[GreenKeys] PC loop current boards

[John Nagle]

On 31 Jan 2009, at 10:02 AM, Ralph Mowery wrote:
 > Here's the schematic for the pc boards I'd like to make.
 >
 > http://rtty.com/wa2hwj.jpg

    I'd suggest using an optoisolator in there.  That
design has a common ground between the high voltage
side and the low voltage side.  This effectively ties
a point in your current loop to the RS-232 ground line.
Back when everyone had desktops, the ground side of RS-232
usually was really grounded to protective ground.  That's not
true of laptops.

    Also, that circuit was designed for "classic RS-232",
with a swing from +12 to -12.  Today, it's +5 to -5.

    Tillson's proposed 4-component solution using a HSR312L
sounds more promising.  That's about what I'm doing, except
that I'm building a bidirectional board.

				John Nagle