AVR Freaks

Jim's right: over many years, we have found very very few cases where RFID tags implanted within animals  fail to work. We *do* find that with time, a chip inserted at the shoulders can migrate within the body to a position where, while it still works, often appears to have very poor or intermittent range from the view of the door/feeder/whatever - it's simply further away. In such a case, a vet will often simply insert a second chip - but that has caused is issues with our latest software/hardware which has improved range over earlier versions and will cheerfully read both chips... so I've had to include code to handle that specific case.

That's not likely to be the case if you have a collar tag since it's external to the animal and constrained in position. Additionally, collar tags as a rule tend to have a much larger activating coil than internal chips and so have increased range - I recently tested a collar tag with over a half-metre detection range. However, I suppose it's possible that a collar tag could suffer damage to the coil?

Other thoughts: units returned to me for fault analysis (for short read range) tend to have circuit failure to some degree resulting in mistuning of the transmitter coil which can have a significant effect on range. Might be worth checking your transmit frequency; the tags can have rather a variable Q and might respond one better than another frequency.

But I don't think I've ever personally seen a chip that, once tested, has failed in service.

If you ping me a land mail address I'll be happy to send you a range indicator widget. It lights up when it sees a scan and will give you an idea of your transmit range. Batteries not required... Or I can send you the Gerbers and you can roll your own.

Neil

If they are on teh same 125Khz carrier you cannot.

Sure does!!

You cannot.  Just like I replied in the last two sentences you cannot unscramble the two eggs.

Might I ask why you need to put a door on the feeder?  Is there another creature in teh place that would think nothig of emptying the feeder?

Jim

That does not mean that he can read two tags on the same frequency and unscramble the ones and zeroes.  I am guessing that he means that his code has a feature that does not cause it to 'crash' when he gets hit with this scenario.    Either way Neil is the only one who can answer this so I am not wasting my time guessing on what his meaning is/was.  Neil can answer if he so chooses tomorrow.  If he indeed has a way of unscrambling two tags coming in on the same reader I would bet there are  lot of interested parties out there that would love to know how he did it.  HID(one of the worlds biggest RFID makers)  would gladly pay to know how that worked.  I have yet to see a reader decode two tags with the same frequency presented at the same time.

You may be looking to start a fight, I certainly am going to avoid one.

Cya tomorrow.

Jim

Oh boy, can of worms...

Ok, quick primer hopefully without revealing company IP that I shouldn't... Disclaimer: I work for a company that produces RFID controlled cat flaps and feeders; that is owned by one of the largest producers of RFID animal tracking products and they are owned by a very large US drugs company. Anything I write here is *strictly* my opinion and not that of any of my employers.

There are, for domestic animals, a handful of competing RFID standards, all working at or around 125-135kHz. All transmit a single unique identifying code as long as they are in range of a sufficiently strong activating RF field of about the right frequency. They consist of an activating coil, around a ferrite bar for internal chips, and a tiny processor that can wake up when there's enough voltage induced in the coil. The processor transmits a single coded binary signal by shorting out its activating coil; that changes the impedance that the transmitter sees and this signal - volts to millivolts - can be detected by the transmitter.

The most common four types are FDX-A, FDX-B, Collar Tag, and Avid; somewhat less common is Trovan (Trovan supply FDX-B parts as well as their proprietary formats, I think). Each uses a different and non-compatible coding standard so the combination of chip type and serial number is sufficient to identify a unique part and therefore animal. In the UK and Europe, the most common type is FDX-B. We supply collar tags with all our products but we recommend that cats are chipped, not collared, to avoid any risk of the collar being trapped. (In the UK all dogs must be chipped by law). Information on the various types is randomly available on the internet; there is an ISO standard  https://en.wikipedia.org/wiki/IS...

Handling each chip type requires different software and different hardware signal chains (which obviously I can't discuss in detail) but when learning a new chip on a product, the process is to scan for a valid reading from each chip type in sequence; when you know what the chip type is, you need only scan for that type (saving both power and time) to make a go/no go decision on whether to release a door latch or open a feeder lid. A chip of a type other than that being scanned will cheerfully return data, but as it's not in the format you're expecting it will always fail to read a valid code.

Fundamental, as discussed above, is that you *can't* read two chips at the same time, whether they are the same type or different. They will stomp all over each others signal... but if one is in a stronger RF field, it will have more effect on the signal and often it is possible, perhaps with multiple attempts, to read that chip. Where a cat has two chips, it's likely that the second has been installed because the first has migrated and it will generally be further away; in most cases that's enough to read the intended chip every time. In good conditions, our hardware can often read a chip that is only an inch or two nearer than an interfering chip. 

But cats get themselves into odd positions. For the basic products, if a second chip is being read, all we can do is recommend trying to learn the animal multiple times. Hopefully one of those times the interfering chip will be read; at that point both chips are valid and the flap/lid will open for either. Learning the same chip repeatedly just rewrites the same table entry, so there's no risk of running out of animals - but it's a bit hit and miss. In Mark's case - very common, where each cat must eat its own diet and nothing else - we'd recommend that each has a dedicated feeder, with one cat learned to each.

However... we're all IoT these days, and now it becomes necessary to associate a particular chip with a particular animal so that e.g. its entrance and exit patterns or its feeding habits can be associated with it. Bear in mind that the whole point of the chip is to keep out unwanted (i.e. unlearned) animals, and there's a lot of clever logic relating to the various ways cats in particular interact. Allow this cat in, but close the lid *immediately* if any other chip is detected, or if another chip of the same type is detected, or train a new animal by leaving the lid initially open but closing it if another cat comes sniffing around... you get the idea. But the users want to see which cat is doing what, so the recent devices report activity to a server which talks to the user's phone app.

If you have two chips in a cat, when either might be detected, you simply need to associate both chips with the same cat. To simplify the server side we make that substitution in the device - it needs our people (usually me!) to tickle the internal data of the device to make the association and the result is that whichever chip is read, only the 'official' one is reported. This means that, for example, you can use a single feeder to track the feeding of two or more cats: if they arrive, eat, and leave at different times we can correctly assign the food eaten to a particular cat, even if they happen to have multiple chips. (If one cat starts to eat and is bullied out of the way by another, it's harder to make a valid assumption about who ate what but we can flag that it's happened; if that's what your cat does we recommend separate feeders (and possibly some custom modes to change the feeder's actions somewhat).

So my comment about 'can read two chips' was not intended to imply 'simultaneously' but rather 'can read one of two chips'... as mentioned above, it may take multiple read attempts before it works. Two collar tags on the same collar will likely never work; and an internal chip will almost always never be read in the presence of a collar tag.

Anyone still awake?

Neil

Well, the 125kHz is pretty arbitrary anyway. Without looking at the source code comments I can't recall the details, but I have an idea that the FDX-B signal chain is used for Collar Tag. If it's an external device it should be a Collar Tag anyway.

Neil

Yeah, they're half duplex chips: they wait a random or programmed time after the energising signal stops before responding. More expensive and sometimes lower range than full duplex chips: all domestic animal stuff is full duplex.

Neil