The Siemens 6ES7138-6BA00-0BA0 is a SIMATIC ET 200SP technology module designed for high-speed counting, measurement, and position acquisition. Siemens names it TM PosInput 1, and its job is simple to describe but surprisingly powerful in practice: it turns encoder signals into clean, deterministic data that your PLC can use for motion and measurement tasks—without adding a separate counter card or a dedicated motion module in another cabinet.
This module supports both incremental encoders (RS-422 differential signals, and also TTL 5 V push-pull for suitable encoders) and SSI absolute encoders via RS-422. That means it can serve as a flexible “position front-end” for common automation needs such as indexing, length measurement, speed monitoring, web handling, packaging synchronization, and general machine position feedback—especially when you want compact I/O with precise timing.
A key point that separates TM PosInput 1 from ordinary digital input modules is performance: it is built for high-frequency pulse processing and for synchronized (isochronous) operation. For incremental signals it supports up to 1 MHz input frequency and up to 4 MHz counting frequency with quadruple evaluation, which is exactly what you want when a fast encoder meets a fast machine.
It also includes 2 configurable digital inputs (DI) and 2 transistor digital outputs (DQ) for auxiliary functions like gating, capture triggers, or simple control tasks close to the sensor.
One practical reality you should know: Siemens lists this specific order number as a spare part, and the main product is discontinued with a successor available. That does not make it “bad”; it simply changes how you plan sourcing and long-term standardization in a project.
Technical Overview (Specification Table)
| Item | Specification |
|---|---|
| Part number | 6ES7138-6BA00-0BA0 |
| Product name / type | SIMATIC ET 200SP TM PosInput 1 (counter and position decoder module) |
| Supported encoder types | RS-422 incremental encoder, TTL 5 V (push-pull only), SSI absolute encoder (RS-422) |
| Incremental input frequency | Max. 1 MHz |
| Counting frequency (quad evaluation) | Max. 4 MHz |
| SSI telegram length | 10…40 bit (parameterizable) |
| SSI clock frequency | Up to 2 MHz (125 kHz to 2 MHz selectable) |
| Digital inputs / outputs | 2 DI (24 V DC), 2 DQ transistor outputs |
| Encoder supply | 24 V encoder supply, max 300 mA, electronic/thermal protection |
| Isochronous mode | Supported |
| Power loss | Typ. 1.9 W |
| Address space per module | Inputs 16 bytes (Fast mode: 4 bytes); Outputs 12 bytes (Motion Control: 4 bytes, Fast mode: 0 bytes) |
| Dimensions (W×H×D) | 15 mm × 73 mm × 58 mm |
| Weight | 50 g |
Encoder Interfaces and Signal Capabilities (Engineering-Focused Table)
| Function area | What TM PosInput 1 supports | Why it matters in real machines |
|---|---|---|
| Incremental encoder (differential) | RS-422 input, 1 MHz input frequency, 4 MHz counting (quad) | Keeps up with high-resolution encoders at high speed while maintaining direction and edge integrity. |
| Incremental encoder (single-ended) | TTL 5 V for push-pull encoders, same frequency class | Useful when legacy encoders are single-ended but you still need fast counting. |
| SSI absolute encoder | RS-422 SSI, 10–40 bit frames, clock up to 2 MHz, binary or Gray, parity option | Lets you read absolute position directly (no homing sequence required in some designs), with configurable framing and error checking. |
| SSI cable length guidance | Up to 320 m shielded at lower clock, down to 8–20 m at highest clocks (example values provided) | Makes the trade-off explicit: longer runs generally require lower clock rates for robust communication. |
| Diagnostics for encoder quality | A/B transition error (incremental), telegram error (SSI), wire-break, short-circuit, group error | Fewer “ghost faults” during commissioning; faster troubleshooting when wiring or signal quality is the problem. |
Built for Deterministic Timing: Isochronous Operation and Data Consistency
When machines get fast, the sneaky enemy is timing jitter—your encoder is honest, but the system reads it at inconsistent moments. TM PosInput 1 supports isochronous mode, and Siemens documentation notes that in isochronous operation you can maintain data consistency across the feedback interface within the bus cycle concept.
In practical terms, that can improve the quality of speed estimation, position sampling, and synchronized events. If you are coordinating cut-to-length, registration marks, label placement, or synchronized transport, “consistent sampling” is one of those boring phrases that quietly prevents expensive scrap.
Digital I/O for Local Control: 2 DI + 2 DQ (Not an Afterthought)
Besides the encoder channel itself, the module includes two 24 V DC digital inputs and two transistor digital outputs. The digital input thresholds and timing options are clearly defined: 24 V rated, with “0” and “1” voltage windows, and parameterizable input delays (including very small minimum times when set to “none”).
For the digital outputs, Siemens specifies transistor outputs with electronic/thermal short-circuit protection and a typical response threshold around 1 A, plus a module-level current limit reference in preserved specification data. These outputs are commonly used for gating, triggering capture actions, or simple control tasks that benefit from being physically close to the sensor/encoder wiring.
Motion Control Integration: Position Input for S7-1500 Motion Control
Siemens explicitly describes using this technology module as a position input in a Motion Control context, including guidance that it can be used for position input with S7-1500 Motion Control, depending on how you configure it in STEP 7 (TIA Portal). This makes the module a strong fit for architectures where the ET 200SP station sits near the mechanics while the controller remains centralized.
Typical Applications
This module is a practical choice when you need accurate encoder-based feedback but want to keep your system modular:
Packaging and converting lines: measuring web speed, registration, synchronization.
Conveyors and material handling: length measurement, slip detection, speed supervision.
Assembly and indexing machines: position feedback for indexing tables or feeder systems.
Test stands: frequency and period measurement, repeatable sampling, structured diagnostics.
Because it includes both high-speed counting and measurement functions (with specified measurement ranges and accuracy characteristics in the datasheet family), it is also attractive in machines where you need both “position” and “measurement” without adding multiple hardware modules.
Conclusion
The Siemens 6ES7138-6BA00-0BA0 (ET 200SP TM PosInput 1) is a compact, high-performance technology module for encoder-based counting and position acquisition, supporting RS-422 incremental encoders, TTL push-pull incremental encoders, and SSI absolute encoders, with up to 4 MHz counting (quad evaluation) and isochronous capability for more deterministic timing.
