[Laser] serial port PDM and PWM

[wa4qal at ix.netcom.com]

>Date: Thu, 07 Dec 2006 11:00:19 -0800
>From: Glenn Thomas <glennt at charter.net>
>Subject: Re: [Laser] serial port PDM and PWM
>To: Free Space LASER Communications <laser at mailman.qth.net>
>Message-ID: <7.0.1.0.2.20061207102448.034289c8 at charter.net>
>Content-Type: text/plain; charset="us-ascii"; format=flowed
>
>uhhh... does anyone on here know how USB really works? I don't.

It's easy enough to find out.  Here's the complete USB 2.0 specification:

http://www.usb.org/developers/docs/usb_20_05122006.zip

>BUT...
>
>The ten foot cable limitation is likely not a signal level issue. On 
>a high speed, multiple node serial system like USB, it may be that 
>there is a timing constraint that is inherent in the protocol. This 
>type of limitation is found in etherNet (~100m) and 802.11 and is due 
>to a sending station waiting only a small amount of time to receive 
>an ack/nak before it concludes that the message wasn't received...

In any case, you'll probably need a USB controller at the end of the
USB cable.  Past that point, well, it depends upon how the USB
controller is programmed (In other words, we're basically back to 
where we started from.).

>BTW, if what you want to do is constant transmissions via a KISS-mode 
>TNC, take a look at JNOS. Frankly, I'm too lazy to look up if JNOS as 
>is will do constant transmissions. However, the source code for JNOS 
>is freely available to Hams for non-commercial use, so you are 
>welcome to modify it so it will do whatever it is you want.
>
>It seems like this thread is about replacing complicated soundcard PC 
>software with equally complicated serial port PC software. Why not 
>build a PWM modulator out of something like a pair of 555 timers? 
>This way you have a (after integration at the receiver) a linear 
>analog channel with significant bandwidth. If you build the same 
>system with hardware that is faster than a 555, you can have a linear 
>analog channel with a bandwidth that is tens of megahertz if not hundreds.

But, I think a lot of us want a digital channel, not an analog channel.

>73 de Glenn wb6w

Dave
WA4QAL