Hi Shane,

I'm very sorry I took so long to reply, I am getting ready to head back up
to school to begin my grad studies. I go back on Monday, so I have been
very busy getting some last-minute stuff done.

Please see my comments below:

At 12:19 PM 8/16/01 +1200, you wrote:
>Hi,
>
>I've read your email, and can offer some more specs on what is needed.
>
>- its low distance, only about 50 to 100 metres.
>- its single unit <-> single unit.
>- the master unit activates and communicates with its slaves one at a time.

I have further questions :-)
Is this indoors or outdoors? Either way, the Linx modules would probably
work for you, but it would allow us to judge what other modules might work,
too.

Could you tell me what it is you are trying to do so I can get a better
picture in my mind?



>I'll look forward to your tutorial. Could you send me the link once you
>have finished?

Certainly. I will also post it on this list.



>Cheers,
>Shane.

Thanks for your interest,

Sean



> > -----Original Message-----
> > From: shb7@cornell.edu [mailto:shb7@cornell.edu]
> > Sent: 16 August 2001 11:07
> > To: Shane Tolmie
> > Cc: shb7@cornell.edu; PICLIST@MITVMA.MIT.EDU
> > Subject: Re: Whats the best radio for a 20kbps transceiver, with
> > variable packet length (1 byte to say 64 bytes) ?
> >
> >
> > Hi Shane,
> >
> > It depends on a number of things, such as how far you need to transmit
> > (i.e., what kind of signal to noise ratio are we dealing with), how much
> > latency you can tolerate, are there going to be multiple stations sharing
> > the same channel, etc. If the ends are being controlled by something
> > simple like a PIC, an XMODEM-type protocol (where each packet must be
> > ACKed) is a good general-purpose protocol. However, your overall
> > throughput will probably only get up to about 80% of your actual transmit
> > speed (so to average 20kbps you would need hardware that could handle
> > around 25 or 30kbps and had very fast transmit/receive turn-around
> > times).
> >
> > If you need to get every last bit of throughput performance and the
> > channel is pretty free of interference, you might be able to get away
> > with a ZMODEM-type protocol, where the transmitter sends long blocks and
> > then just listens for a short time to see if the receiver needs any of it
> > resent. This system can more easily break down in the presence of
> > interference, though.
> >
> > I just recently completed a demonstration project which uses 16F876s
> > along with Linx TR-916-SC modules to do 4800bps with
> > error-correction(retransmission) direct from RS-232 port to RS-232 port.
> > It uses an XMODEM-type protocol and the packets have only about 25 bytes
> > of data (variable length with 25 being the maximum), so the actual
> > throughput is only about 2400 bps.
> >
> > This demonstration project is a part of my tutorial on wireless comms for
> > microcontrollers which is almost completed. I will be announcing the URL
> > on this list when it is ready. As usual in my projects, I have had some
> > delays in getting it ready. At least the Linx module section should be up
> > very soon (within a week).
> >
> > If you add external RAM, an external UART, and make a few simple
> > modifications, you
> > should be able to achieve your 20kbps spec with this system, since the
> > Linx modules can go to 33.6kbps. The reasons why I shot for a much lower
> > goal were that the 16f876's ram limited the packet size (since we also
> > had to have serial receive and transmit buffers in the RAM) and because I
> > had to implement a second UART in software, along with the rest of the
> > fairly complex code.
> >
> > Sean
> >
> > On Thu, 16 Aug 2001, Shane Tolmie
> > wrote:
> >
> > > Hi guys,
> > >
> > > Just imagine I had a board with a 20kbps, di-directional radio
> transceiver, RS232 in, RS232 out.  What sort of radio
> > protocol would
> > > you recommend to communicate between two devices?
> > >
> > > The protocol would have to have elements of the following:
> > >
> > > - variable packet length, 1 byte to say 64 bytes
> > > - CRC for error detection
> > >
> > > Other that that, its an open possibility.
> > >
> > > Regards,
> > > Shane.
> > >
> > > --
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> > >
> > >
> >
>
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