The current loop is, or was, the standard way of connecting a teletype to anything, so all [Marek] had to do was construct a box that translated the signals from a GSM modem to this current loop.

Closing the current loop. Closing the current loop of a totem-pole bridgeless PFC is not as straightforward as in a traditional PFC; it may also vary from controller to controller. This power tip can ...

Over long cable runs, an isolated 4-to-20-mA current loop offers robust noise immunity and tolerance. This characteristic makes it popular for analog data transmission in noisy industrial and ...