Demux problem

[ptheboris]

Hi folks

 

A couple of times over the last few days my demux unit has failed for what seems to be no obvious reason. I had initially put it down to overheating on Saturday as it has been in use for a long while in a room with very little ventilation. However it went again last night.

 

Each time the display comes up with either 257 or 406 on it. Whilst it can be reset to 001, it does not like it when the lead is connected and only recognises 000 or 002 to start with not the usual 001.

 

Does anybody have any ideas about what may be causing the problem? I will try where possible to provide any further information if I can

 

It is a zero 88 Demux unit.

 

regards

 

Paul

[Eric]

Best guess is the internal battery is starting to fail and cause the RAM to corrupted it's internal setting ie the start address etc.

[ptheboris]

Thanks for that suggestion Keith. It doesn't owe me anything so sounds like its a repair or replace job depending on the cost.

 

Paul

[Eric]

Proberly pick up the battery for under £10 form RS or Maplin.

You would be looking to get a 3.6v NiMH with at least a 100 mAH rating on it.

RS part number is 422-393

[Ekij]

If it's losing the information while powered up I would be very surprised if it's the battery.

I would only expect this to be likely if it's forgetting it's settings when powered off (waking up with the wrong values).

[ptheboris]

It is also occassionally waking up with wrong values as I would expect it to reset to 001 once turned off but doesn't

 

PAul

[SGP]

I can say this is a battery problem, mine did this about a year ago and I replaced the battery and it worked fine after that and is still going strong.

[Brian]

If it's losing the information while powered up I would be very surprised if it's the battery.

IIRC the way the battery is wired is that the RAM is fed directly from the battery which is then trickle charged via a diode and a resistor (220R?) from the 5v rail. When the battery fails it holds the RAM power supply line low enough for it not to work properly. The resistor limits the ability of the RAM to be powered by the main 5v rail.

[Ekij]

The way the battery is wired is that the RAM is fed directly from the battery which is then trickle charged via a diode and a resistor (220R?) from the 5v rail. When the battery fails it holds the RAM power supply line low enough for it not to work properly.

What a poor design! We design many things with battery back up and as we know that eventually the battery will fail so we always design so that the thing still works when powered up (it just can't remember its settings when powered down).

Though EEPROM is making this less of an issue these days.

 

Edit: Further thought:

 

The design you mention requires the item to be powered down periodically in order to charge the battery and ensure normal operation. In many applications devices never get powered down in which case this item will fail in a handful of days from new! (Some R&D teams need slapped).