Siemens 6ES72121BE400XB0 — SIMATIC S7-1200 Compact CPU (AC/DC/Relay)
Catalog comparison
The 6ES72121BE400XB0 is an entry-level compact CPU in the SIMATIC S7-1200 family, distinguished by its relay-output configuration. The following table compares it against in-catalog peers sharing the same I/O envelope or functional class.
| Parameter | 6ES72121BE400XB0 (seed) | 6ES72121AE400XB0 | 6ES72121AF400XB0 | 6ES72121BE344YB0 |
|---|---|---|---|---|
| Output type | 6 – Relay | 6 – Pulse (4) | 6 – Pulse (4) | Expandable |
| Inputs | 8 – Digital | 8 – Digital | 8 – Digital | — |
| Analog channels | — | 2 AI | 2 AI | — |
| Memory | 75 KB | 75 KB | 100 KB | — |
| Display | None | None | None | None |
| Encapsulation | Box | Box | Box | Box |
| Mounting | Panel | Panel | Panel | DIN Rail |
| IP rating | IP20 | IP20 | IP20 | IP20 |
| Operating temp. | 20°C – 60°C | 20°C – 60°C | 0°C – 55°C | — |
| Approvals | CE, cULus, FM, KC, RCM, UL | — | CE, cULus, FM, KC, RCM, UL | — |
| Similarity | — | 0.92 | 0.92 | 0.85 |
Why these parts are similar
6ES72121AE400XB0 shares the same S7-1200 CPU 1212C form factor and 8-digital/6-output I/O count. The critical difference is output type: the AE400 variant uses solid-state pulse outputs (4 high-speed), whereas the seed carries six relay contacts, making it suitable for switching AC loads or low-voltage DC without SSR modules.
6ES72121AF400XB0 belongs to the CPU 1212FC variant, sharing the same I/O count but offering 100 KB of memory (versus 75 KB) and a tighter operating range of 0°C – 55°C. Like the AE400, its outputs are pulse-type, not relay.
6ES72121BE344YB0 is a starter kit in the same SIMATIC S7-1200 series. It differs by its DIN Rail mounting form factor and expandable architecture rather than the fixed 8/6 I/O compact layout of the seed.
Technical specifications
| Specification | Value |
|---|---|
| Series | SIMATIC S7-1200 |
| Product type | CPU 1212C |
| Supply class | AC/DC/Relay |
| Digital inputs | 8 |
| Relay outputs | 6 |
| User memory | 75 KB |
| Display | None |
| Expandable | Non-expandable |
| Mounting | Panel mount |
| Encapsulation | Box |
| Ingress protection | IP20 |
| Operating temperature | 20°C to 60°C |
| Approvals | CE, cULus, FM, KC, RCM, UL |
Key selection considerations
- Relay output architecture: The AC/DC/Relay supply class provides six electromechanical relay contacts. This eliminates the need for external SSR modules when driving AC loads or resistive DC circuits, but relay lifecycle and contact rating should be evaluated against load type and switching frequency.
- No onboard display: Operated and configured via TIA Portal software; the compact CPU relies on Ethernet or serial communication for commissioning and monitoring.
- Fixed I/O, non-expandable: The 8 digital inputs and 6 relay outputs are factory-configured. Applications requiring additional I/O or analog channels must move to a higher CPU type (e.g., CPU 1214C or CPU 1215C) or use external signal modules on a separate compact variant.
- Memory: 75 KB is the smallest S7-1200 CPU memory footprint; program complexity and data logging scope should be validated against this limit.
- Operating temperature range: 20°C to 60°C is appropriate for enclosed cabinet installations without active cooling in most industrial environments.
- Approvals: CE, cULus, FM, KC, and RCM cover major global markets; the part is suitable for export-heavy OEM panel builds.
Family context
The 6ES72121BE400XB0 follows Siemens ordering conventions for the S7-1200 compact CPU line: the "C" suffix denotes the compact variant, "AC/DC/RELAY" specifies the supply and output architecture, and the trailing "XB0" indicates the firmware and hardware release. The same base number with an "AE" prefix signals a DC-input/pulse-output variant, while "AF" denotes the FC (fail-safe compact) sub-family. Buyers specifying for a coordinated S7-1200 project should verify that the selected variant's output type aligns with field device coils and the branch coordination scheme.
FAQ
What is the difference between 6ES72121BE400XB0 and 6ES72121AE400XB0? The seed has six relay contacts; the AE400 variant has six pulse outputs (four high-speed). Relay outputs are suitable for AC loads and provide galvanic isolation without external switching hardware.
Can the 6ES72121BE400XB0 be expanded with additional I/O modules? No. The CPU 1212C compact form factor is non-expandable. If additional I/O is required, a larger CPU model (1214C or 1215C) should be considered.
What memory capacity does 6ES72121BE400XB0 provide? The unit offers 75 KB of user memory for program code and data.