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The vast majority of field instruments are connected to controllers by the analog 4 mA to 20 mA current loop. It is the original system of communication between industrial devices many years before digital electronics became a mainstream possibility. As such, it has a powerful incumbent position, because it does not make sense to re-wire factories for digital communications systems, as the costs associated with downtime and re-training are prohibitive. 4 mA marks the minimum range and 20 mA marks the maximum range. The primary advantage of the analog current loop is that the accuracy of the signal is independent of voltage drops in the wiring, thus enabling long current loops to be laid in order to reach more remote process points. The loop can also carry power to the field instruments, allowing them to operate remotely.