The Siemens 6ES7921-3AK20-0AA0 is a front connector module for the SIMATIC S7-300 system, specifically designed for the Compact CPU 312. It provides the physical interface between the CPU and field wiring, carrying up to 1 A per contact at 60 V DC across 40 poles. The module accepts stranded conductors from 0.25 to 0.75 mm² with a stripped length of 6 mm, and uses screw-type terminals for the power supply connection with a plug-in connection for the connecting cable. This direct-mounting part snaps onto the S7-300 rail alongside the CPU.
Rated at 60 V DC and 1 A per contact, with a total current capacity of 4 A across all terminals. The terminal configuration is 2 x 2 x L+ and 2 x 2 x M — eight terminals total, arranged for dual supply feeds. This means the module can distribute power to the CPU and a small peripheral load without an external distribution block. The 30 m maximum cable run between this module and the terminal module gives flexibility for remote I/O placement within a cabinet.
For BOM freeze or long-term support planning, the mature status means Siemens continues to manufacture and support the 6ES7921-3AK20-0AA0. No successor order code has been announced.
Direct mounting on the S7-300 rail — no end cover plate required. The front connector sits at the top of the module, with a 40-pole IDC connector on the first interface for the connection cable. Tightening torque for the screw terminals is 0.4 to 0.7 N·m, using a 3.5 mm screwdriver blade. Wire preparation: strip 6 mm, use end sleeves 5 to 7 mm long without plastic collar per DIN 46228-1. The insulation outer diameter at the supply terminal connection is 2 mm max. Up to two wires per connection are allowed, as long as the total cross-section fits within the ferrule.