Another radio question - star network?

[matherp]

Apologies if this has been covered but the search returns so many threads on radio I didn't quickly find anything suitable.

I want a central picaxe system (this will be GSM enabled) to two-way communicate with 4 (initially but may grow to 8) picaxe outstations - range less than 30m. All communications to be initiated by the central system.

One option would be to use multiple one-to-one links. At the other end of the spectrum it looks like zigbee will do what I want but this looks somewhat complicated.
All ideas and any pointers to examples appreciated.

Thanks

Peter

 

[BeanieBots]

I would (and do) use ERF modules for such a network.
Cheaper than Xbee, 0.1" pitch and 5v tolerant.
Supplied by Rev-Ed and you can even use them for wireless programming with a URF module.

 

[matherp]

BeanieBots

I've just read the ERF datasheet RFA020.pdf and although it mentions "Two (or more) PICAXE project boards can communicate with each other via multiple ERF
modules" there is no information on how to do this. Likewise the example on the ciseco site is only point to point.

Can you give me a clue

thanks

Peter

 

[rossko57]

It's quite easy to manage radio (or wired bus) networks when you have a designated sole 'Master' node. "Slave 7, status please!" - only the addressed slave responds. Keyword; polling.

 

[matherp]

The bit I'm missing is what I need to do to get all the radios to communicate. The implication of what rossko57 says is that if I have 5 ERF and transmit from one the other 4 will all receive it. I have assumed from the datasheet that the ERF has some sort of error correcting transport layer which implies that by default communication only takes place between a paired set of units. The erf datasheet says:

"• Point to point, point to multipoint, or custom addressing available
• Can specify a network identifier PANID to separate communications into separate
networks if desired"

If anyone can point me at the information source that explains how to set up the ERF in multipoint mode it would be appreciated.

If all the ERF does is a dumb broadcast then obviously it is easy to give the slaves addresses and manage the communications in software with each slave junking data for another address. However, if that is the case I might as well use the very cheap radio modules and rfin/rfout.

 

[hippy]

I believe the ERF is basically 'dumb broadcast' but adds filtering on PANID so the receiving PICAXE only sees messages from other ERF devices set to the same PANID. If two ERF are set to the same PANID they are "paired", but there is no reason not to "pair" [sic] more than two devices by putting all of those on the same PANID. In that case it will be as rossko57 says; all ERF's will receive everything sent from any other ERF with the same PANID.

Off-hand I cannot recall exactly what error correction the ERF's provide for but I believe that it ensures the data received matches the data sent, small corruptions being corrected automatically, rather than a full retransmission error-correcting protocol.

The advantage over genuinely dumb transmission is that the ERF's only accepts data on their particular PANID and only passes on validated data packets. This can alleviate having to do additional checks on the data within the receiving PICAXE.

 

[BeanieBots]

It is as the other guys describe.
If you set the PANID to the same value on all devices, when one transmits, all receive.
I've not done much testing for collisions when for example two send at the same time but I have done range tests in noisy environments.
Xbee does a resend up to three times. ERF does not appear to do this.
However, I have NEVER received an incorrect character with ERF.
When they start to get out of range they simply start to drop characters.
Xbee has a few other features such as passing on a message but ERF range is greater and is much easier to use.
(the 0.1" pitch and 5v tolerance are the real winners for me)
I have several projects with a two pole switch that can switch between hserin/out and serin/out which allows me to switch between wireless download/debug and normal use. Makes development a breeze!

There are also networkable Bluetooth devices but they are even more expensive. The cheap ones do not support networking.

I am no expert in this area. Just wanted to share my own experience.
I have many questions myself. Hopefully others may answer them in reply to you or I might start a thread myself.
Good luck.