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I am extremely new to this and I'm learning by building stuff myself so please let me know if this is too vague and I'll elaborate if I can. I'm sure this is something very simple, but can't figure out how to do it.

I have built radios before and know how to modulate a signal as ASK but now I'm trying to send a simple bitstream from a USB port to a radio's input. My idea was to use a serial program to dump bytes into the radio input signal but all the research I have done indicates using rs232 or UART or something. I have no experience with this and I don't want some protocol sticking parity bits in all over the place. I simply want to be able to send, for example, 0x1234 as 00000001000000100000001100000100 at a baudrate of \$2048\$. Can I go further and actually send individual bits like start/stop bits?

My intention is to monitor the frequency with an sdr and then learn how to demodulate the bits from the IQ data.

Is there a simple method outputting bytes directly onto the wire or something I don't know about which will prevent me?

Any help or direction would be greatly appreciated.

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    \$\begingroup\$ USB doesn't work that way -- there is a lot going on in the differential pair signalling, you can't just bitbang one of the USB wires. Need a whole serial interface engine. Just get an Ftdichip.com FT232, you can easily bitbang the Ft232 pins without needing to dig into the USB specs. \$\endgroup\$
    – MarkU
    Commented Feb 8, 2015 at 1:41
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    \$\begingroup\$ USB raw wire is worse than parity bits, its closer to Manchester encoding with bit stuffing. And its always toggling with bus activity, every 1msec a new frame starts. \$\endgroup\$
    – MarkU
    Commented Feb 8, 2015 at 1:43
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    \$\begingroup\$ You should appreciate that the start, stop and (optional) parity bits in an async serial stream are not there for decoration, they serve a purpose. \$\endgroup\$
    – markt
    Commented Feb 8, 2015 at 3:09
  • \$\begingroup\$ Thanks MarkU. That sounds like exactly what I want to do! I already have a pl2302x (usb->ttl) lying around, will that do? and can anyone link me to a good tutorial on this bitbanging stuff? This sounds like my new favourite thing. \$\endgroup\$
    – tozhan
    Commented Feb 8, 2015 at 17:23

2 Answers 2

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Another alternative is using a USB serial port, using the CTS/RTS and DSR/DTR signals as a 2 channel digital I/O.

The good part is that it is well packaged and easy to program, in most languages and OSes, and that the off-the-shelf hardware is readily available.

The drawbacks include that USB, as others have commented, cannot send individual bits, so the overhead is huge and timing and jitter may be a problem if you need precise timing.

Caveat hacker: I haven't done the maths for 2400bps, but I think it's OK, I've done this for at least 1200 bps if memory serves me right.

That said, using an existing protocol like RS232 is probably a simpler and more robust option.

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2048 bits per second is pretty slow. You might try using a USB to parallel converter and bit-banging one of the pins in software. The FTDI UM245R might do the job. It even has a FIFO if you ever want to try higher bit rates.

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