RS485 CSMA/CD protocol

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Hi, I would need to implement an CSMA/CD L1 protocol code for RS485 in my
new AVR project for home automation.
Any link, hint or help for the similar source code?

Re: RS485 CSMA/CD protocol
My hint would be...Don't.

I've been curious about this sort of thing myself, and as far as I can
tell from web research no one out there has ever got it working
satisfactorily for real, as opposed to demonstration projects.

AFAIK it's just not feasible to reliably detect collisions on a
real-world bus with "standard" UART and RS485 hardware.

This is what CANBus is for.

Re: RS485 CSMA/CD protocol
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Hmmm. It is possible, but you have to enable both Tx and Rx, and monitor
that Tx = Rx. If not, back off and try again later.


Re: RS485 CSMA/CD protocol
Yeah, yeah, that's how you would do it *in theory*

In practice I'm not at all confident that "standard" RS485 drivers &
UART Rx. hardware will reliably detect and report the effects of
another RS 485 driver at the other end of a sub-optimal (long, heavily
loaded) real-world bus trying to drive to a conflicting signal level

Re: RS485 CSMA/CD protocol
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That kind of setup doesn't work reliably in my experience. What you
can do is to only drive one level actively and have the termination of
the bus define the other level. Then you connect outgoing data to the
output enable of the RS485 driver. But then we're not within the RS485
spec any more. I have seen this work for synchronous communication at
megabit speeds. At least some members of the QUICC/PowerQUICC family
from Freescale has a mode called HDLC Bus that can be used for
this. It is really designed for a direct connection between MCUs but
with a few tricks it can be used to work with RS485 drivers.



Re: RS485 CSMA/CD protocol
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I got to the same conclusion.

Btw. what are the differences between RS485 / CAN2.0 on physical layer

However, here is a thought:

1. sender waits until bus is idle and then sends SOF (say 0) and
bit-bangs message ID at low speed with software CD.

2. if no collision is detected, the sender continues sending the
content at desired speed using HW UART.

Even with simple mechanism of pulling bus active before sending the
message and checking for idle before doing so decreses the probability
of later collision substantially so the bit-banged priority is just a
fail-safe mechanism in rare case multiple senders decide to pul bus
active within same microsecond.

If system does not need 100% bandwidth and can withstand latency of
occasional 1-2 retries, you may succeed with the active start symbol
and random back-off timer.

Re: RS485 CSMA/CD protocol

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CANBus drivers only actively drive one signal level, relying on
termination resistors to passively assert the other level in absence of
any other nodes trying to actively drive the bus.  This allows CAN's
non-destructive collision arbitration mechanism.

Some early CAN systems used RS485 drivers, but with the TTL Tx. signal
from the higher level controller wired to the drivers chip enable line
rather than the Tx. input.

Re: RS485 CSMA/CD protocol

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This could work quite reliable.

However, you would need a XOR gate between the TTL level Rx and Tx
signals to detect coincidence, but you would still have some spikes at
each bit transition due to the limited rise and fall times and
possibly due to reflections. You would have to filter out these
spikes, but how do set the threshold, in order to quickly detect when
two transmitters starts to send the same character at say 1/4 bit time
after each other.

If the collision is done purely in software after the UART, much more
timing information is lost, especially if a FIFO is used on Rx and Tx.
If the protocol always starts with the same character, say STX, how do
you know, if that STX you got from the Rx FIFO is from your
transmission or not.

Starting the message with _your_ address (not the destination address
as in Modbus) would at least get some assurance that you got your own
ID back, get the ID of an other station or a garbled character
signifying collision, but still there would be false triggering due to
time shifts by 2, 4 or 6 bits. On way around this would be to limit
the number of stations to 8 and have station addresses with only one 0
bit, i.e. EF, DF, BF, 7F, FE, FD, FB, F7 (hex).

With low expected collision rates, it might even be acceptable to
detect collisions from failing CRCs at the end of frame.  


Re: RS485 CSMA/CD protocol
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Too bad the frames in CAN are so tiny.

Yes, I know the reason why, and in some applications it does
make sense.  In applications where higher latencies can be
tolerated, having an option to use 256 or 512 byte frames would
cut way down on the overhead.

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                                  at               lands on my HEAD and I
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Re: RS485 CSMA/CD protocol
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However, you can use CAN bus Transcievers, and UART uC, especially if
it is all your own network.

RS485 drivers have LowZ outputs, and so with real cable impedances, you
cannot reliably sense contentions.

CAN drivers are open-collector, so OR tie, and can detect collisions
within the transit times of cables.

So a vanilla uC can send, checking the RX to verify no differences, and
if it detects a difference, it has to go to collision mode.

UART systems will take longer to detect/respond to a collision than CAN,
but in  a simple network, with low traffic, the lower cost of devices
might make that worthwhile.

Since most uC UARTS have 9 bit modes, you can still use the CAN_Txcvr &
collide detect, but have a master that allocates timeslots, if you need
deterministic performance.


Re: RS485 CSMA/CD protocol

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How many devices and how much comms? I have seen people implement servers
which poll all devices and end up with just as good throughput

Re: RS485 CSMA/CD protocol
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This collision protocol only makes sense with multiple
masters, not in a master-slave setup. And if you have
multiple masters, then better implement a token protocol.
IMO, this collision protocol was one of the biggest
mistakes in history of IT.

Ing.Buero R.Tschaggelar -
& commercial newsgroups -

Ethernet, Aloha and CSMA/CD - (was: RS485 CSMA/CD protocol)
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May be, but it also may have been an unavoidable mistake - I can not
think of a better approach for Hawaii's original ALOHA radio network
on which Ethernet/CSMA/CD where partially based. (Especially when you
consider the technology available at the time.)
Each station in the Aloha network could not listen to the others, both
because of the terrain of the island and because the antennas were
directed to the "common medium", the satellite, so they had to
broadcast blindly and wait for the satellite to echo it back to all
the stations + acknowledge, as an indication that there was not
Of course the satellite or a dedicated ground station could have
worked as masters distributing tokens, allocating time slots, etc. but
that would have required them to adjust to changing network
configurations and use some of the available bandwidth. (9.6
Again, think of the hardware available on the early 70's...

Excerpts from the paper "Ethernet: Distributed Packet Switching for
Local Computer Networks"
Robert M. Metcalfe and David R. Boggs
Xerox Palo Alto Research Center
CACM July 1976 (almost 30 years!)

3. Design Principles
Our object is to design a communication system which can grow smoothly
to accommodate several buildings full of personal computers and the
facilities needed for their support.

Like the computing stations to be connected, the communication system
must be inexpensive. We choose to distribute control of the
communications facility among the communicating computers to eliminate
the reliability problems of an active central controller, to avoid
creating a bottleneck in a system rich in parallelism, and to reduce
the fixed costs which make small systems uneconomical.
Ethernet design started with the basic idea of packet collision and
retransmission developed in the Aloha Network [1]. We expected that,
like the Aloha Network, Ethernets would carry bursty traffic so that
conventional synchronous time-division multiplexing (STDM) would be
inefficient [1, 2, 21, 26]. We saw promise in the Aloha approach to
distributed control of radio channel multiplexing and hoped that it
could be applied effectively with media suited to local computer
communication. With several innovations of our own, the promise is

Ethernet is named for the historical luminiferous ether through which
electromagnetic radiations were once alleged to propagate. Like an
Aloha radio transmitter, an Ethernet transmitter broadcasts
completely-addressed transmitter- synchronous bit sequences called
packets onto the Ether and hopes that they are heard by the intended
receivers. The Ether is a logically passive medium for the propagation
of digit signals Is and can be constructed using any number of media
including coaxial cables, twisted pairs, and optical fibers.

3.1 Topology
We cannot afford the redundant connections and dynamic routing of
store-and-forward packet switching to assure reliable communication,
so we choose to achieve reliability through simplicity. We choose to
make the shared communication facility passive so that the failure of
an active element will tend to affect the communications of only a
single station. The layout and changing needs of office and laboratory
buildings leads us to pick a network topology with the potential for
convenient incremental extension and reconfiguration with minimal
service disruption.

A few highlights:

"to accommodate several buildings full of personal computers..."

    Several buildings, not a city, not the Internet, not the world.

"We expected that, like the Aloha Network, Ethernets would carry
bursty traffic so that conventional synchronous time-division
multiplexing (STDM) would be inefficient [1, 2, 21, 26]. "

    Not teleconferencing, video-streaming, etc.

"we choose to achieve reliability through simplicity"

    And that was indeed achieved.

(x-posted to alt.folklore.computers - I'm sure somebody will have
something interesting to comment over there, before drifting off-topic
for the next 3000 follow-ups... ;-)  )

Roberto Waltman.

[ Please reply to the group,
  return address is invalid ]

Re: Ethernet, Aloha and CSMA/CD - (was: RS485 CSMA/CD protocol)
On Fri, 23 Sep 2005 09:00:04 -0400

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    Indeed I once saw an ethernet installation where the medium was
the open air via a number of stubby ariels. It was a warehouse robot
control system with a number of robots and a control computer communicating
by ethernet with no wires.

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Re: Ethernet, Aloha and CSMA/CD - (was: RS485 CSMA/CD protocol)
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The token ring advocates always pointed out how certain circumstances
could lead to collision detection not working nicely.  They were also
extremely unconfortable with the lack of guaranteed bandwidth for each
master etc.

But the token ring vs collision detection wars for general-purpose
networking were fought and token ring lost.  In real life the concerns
that the token ring advocates had about collisions just don't happen,
even on highly saturated ethernets.

Now, you can make up some really stupid collision detection/back-off
algorithms that just don't work.  CSMA/CD can be stupidly implemented
such that it doesn't work well.  Usually these implementations were
done by committees who thought too hard about a simple problem and
worked hard to come up with an insane list of requirements.  I think
the original poster was looking to avoid such mistakes by asking for
example code that does things the right (not wrong) way.

Curiously I've seen CSMA/CD done the "wrong way" most often when the
committee designing it has a lot of token ring advocates on it.  They
add all sorts of arbitrary and unnecessary requirements about
guaranteed bandwidth etc. and make the result useless.

Token ring still lives on in many special-purpose protocols, not
necessarily because collision detection won't work but just because it
wasn't used.


Re: Ethernet, Aloha and CSMA/CD -

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The point is the behaviour under load. A token protocol doesn't
require a token ring. A bus is sufficient, ARCnet works on
twisted pair and over coax.

Tokenring failed for commercial reasons. It was single source
and the price was beyond reasonable.

When you have a network and it should work under heavy load,
eg all nodes want to transfer huge binaries at the same time,
then a token protocol distributes the bandwidth of the medium
over the nodes while the colision detect system just detects
endless collisions and does endless retries.

It is less that the tokenring advocates were uncomfortable.
A realtime system requires a defined response time that suits
the physical installation this system should control.
A car control system requires responsetimes in the millisecond
region and you wouldn't want your cars cpu to retry some bullshit
while you want the car to stop. Realtime response has nothing
to do with being comfortable, it has to do with lives.

Ing.Buero R.Tschaggelar -
& commercial newsgroups -

Re: Ethernet, Aloha and CSMA/CD -

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Exactly, so for this type of application, it may be a wise engineering
choice NOT to use Enet for communication.  THis does not make it bad!  I
don't think a blanket statement about it being the worst decision ever
is warranted.  If you used Ethernet in a hard RT system and did not
design it properly, thats your fault, not the protocols.....

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Re: Ethernet, Aloha and CSMA/CD -
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Ethernet is used in hard RT vital safety-critical systems all the time.
 It's a matter of system design to make sure that collisions or a
network failure doesn't result in unsafe conditions.  Of course, if a
Token advocate designed a CSMA/CD system you would end up with a
disaster, because they would take the simplicity of working collision
detection/backoff algorithms that work and layer on all sorts of crap
to guarantee timeslots etc and end up with a system that doesn't work.

In fact I would be very critical of a safety-critical system where a
network failure (here I'm lumping "excessive collisions" in with
"network failure" which isn't too far off for Ethernet but of course
the Token advocates will jump all over me for this) results in unsafe

Of course, I work in an industry where we take pride in our vital
relays and vital processors having to meet more stringent failsafe
standards than the detonation systems in thermonuclear weapons :-).
But unlike say aerospace or road vehicle systems, my industry has the
advantage that setting all signals to stop, dropping speed commands,
and applying full-service brakes is the failsafe condition.  It's hard
to claim that shutting down an airplane's engines or applying full
braking on a road vehicle is a failsafe condition...!


Re: Ethernet, Aloha and CSMA/CD -

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When well defined response times are needed, I would not mess with any
tokens but use a traditional fixed single master multiple slave
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For applications, in which the need is for some high priority
emergency messages and ordinary messages, I would use CAN and not mess
with tokens that would require lost token handling etc.


Re: Ethernet, Aloha and CSMA/CD -
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Oops, sorry officer.  I was tuning my radio and I guess I caused the
brakes to stop working for a while...

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