Thread: anyone with 2-wire serial communication experience

Does anyone here have any experience with 2-wire serial chips?
I have a boiling water unit that has a chip embedded in the sealing washer of the filters, and I have to buy "genuine" water filters from the manufacturer of the unit so I have a new chip each time. The filters that they supply are re-boxed filters with the original manufacturer printed on them, and can be bought online for less than $15 for the pair from many distributers but have no chip, but the ones from the boiling water company cost $158 with the chip!!!
I want to read the chip and see if I could make up an external circuit to simulate a chip, and enable me to replace filters with generic ones.

I wonder if the ACCC would be interested? (price gouging etc.)

Originally Posted by fraser130

Mike, I have a chip here, there are only 2 connections on the actual filter, I made a mistake, it's a one-wire EEPROM:

Yep, that makes a lot more sense. It's using the EEPROM as memory space.

Basically, from what I understand, the boiler reads the chip, which has a number of days programmed (you could buy 6 or 12 month filters) and I guess a serial number which I believe the boiler remembers as there are 10 memory spots in the boiler which are related to the filters, but I also think the boiler may write back to the filter as I think there is a system in place to stop you swapping filters between 2 boilers!

Each EEPROM has a unique 48 bit serial number (so you can have more than one EEPROM on the same bus) so yes, it's quite easy for the boiler to recognise when you start swopping things around.

Because it's an EEPROM, which can store pretty much anything, what's on it at the moment is pretty much guesswork, but I'd make an educated guess at the memory space being used to store some form of decremental counter and maybe a couple of other basic parameters, when the counter reaches zero, time to chuck out the filter. Realistically the only way that you are going to achieve your goal of using $15 filters is to hook a few of these chips up to a serial interface and read the data from the memory space to see what has changed. Me, I'd be reading a new filter, a filter after a days use, two days use, and an expired filter. I'd check not only for timing data, but also the possibility of a check bit that gets set to zero (or one) when the boiler has decided that the filter has expired. And after that start looking at making up an adapter containing a similar EEPROM that will take the cheap filters, and writing "new filter" data to the EEPROM. How much effort it takes really depends on the level of communication between boiler and filter - if it's a dumb "is filter present? Yes/No? Is filter expired? Yes/No" then you can connect up a read-only EEPROM and the boiler sees a permanently new filter every morning. However, if the boiler (for example) decrements the counter, and then checks to see if the counter on the chip has decremented, it gets more complex. So on and so forth.

This is worth a read to see the 1 wire protocols and interface - the example given uses a very similar Dallas Semiconductors EEPROM in its examples.
How-to: The Bus Pirate, universal serial interface