What this reserve module actually does in an ET 200S rack
The 6ES7138-4AA01-0AA0 is a mechanical placeholder for unused slots in a SIMATIC DP ET 200S distributed I/O station — it physically fills the gap so the bus architecture stays consistent and dust doesn't collect in empty positions. It carries zero digital inputs and draws only 0.025 W typ. power loss — negligible thermal impact on the enclosure. The 15 mm width matches one standard ET 200S module slot. Each order unit is 5 pieces per packing unit, so a single line item covers five empty slots in the rack. No wiring, no configuration — just snap it onto the DIN rail.
The 15 mm width is critical: if your BOM calls for a populated module at slot 4 but the actual I/O count doesn't need it, this reserve unit fills the mechanical gap and keeps the backplane bus termination correct.
Sourced to order against the BOM quantity through independent distribution.