This usually indicates a ground issue. The printer probably has a grounded plug in which case installing a ground lifter will usually solve the problem.

The RFC-1 power supply must be floating for the telemetry system to function properly. Parallel printers should have separate grounds for data and frame. However, some printers connect the two ground points and cause this to happen.

The RAK-1 uses optical isolation and other techniques to avoid this condtion but it occurs at a small number of installations.