rfid reader start byte confusion

andyschm

According to the documentation I am expecting to see a start byte 0x0A and termination 0x0D. However, I never see that.

This is the pattern I get from tag #1

0x0
0xbd
0x76
0xd6
0x56
0xf6
0x73
0x79
0x99
0x97
0x91
0xe5

And a different pattern from tag #2

0x0
0xbd
0x76
0xd6
0x56
0xf6
0x73
0x79
0x9d
0x7b
0x9d
0xe5

I have two parallax rfid readers and they both behave the same.

They are wired up to a serial port on the computer, standard settings 2400 8N1 (I also get the same data reading from a serial port on a PIC 18f).

Can anyone enlighten me as to what is going on?? Thanks!

Beau Schwabe

andyschm,

It appears to me that the serial data might be inverted. This is a common problem and can often be addressed in software or hardware.

If you look at the data you have presented from a BINARY point of view you can sort of see what I mean... the reason that there appears
to be "shifting" of the bits is because the start/stop bits get confused upon reading the data when the data is inverted.

00000000 10111101    0x00 0xbd    <-Actual   data
       1 010        0x0A            <-Expected data
'''''''''''''''''''''''''''''''''
         11100101    0xe5            <-Actual   data
            1101    0x0D            <-Expected data

...The fact that the data appears to be somewhat consistent from the start and termination sort of gives this away and lead me to believe that
there is an inversion here.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

andyschm

Beau,

Thanks so much for the quick reply. At least I have confirmed that I am not going insane! It seems that the inversion of the stop bit causing the shift is probably why I did not recognize immediately that it was simply an inversion problem.

I have instructions for building an inverting circuit in hardware, however I am curious -- don't the data sheets say that the output of the Parallax RFID reader is non-inverted? Why would I be seeing an inverted output?

Thanks
Andy.

Chris Savage

Andy,

·· The output of the RFID is true (non-inverted) output, whose resting state is HIGH.· I am assuming your device is using inverted mode.· If you have a line driver you could easily confirm the output by piping it through a PC Serial Port.· The driver would invert the data and convert the levels to the PC levels.

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Chris Savage
Parallax Tech Support
csavage@parallax.com