Strange 1-Wire Network Problem

[stevendt]

Hi,

I have a problem with my 1-wire network which someone may be able to shed some light on please ?

I have a number of 1-wire devices, mainly using Hobby boards kit as shown :-


PC Port --> HB 1-Wire Temp Board ---> HB 6 Channel Hub


Hub Ch 1 Main --> HB 1W Temp
Hub Ch 1 Aux --> No connection
Hub Ch 2 Main --> HB 1W Temp
Hub Ch 2 Aux --> No connection
Hub Ch 3 Main --> HB 1W Temp & Hum, HB 1W Temp, DS18B20, DS18B20, DS18B20, DS18B20
Hub Ch 3 Aux --> No connection

All of the temperature detectors are in parasitic mode.

The DS18B20's on CH3 are all taped to my heating pipes and "dropped" of the 1-wire network. They seemed to work reliably until the weekend. We had a power outage and when the power came back, all of the devices on Ch3 Main were not visible on the network. I thought that the power outage had taken out one channel of the hub as the devices became visible again when I connected them to another hub channel.

However, a few hours later, the devices then disappeared. Changing their channel or resetting (power off/on) the hub had no effect so I began to suspect a failed device. I identified the rogue device by adding devices back to the network leg in turn until the devices disappeared again - success ! Having found the bad device, I expected to be able to confirm that by using the one-wire viewer. However, the one-wire viewer successfully reads the device. When put back on the network, the rest of the devices disappear again.

So, at last, a question....

Could there be a failure mode wherby the device could happily communicate on its own but prevent devices sharing the network (including itself) from working ? As I seem to have got round the problem, this is not really an urgent issue - I'd just like to know what might be happening,

regards
Dave

 

[ericvic]

Dave,

I have not see you specific failure but the 1-Wire bus is pretty finicky so if the device in question is right on the edge of the specs then when you add the load of additional devices it might make the 1-Wire waveforms off just enough to take the whole bus down.

Eric

 

[sda]

This may or may not be part of your problem, but its something to consider.

http://www.maxim-ic.com/appnotes.cfm/an_pk/148

To quote:
"When a 1-Wire network fails, the failure often manifests itself as a mysterious "loss" of a device when the searching algorithm is performed. Devices that are physically present may appear and disappear in the search results."

Since you're losing an entire segment, it could be that the device being "lost" is the DS2409 in the hub.

And
"Testing has shown that unswitched star-type network topologies (those with several branches diverging at the master) are the most difficult to make reliable."

If you have more than one hub port enabled at a time, then you've put the 1-wire network into a star configuration.

 

[huggy59]

No idea if this might also be an issue, but what is the total power consumption vs. source on that leg? Also, any chance something is shorting out on the pipes?