The 6ES7138-6AA01-2BA0: This ET 200SP slice takes a 24 V feed with L+ tolerance down to 23.2 V and sips 1 W typ. It fits a 15 mm bay in BU type A0 — no extra DIN rail gymnastics. Input side handles 16 byte of I/O, with 4 byte switched to fast mode for encoder counts that need sub-50 µs edge resolution. Outputs carry 12 byte total, with 4 byte reserved for motion control; fast mode zeroes the output byte allocation. For signal levels: a '0' lives between -5 V and +5 V; '1' needs +11 to +30 V. That gives headroom on 24 V sensor returns even after a 0.8 V drop. Wire-break and short-circuit detection are baked in, and the group error alarm fires without an extra diagnostic module.
Integration and tie-down
Mounts on BaseUnit type A0 — that's the key mechanical constraint. Depth is 58 mm, width 15 mm, height 73 mm. Wait until the 24 V rail is stable before slotting; the module's input threshold below 11 V for a '1' means a sagging bus during panel power-up won't cause false counts. Cable runs: shielded max 1 000 m, unshielded 600 m; longer runs need a line driver or repeater. The module populates 16 input byte and 12 output byte in standard mapping.