RF Module Qualifier, CRC?

[tazntex]

Hello to all,
I am using the Linx RXM & TXM LR (long range) modules. I was sending my data successfully until I change my qualifier to anything but 254. It operated very reliably with only an occasional hiccup (probably due to multipath around the metal buildings) at several hundred feet. I am sending an address (word) broken up into four pieces which is in bytes because I am using Manchester Encoding, then reconstructing and comparing the address in the receiver (constant) and also doing the same with the actual button pressed on the transmitter (key). I am not using USART because I still don't understand it after reading countless post on this forum and reading the datasheet.
So I am doing this on the transmitter:
serout2 serpin,16468,[$AA,$AA,$AA,$AA,$AA,synch,addA,addB,addC,addD,addA ,_
addB,addC,addD,key,key,key,key,key]

And this of course on the receiver:
SERIN2 serpin,16468,500,loop1,[wait(254),address1,address2,address3,address4,_
address5,address6,address7,address8,mydata1,mydata 2,mydata3,mydata4,mydata5]


Then I am decoding the address (manchester) then summing those up, ADD1=ADDA+ADDB<<4+ADDC<<8+ADDD<<12
comparing my address (constant) in the receiver to ADD1
then I check:
IF (address = add1) && (mydata1 == mydata2) && (mydata1 == mydata3) &&_
(mydata1 == mydata4) && (mydata1 == mydata5) && _
(MYDATA1 > 0) && (MYDATA2 > 0) && (MYDATA3 > 0) && (MYDATA4 > 0)&& _
(MYDATA5 > 0) THEN
Index2 = 1
For Index = 0 to 3
mydata.0[Index] = mydata1.0[Index2]
Index2 = Index2 + 2
Next Index
chksum2 = chksum2 + 1
chksum3 = 0
high rf
else
chksum2 = 0
chksum3 = chksum3 + 1
low rf
endif
if chksum2 >=3 then start
until chksum3 >= 1
ADD1=0
chksum2 = 0
chksum3 = 0
mydata = 0
I decode the mydata
then i repeat this three times by going back up to the SERIN2
using that if statement ,if chksum2 >=3 then start
then I assume all of mydata and the address match is good so I continue on to the output I want on.

Life seems good because each output is actually staying on as long as I am pressing the button on the transmitter and the output immediately goes off as soon as I release the button.

BUT:
It does not work if I change the qualifier to anything else, maybe this should not alarm me but I have been trying to figure out why.. call me a perfectionist,...after all I am sending a preamble to balance the data slicer,...right? And I have tried sending more $AA's and even $55's and a combination of those. I am aware of the rx module "noise" read many post on this forum,watched it on my scope and squelch is useless because of range limitations but mainly to stop the noise completey, squelch turn up full, no data is then received

ok, other than changing the qualifier this remote control works almost perfect, that may be an over statement . I have learned a lot from all of you out there, especially breaking down my code and understanding how it ticks.

After doing a search on checksums, found many articles about CRC and a dead link PDF about CRC but as I mentioned, it is a dead link. So any suggestions, preferably in lamen's terms how I can use CRC?

Many Thanks

[languer]

Don't know if you have looked at the following links, but they maybe useful:

• http://www.picbasic.co.uk/forum/show...&highlight=CRC
• http://www.ccsinfo.com/forum/viewtop...&highlight=crc

What you mention truly is puzzling. Before I tried adding any "wireless" comms to my protocols, I always tested them "wired". Have you tried your code changes on a "wired" setup (i.e. TX and RX PICs connected through a wire).

[tazntex]

Thanks Languer, I have tried everything wired before I went to RF, but, I did not make any changes to the qualifier. I was bored last Friday so reading over my code i realized that obviously the qualifier is not Manchester Encoded so I was thinking if I am sending a preamble of %10101010 preceding the qualifier 254,%11111110,what happens to the data slicer so curiosity got the better of me. I then thought that I could keep the data slicer balance by sending 170, %10101010. Thats when I found that I had a problem. when I changed it to 170 or whatever it became intermittent. By keeping it 254 I could operate this way over a couple of hundred feet with only a rare hiccup, stutter or whatever its called, but that is only when I was going close around my metal shop and vehicle. I've been trying to figure out some of the examples I have found on the CRC though. Instead of cut, copy and pasting them to my code I am trying to understand how each line works before I tailor it to my need.

Again, many thanks

[mackrackit]

This might give you some ideas. Not exactly what you are asking for, but it is very reliable.
http://www.picbasic.co.uk/forum/showthread.php?t=12554

[Bruce]

Have a look at this article. The PIC controls a Linx LR transmitter with data received from the PC serial port. It simulates the Holtek 8-bit encoder, and can control a Linx FCTN-RLY4-xxx receiver/relay, or a Linx KH2 receiver module. Both use the Holtek 8-bit decoder IC.

http://www.myamicus.co.uk/content.ph...Remote-Control

And this thread: http://www.picbasic.co.uk/forum/showthread.php?t=6581 has PBP code for decoding a Holtek 8-bit encoder.

[languer]

Those post from Bruce are very valuable. They show very simple methods to make you comms more robust.

Dhouston also has some very good information on similar subject: http://www.picbasic.co.uk/forum/showthread.php?t=6261

in lamen's terms how I can use CRC?

This is my best attempt to address this in particular (full disclosure - I am no expert). CRC is used in many places when you want to validate the contents of a packet (i.e. detect the packet to be error free). A packet can be anything: wireless data, ethernet data, bootloader data, archive data (read WinZIP). It is a very simple way of coming up with a somewhat unique identifier that tells you the packet you have has not been compromised (i.e. corrupted, changed, etc).

How effective is it? - It depends. There are as many algorithms (polynomials) as days of the year (and then some). It all depends on how "unique" you want to define your packet and how much math (read - computer resources) you want to use to come up with the CRC.

How simple is it? - Again, it depends.

Why use it? - It is pretty standard method of validating packetized data; as long as you have the computing power to do it, and select an algorithm/polynomial and stick with it.

What it does? - It gives you confidence on the validity of your packet (i.e. error detection).

What it does not do? - If your data packet is corrupted, it does not tell you how to correct it (i.e. it does not have error correction).

How you could use it? - On the transmit side, you calculate the CRC of your data packet (after selecting the algorithm you want/need) and send it along with your data packet (usually at the end). On the receive side, you receive the data packet and the CRC. Calculate a new CRC value form the data packet (not including the received CRC) and compare it to the received CRC. If they match you have a higher degree of certainty the packet is free from errors.

Good reads:
http://en.wikipedia.org/wiki/Cyclic_redundancy_check
http://www.hackersdelight.org/crc.pdf
http://www.repairfaq.org/filipg/LINK/F_crc_v3.html