For serial communication, the current loop is a communication interface that uses current not voltage for signaling. The current loop can be used for long distances (tens of kilometers), and can be connected to optically isolated links.
Video Digital current loop interface
Histori
Long before the RS-232 standard, the current loop is used to send digital data in serial form for teleprinters. More than two teleprints can be connected to a single circuit that allows simple network shapes. Older teleprinters use 60 mA current loops. Later machines, such as Teletype Model 33, operate at a lower 20 mA current level and most of the initial minicomputers feature a 20 mA current loop interface, with RS-232 ports generally available as more expensive options. The original IBM PC serial port card has provision for a 20 mA current loop.
A digital current loop uses absence of current for height (space or rest), and the presence of a current in the loop for a low (mark). This is done to ensure that under normal conditions there is always a flowing current and if there is a cut line, the flow stops indefinitely, immediately raising the alarm from the event usually because the heavy voice of the teleprinter is not synchronized, something that would be impossible if the silence does not exist current flowing.
Maps Digital current loop interface
Electrical characteristics
Maximum resistance to the current loop is limited by the available voltage. The current loop interface typically uses much higher voltages than those found on the RS-232 interface, and can not be connected to a voltage type input without some form of level translation circuit.
For full-duplex communication between two devices, two pairs of cables will be used. There is no general standard for the current loop interface, so details like time, connector, wire color code, and so on, are application specific.
See also
- MIDI, the digital loop current interface is limited to 5 milliamps and 5 volts.
References
Source of the article : Wikipedia
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