[Laser] serial port PDM and PWM

[Chuck Hast]

On 12/7/06, Art <KY1K at verizon.net> wrote:
> I'm not sure how useful a data link will be for weak signals. And, if
> the signals are strong, you probably want to just do USB to USB anyway::>
>
No, this is not weak signal, it is all LOS, the idea is to look at
alternate links
and optical links are a interesting alternative. If I can get 3-5
miles out of them
I am good. My next step is to test a 3 mile link between my home and a building
in down town Tampa.

> But, I did set up tnc's over laser years ago. There is a problem that
> might not seem obvious to those who haven't tried it.
>
> If you use packet tnc's, you will need software for the computers. To
> the best of my knowledge, there is no kiss based software that will
> allow you to send a constant stream of beacons or connect attempts.
> If you use cmd mode, the most often you can make a connect attempt is
> once per second and once every 10 seconds for beacons. That's becasue
> these are made for shared channels, and the vendors don't give you
> absolute control of the duty cycle on trasnmit.
>
Here in Florida we are in the process of changing our packet switches to
Linux based machines, the network is something called FPAC and one
of the test features is the ability to send a continuous string of data or run
full dux links. I can make the thing send a beacon once/sec or even faster,
or I can cause it to send a continous string of packets, the tools have all
manner of softknobs to twist to I can get a constant stream if I so need.
For the TNC based test, replace the eprom in the TNC (needs to be a
TNC 2 type machine) with a 6pack eprom and you can make it do all
of that fast stuff at whatever timing you need. By using 6pack or KISS
all of the timing parameters are passed to the PC and so you are no
longer tied to the TNC internal parameters.

> So, just beware that alignment and signal acquisition will be limited
> by the hardware and you'll spend long periods standing around waiting
> for the units to transmit.
>
See above, since these links will be full dux anyhow (there should be
no reason not to run them FDX) I can insert commands in the data stream
to the far end and control it too.

> On the other hand, USB might be more practical. The usb transceivers
> are simple plug in units with the transmitter LED under one 'bubble'
> and the receiver under the same type of 'bubble' on opposite ends of
> a DIP chip package.
>
Which USB device are you talking about here? I have thought about
getting my hands on a couple of the cheap RS-232 comm port devices,
the ones that in many cases only have TXD/RXD and ground active
and using two of them as the data source to run the laser TX/RX head.

> The receiver can be used as is.....just focus your receiver lens on
> the bubble. For the transmitter, I think you can mount a high speed
> photodiode over the LED bubble to intercept the transmit data, and
> then use that output to drive your laser transmitter.
>
I am interested in the device you are describing, is it like a IR transceiver?

> I'm not sure how far you can go on USB. But, I know that 10 feet of
> cable is about as far as you can go when using hardwired links over
> USB. So, you probably need a honkin' big signal to make USB work from
> computer to computer.
>

The first step is just to use the serial port, it could be a USB serial port
or the real thing (getting rarer and rarer these days). The trick is to get the
bits into the laser and out of the receive end. Then see just how far it will
talk at reasonable data link speeds. Remember that most packet stuff
(for those of you who are still doing it) is fast if it is above
9k6... So if I can
get it to go at 96kb then I will be happy. Of course we are running parallel
radio links, but getting these things going is just part of amateur radio, and
a part that a lot of hams do not think of, but it is all RF, just in some cases
high enough to see B=]. Besides if I can get a few miles out of a laser as
opposed to a RF link well, it is one more path I have available in the event
that we need to have data links.

Also some will point out the issues with optical links such as rain, fog etc,
well if the sites are still standing the air after a hurricane (the major issue
in Florida) is usually clear and rather dry, so as long as the two sites can
see each other they should be able to pass data. I also figure that the size
of the piece exposed to the weather is smaller for the optical device as opposed
to the RF device, antennas in order to get the gain at lower frequencies tend
to be big wind catchers.



-- 
Chuck Hast  -- KP4DJT --
To paraphrase my flight instructor;
"the only dumb question is the one you DID NOT ask resulting in my going
out and having to identify your bits and pieces in the midst of torn
and twisted metal."