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.

Figure 4 shows the efficiency for the current-loop solution shown in Figure 2, with the 5.5-V OUT output rail tied to the BIAS pin of the LT8618.

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 ...