Your message is obviously being blocked by nybbles. There are several possibilities for the final nybble: 1. It's parity, even or odd. 2. It's a checksum or the complement of a checksum. 3. It's a CRC or the complement of a CRC, in which case there are only 16 possible polynomial nybbles to use. 4. It's none of the above. The brute force method of cracking the code is to check each one of these possibilities on your test set to see which works. At 08:21 PM 1/12/2010, Matt Callow wrote: >Hi, > >I've been trying to decode the signals from an RF temperature and >humidity sensor. The sensor is a small battery powered sensor which >comes as part of an home 'weather station' >The messages are all 36 bits long, I've worked out most of it, but it >looks like there may be a 4 bit checksum or crc at the end, which I >have been unable to work out. > >Here are some examples, with the information I know. Each message >contains a ID (which changes every time the sensor is reset and it >used to 'pair' the sensor to the display unit), channel id, 12 bit >signed temperature reading and humidity as 2 BCD digits. I think the >last 4 bits are some sort of checksum/crc > >This is what I know of the protocol so far > >ID? CH ?????? Temperature Humidity CRC? >0001 01 001010 010100110000 1100 0010 1100 >0001 10 001010 111100110000 0010 0010 0111 >0001 11 001010 011100110000 1100 0010 0111 >0001 10 001000 111000110000 0010 0010 0101 >0001 10 001000 000100110000 0010 0010 1001 >0001 10 001000 100100110000 0010 0010 0001 > >I think the bits marked ?????? include battery level, and I'm not sure >what else. > >I have lots of data, so I'm thinking that it should be possible to >work out the CRC algorithm, however I've tried various crc calculators >such as http://www.lammertbies.nl/comm/info/crc-calculation.html but >have not been able to find the crc parameters. > >I've also tried crcbfs.pl to find the crc parameters, however this >method requires that I have at least 2 strings of different lengths >(which I don't have). > >Does anyone have any suggestions of how I might work out this CRC (if >indeed it is a CRC)? > >Regards > >Matt > >-- >http://www.piclist.com PIC/SX FAQ & list archive >View/change your membership options at >http://mailman.mit.edu/mailman/listinfo/piclist ================================================================ Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: ral@lcfltd.com Least Cost Formulations, Ltd. URL: http://lcfltd.com/ 824 Timberlake Drive Tel: 757-467-0954 Virginia Beach, VA 23464-3239 Fax: 757-467-2947 "Vere scire est per causas scire" ================================================================ -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist