The Siemens 6ES7521-1BH10-0AA0 is a SIMATIC S7-1500 digital input module with the product type designation DI 16 x 24 V DC BA. It provides 16 digital inputs, arranged in one group of 16, and is designed for 24 V DC signals with an input characteristic curve that complies with IEC 61131 Type 3. In practical terms, this is a mainstream, cabinet-friendly input module used to acquire signals from proximity sensors, photoelectric sensors, pressure switches, limit switches, relay contacts, and other 24 V DC field devices in S7-1500 architectures.
What makes this module a solid choice in automation cabinets
Clear thresholds for “0” and “1” signals
Field wiring is messy, so thresholds matter. Siemens specifies the input voltage ranges used to interpret logic states: for signal “0” it lists -30 to +5 V, and for signal “1” it lists +11 to +30 V (at rated 24 V DC). That gap between +5 V and +11 V provides noise margin that helps prevent borderline voltages from being misread during switching transients or long cable runs.
Deterministic input delay (debounce / filtering behavior)
For standard inputs, the module’s input delay is not parameterizable and is specified at 3 to 4 ms for both rising and falling edges (0→1 and 1→0) at rated input voltage. This is a practical “set-and-forget” delay that helps suppress short pulses and contact bounce without requiring per-channel tuning.
Designed for long cable runs and common sensor types
Siemens lists maximum cable lengths of 1000 m shielded and 600 m unshielded. It also states 2-wire sensors are connectable, with a maximum permissible quiescent current of 1.5 mA for a 2-wire sensor. Those details matter when you’re mixing classic 2-wire proximity sensors with modern three-wire devices across different machine generations.
Technical Specifications (from Siemens datasheet)
| Item | Specification |
|---|---|
| Order number | 6ES7521-1BH10-0AA0 |
| Product type designation | DI 16 x 24 V DC BA |
| Number of digital inputs | 16 |
| Channel grouping | 16 channels in groups of 16 |
| Input characteristic | IEC 61131, Type 3 |
| Rated supply voltage | 24 V DC |
| Permissible supply range | 19.2 to 28.8 V DC |
| Input voltage thresholds | “0”: -30 to +5 V, “1”: +11 to +30 V |
| Input current (signal “1”) | Typical 2.7 mA |
| Input delay (standard inputs) | 3–4 ms (0→1), 3–4 ms (1→0), not parameterizable |
| Max cable length | Shielded 1000 m; unshielded 600 m |
| 2-wire sensor support | Yes; quiescent current max 1.5 mA |
| Potential isolation | Channels to backplane bus: Yes; between channels: No (group of 16) |
| Isolation test | 707 V DC (type test) |
| Engineering software | TIA Portal from V13; STEP 7 from V5.5 SP3 |
| Fieldbus integration notes | PROFIBUS GSD V1.0 / V5.1; PROFINET GSD V2.3 |
| Diagnostics / alarms | Diagnostics function: No; wire-break: No; short-circuit: No |
| LED indication | RUN LED (green), ERROR LED (red), channel status LEDs (green) |
| Power consumption | Backplane bus 1.05 W; power loss typ. 1.8 W |
| Dimensions (W×H×D) | 25 mm × 150 mm × 129 mm |
| Weight | 316 g |
| Delivery contents | Supplied incl. 40-pole push-in front connector |
| Safety functions | Suitable for safety functions: No |
How it performs in typical applications
Sensor inputs in packaging, assembly, and material handling
Most 24 V DC sensors produce clean switching, but real installations add noise: inductive loads switching nearby, long cable trays, and imperfect grounding. The Type 3 input characteristic and defined thresholds help keep signal interpretation consistent. The fixed 3–4 ms delay is also well suited to typical mechanical switch bounce and short interference spikes.
Limit switches and relay contacts in legacy-retrofit machines
When you retrofit older machines into an S7-1500 cabinet, you often inherit dry contacts and relay logic. A stable input delay plus clear “0/1” voltage regions reduces the chance that a contact’s bounce becomes multiple “events” in software, especially when your PLC logic counts edges.
Long cable runs to field devices
Siemens explicitly provides max cable lengths for shielded and unshielded wiring. That doesn’t mean you should always push the maximum, but it’s valuable engineering data when you have remote sensors on large equipment (conveyors, cranes, long transfer lines) and you need to estimate signal integrity margins.
Installation and commissioning notes that prevent headaches
| Topic | Practical guidance | Why it helps |
|---|---|---|
| Use the included push-in connector | The module ships with a 40-pole push-in front connector | Faster wiring, consistent contact quality, less rework |
| Understand diagnostic expectations | Many advanced diagnostic features are not supported (wire-break: No, short-circuit: No) | Avoid designing a maintenance strategy that assumes per-channel fault detection |
| Plan for the fixed input delay | Delay is not parameterizable, 3–4 ms | Edge timing and high-speed pulse capture should use different hardware if needed |
| Use shielding where EMC is tough | Siemens provides different length limits for shielded vs unshielded wiring | Better noise immunity and fewer intermittent “ghost inputs” |
If your application depends on very fast pulse measurement or high-frequency counting, the fixed 3–4 ms delay is the big clue that this module is meant for standard discrete sensing, not high-speed metrology. In those cases, a dedicated high-speed counter module or technology input approach is usually the right tool.
Environmental and mechanical suitability
Cabinet space and thermal planning are the unglamorous parts of automation that keep systems alive. This module is a slim 25 mm wide and dissipates a typical power loss of 1.8 W, with 1.05 W drawn from the backplane bus. Siemens lists operating temperature limits up to 60 °C in horizontal installation (and up to 40 °C in vertical installation), as well as altitude guidance up to 5000 m with restrictions above 2000 m. These are useful when you’re building high-density panels or shipping skids to higher-altitude facilities.
Summary
The Siemens 6ES7521-1BH10-0AA0 is a SIMATIC S7-1500 DI 16 x 24 V DC BA module providing 16 digital inputs in a group of 16, with IEC 61131 Type 3 input behavior, defined “0/1” voltage ranges, and a fixed 3–4 ms input delay that suits mainstream industrial sensing. It ships with a 40-pole push-in connector, offers clear LED status indication, and fits neatly into modern S7-1500 cabinets where reliable 24 V DC signal acquisition matters more than fancy diagnostics.
