KEYENCE FS-N10 Series - Manual (Page 39)

3-4 Setting the Current Received Light Intensity to 0 (Zero Shift)
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3-17 - Digital Fiber Sensor FS-N10 Series User's Manual -
The zero shift function cannot be used in combination with the preset
function. To use the zero shift function, make sure that the preset
function is disabled (the PST indicator is not lit).
Zero shift of the expansion unit can be set from the main unit.
"Common Key-Operations Function" (page 4-34)
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Reference