receiving serin data when not connected

M

martinn

Member
I am playing around with 315mHz wireless units, I am having problems with Picaxe receiving a signal via serin when not attached to the receiver, which I assume is interference.

I have used both an 18x in the datalogger unit AXE110 and an 8m on a Picaxe 80 Proto board.

Currently I have a 10cm wire connecting the 8m’s input 1 to the receiver, and without a qualifier a stream of random data numbers pours in even when the wire is not connected to the output of the receiver and no transmitter is in use. Even a short 25mm piece of wire will pick up the interference.

- I am using batteries as a power source and have put a capacitor across the power supply
- I have grounded all of the Picaxe pins not being used (using 10k resistors)
- It occurs both in the house and in the middle of the backyard
- Changing to the 18x and datalogging board did not change the problem
- Disconnecting the serial cable and using a led to indicated in coming data did not change the problem

Code: 
'stripped basic code (pinched from something Hippy posted)
#picaxe 08m
RxLoop:
SerIn 1, N2400,b0
sertxd ("b0= ",#b0,cr,lf)
Goto RxLoop
Any suggestions?

Thanks
Martin
 
M

moxhamj

New Member
Interference is not good. Leave out the picaxe for the moment and feed the radio unit into a CRO and see what is out there. If you don't have a cro, feed it into an audio amp. If you don't have one of those, feed it into the audio input at the back of the PC. See what you can hear.

The probable solution will be to make sure your transmitter is more powerful than the interference. If you add qualifiers then it becomes less likely that the data will be corrupt and it is even better if you have a simple checksum - eg the last byte has to be a certain value as well.
 
C

ciseco

Senior Member
Hi,


Theres a 15 day trial of a PC based oscilloscope that uses the sound card, it's on the net I used a month odd ago, he also sells the software on your least favorite auction site for something like ?20, off hand cant remember what it's called, was in the first few lines when I searched google from memory.




Cheers

Miles
________
herbal vaporizers
 
Last edited:
hippy

hippy

Ex-Staff (retired)
Qualifiers ( and preferably checksums as well ) will be needed to deal with interference when nothing is transmitting as the receiver will detect random noise. There's not a lot that can be done about that, you can only filter it out and throw away corrupted data.
 
C

ciseco

Senior Member
Hippy is right, which is why I now do everything with Xbee's, it queues/buffers packets upto 100 bytes so they allways come through in order and deals with CRC etc, without any need for programming. My first attempts with 433 worked fine but needed checksums etc and they trashed my X10 / bye bye standbye devices on the same frequency, I could have improved it to stop this but without serious code and there aint much room in a picaxe to do it, two way comms was far easier if 40-50% more expensive (?10 for 433, ?14-15 for xbee)

Miles
________
find headshop
 
Last edited:
premelec

premelec

Senior Member
Some resceivers have a received signal strength pin out [RSS] which you can use to only receive the strongest signals... another common technique is to use tone modulation of the transmitter and sharp filtering for only that tone on the receiver - when the PICAXE DSP units come out [?] that will be easier.... A combinatoin of these methods and Frequency Shift Keying is also useful.... anyhow noise is with us and getting more prevalent...
 
P

papaof2

Senior Member
I think some of the responders missed this: "when not attached to the receiver"

That would indicate that serin is floating, which means that the length of wire connected to serin is acting as an antenna for every electrical signal in the area.

Add a 22K from serin to ground and see if the problem goes away (or is reduced).

John
 
D

Dippy

Moderator
Right on Dad.

And when connected, sometimes receivers will cause unqualified Serin to popout. Another reason to spend the extra quid and get proper stuff with RSSI. There can be quite a lot of mush coming out those little modules esp when you have AGC kicking about. And most of those things disappear using RSSI.
Another excuse to get a 'scope though eh :)
 
You must log in or register to reply here.
Top