RS485 bus and auto-addressing

I have a small RS485 two-wires network with about ten nodes. I have a 
master that initiates the communication on the bus. All the other nodes 
are always in receiving state and start transmitting only if the message 
from the master is addressed to them.

A simple method to assign an address to a slave is to have a set of 
jumpers/dip-switch on the board, but during the installation/setup 
phase, the technician must be careful to avoid duplicate addresses.

I'm thinking about an alternative method to implement in the master 
that, during the start-up phase, assigns automatically the addresses to 
the slaves.
However, at the moment, I didn't find a good solution.

Any suggestion or experience on this topic?

On Sat, 27 Apr 2013 23:33:36 +0200, pozz <pozzugno@gmail.com> wrote:

>I have a small RS485 two-wires network with about ten nodes. I have a 
>master that initiates the communication on the bus. All the other nodes 
>are always in receiving state and start transmitting only if the message 
>from the master is addressed to them.
>
>A simple method to assign an address to a slave is to have a set of 
>jumpers/dip-switch on the board, but during the installation/setup 
>phase, the technician must be careful to avoid duplicate addresses.
>
>I'm thinking about an alternative method to implement in the master 
>that, during the start-up phase, assigns automatically the addresses to 
>the slaves.
>However, at the moment, I didn't find a good solution.
>
>Any suggestion or experience on this topic?

I don't believe there are any techniques that are guaranteed to
produce unique addresses for each slave if the slaves are otherwise
identical.

An approach that will work most of the time is:

master to slaves: Send "First one to answer is assigned address #1."

slaves: All wait a random interval while listening for another to
claim #1. One (or more!) with the shortest wait time claim #1. 

Since there's a non-zero chance that two (or more) slaves replied at
exactly the same time:

master to slaves: Send "Okay, confirm address #1."

slaves: The one (or more) that think it/they has/have #1 wait for
another random interval. The first to speak, gets #1 and the possible
others relinquish it.

Repeat for the remaining addresses. However, I don't believe there is
a way to make the probability exactly zero that more than one slave
does not behave in an identical fashion throughout the process (even
given an arbitrary number of confirmation cycles).

[This followup was posted to comp.arch.embedded and a copy was sent to 
the cited author.]

In article <klhg1d$4u7$4@dont-email.me>, pozzugno@gmail.com says...
> 
> I have a small RS485 two-wires network with about ten nodes. I have a 
> master that initiates the communication on the bus. All the other nodes 
> are always in receiving state and start transmitting only if the message 
> from the master is addressed to them.
> 
> A simple method to assign an address to a slave is to have a set of 
> jumpers/dip-switch on the board, but during the installation/setup 
> phase, the technician must be careful to avoid duplicate addresses.
> 
> I'm thinking about an alternative method to implement in the master 
> that, during the start-up phase, assigns automatically the addresses to 
> the slaves.
> However, at the moment, I didn't find a good solution.
> 
> Any suggestion or experience on this topic?

The way I've implemented auto addressing in the past is to arrange the 
serial bus connection to pass through the slave units on a gated pass 
through. Initially all slaves wake up with the pass through gated open. 
This causes just one slave to "see" the master. He gets the first 
address and then after a confirmation step with the master closes his 
pass through. The process repeats till the master realizes that no 
further devices respond on the serial chain. This works like a champ and 
is reliable.

-- 

Michael Karas
Carousel Design Solutions
http://www.carousel-design.com

On Saturday 27 April 2013 23:33 pozz wrote:

> I have a small RS485 two-wires network with about ten nodes. I have a
> master that initiates the communication on the bus. All the other nodes
> are always in receiving state and start transmitting only if the message
> from the master is addressed to them.
> 
> A simple method to assign an address to a slave is to have a set of
> jumpers/dip-switch on the board, but during the installation/setup
> phase, the technician must be careful to avoid duplicate addresses.
> 
> I'm thinking about an alternative method to implement in the master
> that, during the start-up phase, assigns automatically the addresses to
> the slaves.
> However, at the moment, I didn't find a good solution.
> 
> Any suggestion or experience on this topic?

There may be some methods that will work, like some random scheme to assign 
addresses.
But you still face the problem how your master will have to know who is who. 

If your nodes are otherwise identical, i.e. several temperature sensors, you 
need to know which reported temperature belongs to which physical point.

If your nodes are not identical then you could of course assign addresses 
using the character of the node.

-- 
Reinhardt Behm, Bodenheim, Germany
rb@rbehm.de

On Sat, 27 Apr 2013 23:33:36 +0200, pozz <pozzugno@gmail.com> wrote:

>I have a small RS485 two-wires network with about ten nodes. I have a 
>master that initiates the communication on the bus. All the other nodes 
>are always in receiving state and start transmitting only if the message 
>from the master is addressed to them.
>
>A simple method to assign an address to a slave is to have a set of 
>jumpers/dip-switch on the board, but during the installation/setup 
>phase, the technician must be careful to avoid duplicate addresses.

In addiction DIP switches are unreliable in harsh conditions.

>I'm thinking about an alternative method to implement in the master 
>that, during the start-up phase, assigns automatically the addresses to 
>the slaves.
>However, at the moment, I didn't find a good solution.
>
>Any suggestion or experience on this topic?

If the master has persistent memory and each slave is initially known
slave address (say 255), build the network by adding one node at a
time (e.g. by powering up the devices one by one). The master checks
for slave 255 and assigns it an other slave address. This is repeated
for all slaves. 

If a faulty node needs to be replaced, the new replacement unit is
again at address 255 and the master then commands it to the same slave
ID as the original device.

If each slave has a unique vendor:serial_number, either readable
through the ordinary bus or some configuration port  there are various
methods.

The address data base could be done by adding one device at a time, as
in the previous case or using some off-line configuration (running on
a laptop) to run a point-to-point connection with a new slave, either
to the network port or through some configuration port and command the
slave address.

Take a look at CanOpen Layer Setting Services (LSS).

-----

If the slave has a unique vendor:sertial_number, generate a hash code
and use it for slave address. The master runs through all possible
slaves by asking e.g.  "Slave 37, report your serial". If there is a
timeout, none is there, if there is a garbled reply, there is a
duplicate, there is a collision and if there is a clean response,
assign the address. Repeat the scans but ask "slave 37, report serial
except serial37" and if you got a response, force the slave address to
a free slave address. 

If there still are garbled responses, refine your search "Slave 37
report your serial ending in 0" up to serial ending in 9. Of course,
this requires that you are able to differentiated between no response,
garbled response and valid response.

On 04/27/2013 11:33 PM, pozz wrote:
> I have a small RS485 two-wires network with about ten nodes. I have a
> master that initiates the communication on the bus. All the other nodes
> are always in receiving state and start transmitting only if the message
> from the master is addressed to them.
>
> A simple method to assign an address to a slave is to have a set of
> jumpers/dip-switch on the board, but during the installation/setup
> phase, the technician must be careful to avoid duplicate addresses.
>
> I'm thinking about an alternative method to implement in the master
> that, during the start-up phase, assigns automatically the addresses to
> the slaves.
> However, at the moment, I didn't find a good solution.
>
> Any suggestion or experience on this topic?

Assign each unit a unique serial number. This can be done during 
manufacturing, or using an external ID chip. If that's too expensive, 
you can use a random number. For instance, you can use the LSB of an ADC 
input to generate a random number. If you make the serial number big 
enough, the chance of duplicates is very unlikely (32 bit for instance). 
Once you have a known good serial number, program it into the 
EEPROM/Flash if you have it.

The master then broadcasts a probe:

"any device that's not initialized, please respond with your serial 
number".

To avoid collision, clients use a random backoff time before sending, 
and only send when the bus is idle.

If the master receives a valid response from serial number X, it sends a 
packet: "device with serial number X, you are now initialized with bus 
ID Y". This device then stops responding to the probe broadcasts.

The master repeats this process until it gets no response. After that, 
it can send an occasional probe during normal operation to detect new 
attachments to the bus.

Of course, like somebody else remarked, you still may need to figure out 
which device is which, depending on your application.

pozz wrote:
> I have a small RS485 two-wires network with about ten nodes. I have a
> master that initiates the communication on the bus. All the other nodes
> are always in receiving state and start transmitting only if the message
> from the master is addressed to them.
>
> A simple method to assign an address to a slave is to have a set of
> jumpers/dip-switch on the board, but during the installation/setup
> phase, the technician must be careful to avoid duplicate addresses.
>
> I'm thinking about an alternative method to implement in the master
> that, during the start-up phase, assigns automatically the addresses to
> the slaves.
> However, at the moment, I didn't find a good solution.
>
> Any suggestion or experience on this topic?

I've never seen a case where the address of a 485 node wasn't part
of the node's configuration identity - we always had to know what
the role of the thing at address <x> was. It therefore had to be
static.

It's also usually preferable to have the address stored on the device
in some way other than  dip switches.

-- 
Les Cargill

On 27.04.2013 23:33, pozz wrote:
> I'm thinking about an alternative method to implement in the master
> that, during the start-up phase, assigns automatically the addresses to
> the slaves.

For truly identical slaves, that would be somewhat obviously either 
pointless, or impossible.  There must be _something_ that differentiates 
the slaves from each other --- otherwise, why would you have more than 
one in the first place?

How best to address individual slaves depends on what that 
differentiating "something" is.  It could be the type of node (--> I2C 
addresssing by device type), a physical ID assigned to the node 
beforehand (--> dip-switch, configurable EEPROM), a universally unique 
ID like in Maxim's 1-wire bus devices, or even just its position along 
the cable connecting all nodes (--> "auto-addressing", "cable select").

So it's really your choice to make, depending on the specifics of your 
situation.

On 4/27/2013 5:35 PM, Rich Webb wrote:
> On Sat, 27 Apr 2013 23:33:36 +0200, pozz <pozzugno@gmail.com> wrote:
>
>> I have a small RS485 two-wires network with about ten nodes. I have a
>> master that initiates the communication on the bus. All the other nodes
>> are always in receiving state and start transmitting only if the message
>>from the master is addressed to them.
>>
>> A simple method to assign an address to a slave is to have a set of
>> jumpers/dip-switch on the board, but during the installation/setup
>> phase, the technician must be careful to avoid duplicate addresses.
>>
>> I'm thinking about an alternative method to implement in the master
>> that, during the start-up phase, assigns automatically the addresses to
>> the slaves.
>> However, at the moment, I didn't find a good solution.
>>
>> Any suggestion or experience on this topic?
>
> I don't believe there are any techniques that are guaranteed to
> produce unique addresses for each slave if the slaves are otherwise
> identical.
>
> An approach that will work most of the time is:
>
> master to slaves: Send "First one to answer is assigned address #1."

[...]

Assigning addresses in Plug-and-Play manner is only a half of the 
problem. You need to make sure that devices would always be enumerated 
in the same way. Also, the master should somehow identify the roles of 
the devices; i.e. which device does what.

One obvious solution is assigning globally unique ID to every device. 
Like EEPROM with pre-programmed MAC address or serial number.

The other solution is token ring network topology. The devices are 
automatically numbered in their order in the ring. Done that with 
RS-232; very simple and elegant.


Vladimir Vassilevsky
DSP and Mixed Signal Designs
www.abvolt.com

On 4/27/2013 10:14 PM, Michael Karas wrote:

> The way I've implemented auto addressing in the past is to arrange the
> serial bus connection to pass through the slave units on a gated pass
> through. Initially all slaves wake up with the pass through gated open.

This could be done even simpler: use connectors with built-in switch. If 
nothing is plugged into the output connector, this device is last in line.

> This causes just one slave to "see" the master. He gets the first
> address and then after a confirmation step with the master closes his
> pass through. The process repeats till the master realizes that no
> further devices respond on the serial chain. This works like a champ and
> is reliable.

Yes, this is simple and works great. But, if a device somewhere in the 
middle of the chain is broken, it could cut off all netvork devices behind.

Vladimir Vassilevsky
DSP and Mixed Signal Designs
www.abvolt.com