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Topic starter
2023-01-08 9:11 pm
Hello DroneBotWorkshopForum!
How would you approach reading a digital signal if you have no documentation on how it is encoded?
I have a door opening device with RFID cards and they all have a unique number to open a magnetic mechanism. The device with the RFID reader has two digital pins that I'd like to get the RFID card number from, hopefully. These are two pins (D0 and D1) and I hooked them up to my oscilloscope to get this image:
https://drive.google.com/file/d/1mgGFZ5JWFrVk2x2oYDiMiCwowVWqY16j/view?usp=share_link
Both signals appear to be the same. There is a sine wave which is steady and this transients when a card is triggering the sensor. Can you help me with feedback on what you guess this signal could be, according to how it looks?
Thank you
Horst
2023-01-08 11:57 pm
@klangkulisse Does this help? https://blog.pepperl-fuchs.com/en/2022/how-to-decode-rfid-tags/
First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
Sure you can learn to be a programmer, it will take the same amount of time for me to learn to be a Doctor.
2023-01-09 3:04 am
Hi Holst @klangkulisse,
Sorry, but at the risk of either misunderstanding your description, or pointing out the obvious, maybe both, your scope picture does not look like a 'digital signal'. At a rough guess, it might be the signal from the pickup coil (maybe amplified).
I haven't worked with them, but there are a number of readily available boards/kits for RFID experiments. I would suggest you obtain a suitable kit and get some familiarity.
A quick look at your picture suggested a period of about 7.5 microsecs ... 133.3kHz ... I note from
https://www.rfidjournal.com/question/what-are-the-typical-rfid-tag-frequencies
that one of the common frequencies is 134 kHz, or maybe 125 kHz as I didn't measure accurately, which could be the appropriate system for you to look at.
I would expect the receiver 'chip' to do the first level of 'interpretation' of the kind of waveform you a are looking at, and hopefully provide a more 'digital' signal output for you to interpret.
Good luck. Dave
Topic starter
2023-01-09 6:41 pm
@zander Thanks Ron, I will read this article as it seems like to be good basic understanding of how a RFID card is related to a unique ID.
Topic starter
2023-01-09 6:46 pm
@davee Thank you Dave. I could be misinterpreting the "D" on D0 and D1, maybe it is a signal path to be decoded. And thanks, your description that this is an analogue signal seems to be working for me too. I thought that the digital signal would be encoded, like it is in FM radios. I really hope that the manufacturer answers my questions.
What you suggest is that I treat this signal as a raw source of 133.3 kHz signal and decode it on my own, say with a Raspberry Pi and software. Correct?
2023-01-10 1:28 am
Hi @klangkulisse,
Sorry, I haven't played in this area ...I just read a little, a while ago. So this is vague and maybe in error.
I don't know what D0 & D1 imply in your case, but it looked a bit like the kind of waveforms I have seen in pictures relating to the voltage across the sense coil, though (naively?) I would have expected to see an excitation phase, when the coil was being driven and a sensing phase, when the RFID chip was replying.
(Please note, I may have misunderstood your scope trace .. it just seemed to be a 'strong coincidence' that the most obvious frequency visible matched to one of the common frequencies used for RFID.)
I think you will struggle trying to decode the raw signal across the coil terminals ... there is an entire industry producing chips to do that .. I was thinking of deploying one of the appropriate chips and maybe using an Arduino to make sense of the output of one of these chips, which should indeed be digital, and probably using one of the well known interface types, like I2C.
If I were to start down this road, I would get one (or more) of the 'off-the-shelf' demo/experiment boards and just play with it (scope in hand), to figure out how it works, etc.
You will need to do some Googling ... the first couple I found is, which I only offer as illustrative examples ... I have no idea if they work or not!! :
https://mschoeffler.com/2018/01/05/arduino-tutorial-how-to-use-the-rdm630-rdm6300-rfid-reader/
https://www.aliexpress.com/item/33020296300.html
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I did notice a 'lack' of 125kHz writers in AliExpress ... I don't know why ... maybe lack of chips in the present drought ... 13.56MHz seemed to be better supported.
125 KHz seems 'better known' than 134 kHz ... I would expect any readers or writers for these frequecies, to look very similar, except possibly small changes in inductance and capacitance in any tuned circuits, and maybe a slightly different crystal frequency.
Maybe you will have better luck if you do a proper search - I am only trying to introduce a possible direction to explore.
NB, for a simple start, these guys usually do their research ... though 13.56MHz, not 125 kHz
https://randomnerdtutorials.com/arduino-time-attendance-system-with-rfid/#more-61725
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All vague, I am sorry to say, but hopefully you can do some experiments and make some progress.
Best wishes,
Dave
Topic starter
2023-01-10 5:08 am
@davee Hello Dave, thank you for your suggestions. The links seem to be good starts for taking my first look into setting up such a system on my own. I have the strong feeling that I need to take a different approach to it as my understanding is too limited to read the unique IDs from the cards coming out of the D0 and D1 pins from a raw signal.
DaveE reacted