[GreenKeys] TTY-Connect boards/micros all gone (maybe)

[Bob Camp]

Hi

The simplest example of a polar loop is good old RS-232. Instead of  
on/off (current / no current) in the loop you have positive/negative in  
the loop.

The idea goes *way* back like to the 1830's or so.  The original polar  
circuits ran by switching the loop battery back and forth on the single  
wire telegraph line. The reception end then just looked for positive /  
negative relative to ground. This was by no means a universal practice  
but there were some circuits set up that way.

Once they started doing things like submarine cables (coax ....) the  
whole issue of capacitance / inductance and bandwidth got real  
important in a big hurry. They found that you simply could not key an  
undersea cable with on/off keying. You put a voltage on one end and it  
might stop rising on the other end the next day (at least that was the  
story ...).  Polar circuits turned out to be the only answer that  
worked. A couple of Scottish guys figured out termination impedance and  
then things started to work a bit better still.....

Since this was still the mid 1800's tubes and transistors were a bit in  
the future. All this stuff had to be done with relays and batteries.  
Rather than looking at a relay as a voltage driven part the tradition  
was to look at it like a current driven part. This made a lot of sense  
if you had it on a great big long chunk of wire with who knows how much  
resistance involved.

A polar circuit always makes a decision as close to zero as it possibly  
can on the receive end. Since you load the thing with a resistance it  
does not matter a whole lot weather you call it zero voltage or zero  
current. This is pretty close to what a RS-232 computer interface does.  
The only difference is that the actual zero voltage point is defined in  
RS-232 rather than being ambiguous in a polar loop.

With a polar circuit on the transmit end you always supply a balanced  
feed to the line. The mark voltage (or current) is always the same  
magnitude but opposite in sign compared to the space voltage. Since we  
know the characteristic impedance of the line we match the source  
impedance of the drive as best we can to make things go as fast as  
possible.

The net result is that if you are going to try for a good long wire  
circuit a polar system is a better bet than a current loop. You have  
some other ways to go today but that's the best choice with pre 1920's  
technology.

Hopefully that helps a little ....

	Enjoy!

		Bob Camp
		KB8TQ


On Jun 15, 2004, at 8:42 PM, gil smith wrote:

> Hi folks:
>
> Looks like the boards are all spoken for.  I am going to check with  
> the pcb house tomorrow to see if there is still time to up the  
> quantity, and I'll let everyone know.
>
> thanks,
>
> gil
>
>
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