Manchester or not ?

[Fabri]

Hi Everybody,

I need to transmitt just some data wireless using a cheep radio on 433 Mhz. I don't want to spend to mutch time to write code for manchester.

By your experience is better to use manchester encoding system or shall I use rs232 direct on radio module ?

Thanks,

[arunb]

What Radio module are you using ???

thanks
arunb

[Fabri]

Hi Arunb,

In tx I use TX-SAW/433 S-Z Aurel
In rx I use RX BC BNK Aurel

This are 2400 baud but I'm not interest in speed.

Tanks,
Fabri

[Humberto]

If the string to transmit is not very long, the use of Manchester codification can be avoided.
There are some very interesting threads:

http://www.ccsinfo.com/forum/viewtopic.php?t=27263&highlight=manchester
http://www.ccsinfo.com/forum/viewtopic.php?t=19141&highlight=manchester

Humberto

[Guest]

One other option, is to take advantage of the need for balance characters. If you send a couple of initialisation bytes (say 0xAA 0x55), then the string of data, but for each character, send it once, then send the bitwise 'not' of the byte, the transmission will inherently be balanced. Even more usefully, if you are sending only 7 bit data, and add a parity bit to the byte, then send this, followed by the 'not', if you check the parity on receive, you can reject a byte, if it's parity is wrong, and take the other 'copy', giving one bit error recovery (not efficiently, but it does make 'good use' of the padding data :-).

Best Wishes

[rnielsen]

If you're simply sending data via the serial port you might want to try looking at the XBEE and XBEE Pro at www.maxstream.net . The XBEE is only $19USD and is rather simple to use.

Ronald

[newguy]

Try this first:

http://www.ccsinfo.com/forum/viewtopic.php?t=23953

If that doesn't work, then you can start to balance the signal by sending the inverse of each data byte immediately after each byte.

Anyway, try the code/idea in the link first. Then get fancy if it doesn't.

[Bill Legge]

I would welcome comments/arguements on the business of using Manchester code for radio links. I don't think it is generally necessary because:

1. Most (I would guess very nearly all) 433MHz type radio modules use frequency shift keying. A '1' is represented by a 'tone' and a '0' by a different tone.

2. If the Tx data pin is set permanently 'high' a continuos tone will be transmitted - the output of the Tx is not a 'dc' level.

3. So, 'balancing' is unnecessary for the radio link.

4. Where 'balancing' is important is when data is sent by different 'dc' voltage levels - say along a pair of wires - and the capicatance of the wires distortes the waveform; typically rounding the pulses until the Rx can not detect them.

5. If the wire from the MCU to the Tx or Rx modules are long/longish then go for Manchester. Otherwise don't bother.

Regards Bill Legge
_________________
Denmark in West Australia
'Where the forest meets the sea'

[dbotkin]

That depends a great deal on the radio module being used. For one project I worked on, the modules specifically required Manchester or something similar. The transmitter could not be keyed more than a few tens of milliseconds - or maybe the receiver was the issue. It's been a few years, so I don't remember all the specifics, but I do remember it being enough of an issue that I had to write Manchester encode/decode routines from scratch.

Sending Manchester is almost trivially easy. Receiving it is much less so. Once it works, it works very well. I'll see if I can find that code.

[FvM]