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When copying a URL to external programs, make sure that you are using `Edit > Copy As > Plain Text` (Cmnd-Shift-C on Mac)... or otherwise remove the line continuation characters ("`\`") that the Front End inserts in the copied text. As yet...
Sorry for being late to the party. To reduce overhead, try using  DeviceWrite on a pre-discovered device, and once per array: $gpio = DeviceOpen["GPIO"]; setStep[array_] := DeviceWrite[$gpio, Thread[$coils -> array]]