What the slice does
The Siemens 6ES7138-4DB030 is a SIMATIC DP electronics module for ET 200S that reads an SSI absolute encoder and returns its position word to a PROFIBUS-DP master. It is a single-width slice, 1.5 x 8.1 x 5.2 cm, that drops into an ET 200S rack beside the standard power and interface modules. The 24 V encoder supply is fed out to the transducer and the module reports short-circuit protection on that output, so a wiring fault on the field side will not collapse the backplane.
Timing, cable reach, and the specs that actually govern fit
The position-decode window is set by the monoflop time, selectable at 16/32/48/64 µs, and that figure plus the 125 kHz SSI rate cap the maximum shielded cable length at 320 m. The supply rail tolerates the L+ drop-out to L+ (-0.8 V), so derate the panel-side 24 V budget by that margin when more than one slice shares a feed. Power dissipation is 1 W typical, which is what to enter into the ET 200S thermal calculation for the station.
Sourcing posture and lifecycle
Hold one in the cabinet for any ET 200S station that depends on it — an SSI slice failure does not give the operator a graceful fallback, and the encoder stops reporting position until the module is swapped.