[GreenKeys] [Bulk] Re: [Bulk] Re: A reliable current-loop-to-RS232 converter

[Jeffrey D Angus]

On 10/6/2015 6:46 PM, drlegendre . wrote:
> Cool work, Jeff - very tidy job.
Thankies
> Question.. why did you include the RS-232 to TTL module? It seems to
> be (almost) totally unnecessary.. For instance, I'm running the circuit
> from http://www.aetherltd.com/connecting.html and it connects the
> DB-9 directly to the optos.
No offense to John, but I don't trust an RS-232 level to be "right" every
time. The specification calls for a voltage level of 3-15 volts. Kind of 
hard
to guarantee you're not under or over driving the opto-coupler. Equally
iffy to guarantee proper output levels on the RS-232 as well.

However, John's application is the one that "doesn't require an external
supply to make it work." So it's quite popular.
> The latter circuit is used with 120V / 60mA @ 45.45 baud if that makes
> any difference, but it shouldn't.
Another difference with my design is using a bridge rectifier for the keying
sensing. Both the TX and RX on the loop side don't care about loop polarity.
There is a jumper on the bridge to change from 60 mA sensing to 20 mA
sensing.


-- 
Jeff-1.0
wa6fwi
http://www.foxsmercantile.com

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