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Re: [Discuss-gnuradio] Synchronous bitstream output using ppio_ppdev?

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] Synchronous bitstream output using ppio_ppdev?
Date: Wed, 22 Aug 2007 17:08:23 -0700
User-agent: Mutt/1.5.9i

On Wed, Aug 22, 2007 at 07:04:29PM -0400, Ed Criscuolo wrote:
> I'm working on a GNU Radio application that demodulates a synchronous 
> bitstream from a spacecraft telemetry downlink.  Once I have the 
> bitstream, I need to get it back onto a wire in order to feed it into 
> our existing equipment.

What's the protocol for talking to the equipment?  Is there any
handshaking, etc, or do you just need a raw synchronous stream?

> Is it possible to do this by using the ppio_ppdev?  I would get the 
> bitstream unpacked into the LSBits of a stream of bytes, and write these 
> using ppio_ppdev.write_data().
> The bitstream is running at 76800 bits/sec, so my questions are:
> 1) Is ppio_ppdev fast enough to keep up with this bitrate?

I doubt it.  It just bit bangs the serial port.  Thus you've got no control
of the data rate, etc.

> 2) Can this be done from Python, or does it need to be embedded in a C++ 
> block?
> 3) Are there other, better or easier ways to do this, such as using one 
> of the debug pins on a USRP daughterboard?

If you're willing to hack verilog, you can get it out the debug pins.

If the output needs to be synchronous serial output, the easiest way to
get it out may be to use a serial card with a USART on it.  E.g.,
something that's capable of synchronous serial transmission.  Look for
a card that'll do HDLC, then see if there's a "raw" or "unframed"
mode.  Generally the hardware that'll can do HDLC can also do

Good luck!


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