Multifunction Passive 7400 RFID Tag

Posted on Feb 7, 2014

Some serious overengineering was required to match my previous capacitive scanner. At the local hackerspace OSAA I went on to start a brainstorm with pedersen, AsbjG rn and Flemming, who promptly came up with a lot of funnyideas. All nice and well until Flemming mentioned RFID (he is the creator of the hackerspace`s access system, HAL900, which is based on RFID). Now that is a thought.

Multifunction Passive 7400 RFID Tag
Click here to download the full size of the above Circuit.

Of course, the first thoughts were with an RFID reader, but there have been made many of them already and that would be boring. However, the idea materialized to make an RFID Tag. I don`t know who mentioned it first and, as with brainstorms, the ideas materialize as part of the joining of minds. The idea was born, make a tag based entirely on 7400 logic. An RFID tag, sends a (unique) code modulated on a carrier wave. Most tags are passive, which means that there are no batteries and the tag only sends some ID data. Common tags use the EM4100 protocol. Many EM4100 compatible use the same internal transponder chip and are available in several configurations which differ in encoding protocol and baud rate. A general tag`s transponder has the following features: The EM4100 transponder uses 125kHz and AM modulation ( ASK ). They are available in all kinds of encoding, but the Manchester encoding type with a 2kbs baud rate is most common. The EM4100 transponder sends 64 bits of data which encapsulate a 32 bit unique ID, an 8 bit version/manufacturer number, 9 bits header, 14 bits interspersed checksum (parity) and a 1 bit trailer. The data from the tag is encoded such that a unique pattern is embedded for synchronization to the data stream. EM4100 transponders send nine bits of `1` in sequence which cannot occur anywhere else than at the start of the data stream. The main feature of a transponder is that it derives power from...

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