Current loop serial data transmission
- an overview of the current loop technique that was
used for the transmission of speed serial data
Although the RS-232C interface standard was devised with
voltage signalling in mind, some equipments including older teleprinters and the
like use a current loop form of interface.
Instead of using the voltage levels for signalling, the
current loop system uses a current that is switched on and off. This has the
advantage for a long line with losses that a current generator can be used and
this will keep its levels whatever the resistance involved. Normally a current
of 20 mA is used, although sometimes a 60 mA current can be used. This technique
can also have the advantage that the current loop can be isolated from ground
using opto-couplers.
As current loop signalling is not defined under RS-232C,
there can be some ambiguity in which equipment supplies the current source.
Often one will be available at either end, but it is necessary to decide which
end will provide the current when making up the lead for the installation. This
current source can be created by simply having a fixed resistor from a 12 volt
source in the equipment.
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