - so I'm referring to RS-232.

http://www.interfacebus.com/Design_Connector_RS232.html

> Also, how do serial, SPI, and I2C compare in tolerance of noise?

I2C sucks because of the open drain.
SPI is somewhat better.
RS-232 is much better.

> On a related note - anybody know how RS-232 and CAN compare in
> handling noise?

They are comparable. It depends on the PHY and the system layout.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

On Jun 13, 3:27 am, Paul Keinanen <keina...@sci.fi> wrote:
>
> A much better serial interface form is the RS-422/485 standard. It is
> differential and the impedance levels match with the transmission line
> (twisted pair) impedance. You should be able to run more than 100 m at
> 1 Mbit. Profibus-DP (which is essentially RS-485) can reach 100 m at
> 12 Mbit/s even in a multidrop configuration with some attention to
> detail.

RS-485 is another that we have been considering. I have read that
multi-master systems are possible with 485. What sort of protection is
there from collisions with 485? As far as I can find, there are none -
so everything would have to be done in software. Also - how exactly
would one achieve RS-485 communication with a typical microcontroller?
Most uCs have tx and rx lines for serial communication. From my
understanding these same lines could be used for RS-485. Is that
accurate? If so - if you have a half duplex RS-485 link, how does the
transceiver know whether to drive the line or listen? Is there a
direction control on 485 transceivers or something?

Thanks,

-Michael

On 2007-06-13, Michael <nleahcim@gmail.com> wrote:

> RS-485 is another that we have been considering. I have read
> that multi-master systems are possible with 485.

Indeed they are.

> What sort of protection is there from collisions with 485?

You design a protocol such that only one device transmits at a
time.

> As far as I can find, there are none - so everything would
> have to be done in software.

Yup.

> Also - how exactly would one achieve RS-485 communication with
> a typical microcontroller? Most uCs have tx and rx lines for
> serial communication. From my understanding these same lines
> could be used for RS-485. Is that accurate?

Yes.

> If so - if you have a half duplex RS-485 link, how does the
> transceiver know whether to drive the line or listen?

You tell it.

> Is there a direction control on 485 transceivers or something?

Yes.

Have you heard of google?

  http://www.google.com/search?q=rs-485

In particular these links are good ones:

  http://en.wikipedia.org/wiki/RS-485
  http://www.lammertbies.nl/comm/info/RS-485.html

-- 
Grant Edwards                   grante             Yow!  I want to kill
                                  at               everyone here with a cute
                               visi.com            colorful Hydrogen Bomb!!

On Wed, 13 Jun 2007 23:34:38 -0000, Michael <nleahcim@gmail.com>
wrote:

>
>RS-485 is another that we have been considering. I have read that
>multi-master systems are possible with 485. 

Do you mean multi-master or multi-slave ?

For multi-slave just use any master-slave protocol with addressable
slaves. The master must allow sufficient time for each slave to
response, including the response transfer time and any request
processing latency in the slave. 

Thus, in order to keep a decent throughput, the slave should respond
quickly to the request e.g. doing the processing in a high priority
task or even in the interrupt context.

For a true multi-master system, just pass a token message to an other
master and remain silent, until some other master passes the token
message back to you. Some protection against lost tokens must be used
in the protocol.

>What sort of protection is
>there from collisions with 485? As far as I can find, there are none -
>so everything would have to be done in software. 

It should not be needed with a proper protocol. Anyway, the
transceivers are short circuit protected, so conflicts do not harm the
hardware (although the chips may run quite warm, if the cable between
the offending transceivers is short). 
 
>Also - how exactly
>would one achieve RS-485 communication with a typical microcontroller?
>Most uCs have tx and rx lines for serial communication. From my
>understanding these same lines could be used for RS-485. Is that
>accurate? If so - if you have a half duplex RS-485 link, how does the
>transceiver know whether to drive the line or listen? Is there a
>direction control on 485 transceivers or something?

Any decent UART should have a RTS (RequestToSend) that can be
programmed to be active, while bits are still being transmitted. This
RTS line is used to control the data direction on the RS-485
transceiver chip.

If such automatic control is not available, you have to control the
RTS line by software (as with the common 14550 series UARTs) or use a
separate parallel uC port for it. 

However, there are a few pitfalls with a few such UARTs that need
software RTS control. It is important, that the control line is turned
off immediately after the last (stop) bit of the last character has
actually been sent, since the other end may start to send a response
message. However, if you turn it off too early, the most significant
bits of the last character is cut off and the receiver will receive a
character with the high bits set. 

If you get "All_bits_sent" interrupt, then it is easy to turn of the
transmitter from the interrupt service routine, however, for the 14550
series UARTs you only get an interrupt when the last character was
transferred to the Tx shift register, but it is just starting to be
shifted out from the shift register. You have to poll the shift
register status register very frequently (a bad idea in multitasking
systems) to see, when the character has been fully sent.

One other way of doing this on stupid UARTs, such as the 14550 series,
is to keep the receiver active and listen to the echo of your own
transmission and when the last character echoed generates an
RX-interrupt, turn off the transmitter from the interrupt service
routine. Of course, the RX-FIFO (if exist) should be disabled to get a
quick response to the RTS line.

Paul