Long Circuits
Analog current loops were occasionally carried between buildings by dry pairs in telephone cables leased from the local telephone company. 4–20 mA loops were more common in the days of analog telephony. These circuits require end-to-end direct current (DC) continuity. DC continuity is not available over a microwave radio, optical fiber, or a multiplexed telephone circuit connection.
Basic DC circuit theory shows that the current is the same all along the line. It was common to see 4–20 mA circuits that had loop lengths in miles or circuits working over telephone cable pairs that were longer than ten thousand feet end-to-end. There are still legacy systems in place using this technology. In Bell System circuits, voltages up to 125 VDC were employed.
Read more about this topic: Current Loop
Famous quotes containing the words long and/or circuits:
“Nor now the long light on the sea—
And here face downward in the sun
To feel how swift how secretly
The shadow of the night comes on . . .”
—Archibald MacLeish (1892–1982)
“The Buddha, the Godhead, resides quite as comfortably in the circuits of a digital computer or the gears of a cycle transmission as he does at the top of a mountain or in the petals of a flower.”
—Robert M. Pirsig (b. 1928)