[GreenKeys] RS232 to current loop dongle

[Jeffrey D Angus]

On 12/8/2014 8:16 AM, digitalham at gmail.com wrote:
> Anyone used this or a similar device to talk to their loop from a PC?
> This one isn't that expensive. I'm wondering if I should give it a try.
Personally, I don't see $42 as "inexpensive."
> http://datainterfaces.com/rs232_female_current_loop_interface_converter.aspx
The real issue here is what voltage can the loop side handle? From the
looks of it, since there are NO levels specified in the data or application
sheet, I'm thinking it's limited to maybe 15-20 volts. Looks like hitting it
with a 120-170 v loop supply and the resultant selector magnet spikes
would be sudden death and possible catastrophic failure to the PC.

A side note on TTY circuits. The distance a 60 mA loop is capable of is
limited by the total loop resistance. For 24 AWG wire, this is roughly
50 ohms/1000 ft. (for a pair) For a 120 v loop supply, the total loop
resistance has to be under 2000 ohms. This translates to just under 7.5
miles. Hardly the 0.6 miles the above mentioned converter is rated at.
Raising the loop voltage to 170 v could increase the range by 40%.
Interestingly enough, the distance ratio is consistent with 120 volts vs
the 9 volts supplied by the external supply.

By the way, the "Black box" converters mentioned previously are NOT
rated at the full 120-170 volt loop supplies, but are limited to about 35
volts. Which also limits their range and the ability to deal with "old 
school"
hardware running the considerably higher loop voltages.





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