# RFID module always finds the same non-existent tag when seeking

I'm wondering whether anyone has any experience with the SM130 Mifare module. I've managed to get my hands on one, sans antenna of course, and seem to be having a few issues with it.

I've got the module connected to an FTDI 5V USB to TTL converter via the UART lines. I've added capacitors over the power rails for good measure. I've got the SM130 module connected to a single wire, a home-brew PCB antenna and a whole 50m roll of thin hookup wire as an antenna. In all cases, I've used the SMRFID software from SonMicro directly, which identifies the device correctly and can enable the RF field and disable it just fine. I took out my SSB shortwave receiver and I can hear the field being energized and de-energized.

Here comes the problem:
I've ordered a fair amount of the Mifare transparent and ABS tags, however, when I select seek for tag, SMRFID always reports finding TAG TYPE: 55 SERIAL: 2E31204D, despite there being no tag in the field. Even with a tag in the field, it refuses to find it and reports the same details. I've tried relocating the device and antenna somewhere else, thinking it's interference, but still it didn't help. I also tried toggling the reset pin a few times; still no change.

The datasheet says that it should return an L representing "Seeking for tag" right after a seek command and sit there until a tag enters the field. Instead, my module seems to be reporting an L, then barely 10 seconds later, will report the same data.

At this point a short list of possibilities would be helpful; a total solution is not expected.

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well, obviously big brother injected you with a tracking tag the last time you went to the doctors and thats what your reading! :p –  Mark Feb 28 '11 at 6:33
Search in your code for "2E31204D" maybe its a debug setting anywhere? –  powtac Feb 28 '11 at 11:55
You've got a whole 50m reel of wire for the antenna? –  Jim Feb 28 '11 at 15:42
The code I'm using is the sample application from SonMicro called SMRFID - I don't have the sources for it, but as far as I can tell, it's what the module is sending back over the RS232-TTL link. –  user2998 Mar 6 '11 at 4:07
And yes, I tried a 50m reel of solid core wire just because it's a lot of wire and it gives a very strong field with it when judged with my SSB receiver. Anyway, I have given up on this module and RMA'd it on suspicion that it is faulty. Thanks for all your replies! –  user2998 Mar 6 '11 at 4:08

I have spent the start of time looking into this, but I need to spend more. This is what I have found so far.

The fact that your tag type is 55 is a big deal. The acceptable tag types are:

0x01 – Mifare Ultralight

0x02 – Mifare Standard 1K

0x03 – Mifare Classic 4K

0xFF – Unknown Tag type

Now, to continue from here. There are some error codes that you could get querying a tag type.

0x4E ‘N’ - No Tag present.

0x55 ‘U’ – Access failed due to RF Field is OFF

There are other commands that also get responses of this type, so I need to research the software to find more, but as of right now it seems that it is sending you back an error code as the tag type cannot ever be 0x55, however almost every command has an error option that can return 0x55.

I hope this gives you some direction, either I will find something or you will find more as you debug to make it possible.

I could be helped if you had a debugger and could show me everything being sent over serial to the device, this would allow me to determine the exact state the error is being generated from.

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Thanks for the reply. I've already given up on the module - I've sent explicit RF On commands and I verify with an SSB receiver that the field was on, and yet, it kept giving me the same reply. I've returned the module for RMA on the suspicion that it's faulty. I was using the SMRFID software that was provided by SonMicro (the guys who made the module) so I would presume they should have got the commands and sequencing right. –  user2998 Mar 6 '11 at 4:04
@GoughLui, Yes, I would assume they were doing it properly, but knowing at which specific command they are receiving the 55 can change what the error means. I am not completely impressed by the design, but it does seem to be better than some I have seen. –  Kortuk Mar 6 '11 at 4:12
@GoughLui, does it use the RTS and CTS signals during comm? I would like to note that I would consider you one of the question askers here whom actually gave solid work before asking. –  Kortuk Mar 6 '11 at 4:13
@GhoughLui, it is possible that the software you are using is running so quickly that it energizes the RF field and before it has fully reached steady state attempts to read. This would result in the tag being read as state 0x55, or RF field is off. There is not good information on the datasheet about this, but if you were to send the 0x90 to start it and wait and then try a select tag it may work. –  Kortuk Mar 6 '11 at 7:50