The Siemens 6ES7921-3AH20-1AA0 is a 40-pole front connector for the SIMATIC S7-300 system, designed to terminate I/O module wiring via crimp contacts. It accepts stranded conductors from 0.5 to 0.75 mm² and is rated at 60 V / 1 A. The connector mounts directly onto the S7-300 rail and positions the terminals at the top for straightforward panel wiring.
The connector uses a crimp-contact termination for the power supply and a plug-in connection for the I/O cable. The 40-pole IDC interface at the first connector position mates with the S7-300 I/O module's backplane. Maximum cable length between the front connector module and the terminal module is 30 m. The insulating body material is listed as 'other' — not standard thermoplastic — so verify compatibility with your panel's environmental rating if the assembly sees aggressive solvents or continuous elevated humidity.
The connector is supplied without crimp contacts, so order those separately. VPE is 100 units, which matters for project planning: one bag covers a rack of 20 modules with two spare connectors.