What it is and why it matters
This is the Siemens 6ES7321-1CH20-0AA0, a 16-point digital input module from the S7-300 SM 321 family, designed for 48-125 V DC control circuits. It accepts two-wire sensors and provides galvanic isolation in groups of 8 channels via optocouplers — a critical detail when wiring field devices that share a common potential on separate groups. Signal thresholds are clearly defined: a logic '0' spans -146 V to +15 V DC, while a logic '1' requires 30 V to 146 V DC (typical input current 3.5 mA). Switching times are in the sub‑millisecond range, with minimum rise/fall below 0.1 ms.
Lifecycle reality — act now for spares
There is no official Siemens replacement listed for this order code; any functional equivalent would need to accept 48-125 V DC and maintain the same 20-pin front connector. We source this module on a quoted-to-order basis — submit an RFQ and we will confirm availability and lead time from our surplus and redistribution network.
Mechanical fit and wiring notes
The module occupies a single S7-300 width: 40 mm wide, 125 mm high, 120 mm deep. It requires a 20-pin front connector (not included). Standard S7-300 rail mounting applies. Channels are in isolated groups of 8. Each group is optocoupler-isolated from the backplane bus. No diagnostic alarms, interrupts, or parameterization. Maximum cable lengths: 1 000 m shielded, 600 m unshielded. For two-wire sensors, the permissible quiescent current is limited to 1 mA.