27 Aug
2011
27 Aug
'11
10:49 a.m.
Never seen a USB-20mA (for example). Everything seems
to be based on
voltage thresholds these days, and the shortcomings of that were brought
Is it? I've seen analogue transducers with a 4-20mA analogue output (i.e.
a current source output) very recently. I assume they still exist.
home to me today in discussion with a building
management systems
installer advising that the signaling from a leak detector would not
travel several hundred feet without an (expensive) booster box at each
end. We're not talking about fast baud rates here, just some remote
sensing device with an output consisting of voltage-free relay contacts,
feeding equipment that provides a voltage to distinguish "on" this
morning from "off" this afternoon. The cable resistance has an inherent
voltage drop of course, so they can't guarantee that closing the relay
contacts will cause the voltage to drop sufficiently at the active
equipment end. A current-sensing input could deal with that no problem,
but they'd never come across the concept of current-sensing.
ARGH!. But that's what you get nowadays. So-called 'engineers' who belive
problems are solved by sticking complex modules toghter and who don't
(and many can't) really think about the probkem. I've see it all too
often.
-tony