RFID Proxmity Circuit

Projects are always harder than you first think they will be. That's the only constant in life :)

It looks to me like the code you're using is not ideal because it's looking for a fixed 8 byte output from the reader. Unless the firmware has changed since I first reviewed it, the length of event packets output by the RF9315R is not fixed, it changes with RSSI value.

First, download the Parallax STAMP programming manual. Page 397 of the PDF (page 393 in print) is the start of the SERIN command details section. Since the RF9315R outputs each data tag ID and RSSI value packet followed by a space, I think you'll want to modify your SERIN command to include a "end character";

    SERIN 1,84,[STR TagID\7\" "]

You'll notice I also decreased the size of the byte array input from 8 to 7. This leaves room for the STR formatter's use of the end-of-string char (ASCII 0 or NULL). Because RSSI values max out at 3 bytes in length, 7 is enough to store tag ID and the longest possible RSSI value.

The nice thing is that it will automatically terminate the SERIN command upon receipt of a space character. This allows you to easily handle variable length input without looping and shifting your data frames in a fixed length SERIN command when RSSI values are less than 3 characters.

Give that change a try and post the new output values. Also, try outputting the captured data using STR instead of HEX (DEBUG STR TagID(0), CR). Post the output you're getting and we'll see if it looks anything like normal data. Don't quote me on this, but you might be able to just use DEBUG STR TagID, CR (with no index on the array) and DEBUG should output the entire array on one line.

Give it a try and post your outputs.

Hmm, what does the o-scope say about the TTL voltage levels you're shunting?

The reader's RF range is 350mhz but you're only concerned with reading the voltage levels of the data coming from the shunts you've inserted between microprocessor and RS232 conversion circuitry. Your 50mhz scope should have plenty of horsepower to sample the serial communication signals between STAMP and reader.