Product Overview

The Siemens 6ES7551-1AB00-0AB0 SIMATIC S7-1500 Counter and Position Detection Module, identified as TM PosInput 2, is a two-channel technology module for acquiring encoder signals, processing high-speed pulses, and measuring absolute or incremental position in SIMATIC S7-1500 automation systems.

Unlike a conventional counter module that primarily evaluates 24 V pulse signals, the TM PosInput 2 supports several encoder technologies, including RS422 differential signals, 5 V TTL signals, and SSI absolute encoder interfaces. This makes it suitable for machines that require accurate position feedback from rotating shafts, linear measuring systems, servo-driven mechanisms, and other motion-related equipment.

A typical application is a material feeding machine equipped with an incremental encoder. As the feeding roller rotates, the encoder generates A/B phase-shifted signals corresponding to the movement of the material. The 6ES7551-1AB00-0AB0 evaluates these signals and transfers the resulting position information to the S7-1500 CPU. The controller can use the measured position to coordinate feeding, cutting, and machine synchronization.

The module also supports SSI absolute encoders. Unlike incremental encoders, which determine position by evaluating movement pulses, an SSI encoder transmits an absolute position value through a synchronous serial interface. This is particularly useful in positioning applications where the machine needs to acquire the actual position directly rather than reconstructing it from accumulated pulses.

Each channel has dedicated digital inputs and outputs. The inputs can be used for functions such as hardware gating, capture, and synchronization, while the outputs can switch in response to configured comparison values. These functions allow certain time-critical reactions to take place directly in the technology module instead of depending entirely on the PLC program scan.

For example, in an automated cutting system, the module can monitor encoder position and switch an output when a predefined position is reached. The PLC can then coordinate the cutting mechanism with the material movement. This arrangement is useful when the machine needs repeatable position-based switching.

Another important capability is its integration with S7-1500 Motion Control technology objects. The module can provide position information to compatible motion applications, allowing the controller to use encoder feedback in positioning and movement-related tasks.

The TM PosInput 2 also supports frequency, period, and velocity measurement. Engineers can therefore use the same module for both position acquisition and monitoring the operating speed of rotating equipment.

With two independent channels, multiple encoder signals can be evaluated within one technology module. This is useful in machines with two measuring points, such as separate feed rollers, independent conveyor sections, or two rotating shafts.

For automation engineers working with S7-1500 systems, the 6ES7551-1AB00-0AB0 is a specialized interface for applications where encoder compatibility, position acquisition, and fast signal processing are more important than ordinary digital input functions.

Technical Specifications

Product Features

• Two Independent Channels: Processes encoder and pulse signals through two separate counting and position acquisition channels.
• Multiple Encoder Interfaces: Supports RS422 differential signals, 5 V TTL signals, and SSI absolute encoder communication.
• High-Speed Counting: Supports counting frequencies up to 4 MHz with quadruple evaluation.
• 32-Bit Position Range: Handles position and counting values within a configurable 32-bit range.
• Incremental Position Detection: Evaluates A/B phase-shifted signals to determine movement and direction.
• Absolute Position Acquisition: Reads position information directly from compatible SSI absolute encoders.
• Frequency and Period Measurement: Measures changing pulse frequencies and cycle durations.
• Velocity Measurement: Provides measurement information for monitoring rotating or moving equipment.
• Capture Function: Records the current position when a configured digital input event occurs.
• Synchronization: Aligns counting operations with external reference signals.
• Integrated Comparators: Provides two comparators per channel for position-dependent switching.
• Digital Input and Output Channels: Includes two digital inputs and two digital outputs per channel.
• Motion Control Compatibility: Supplies encoder position information to compatible S7-1500 Motion Control technology objects.
• Encoder Supply Protection: Provides 5 V and 24 V encoder supplies with short-circuit protection.
• Channel Diagnostics: Monitors encoder connections and identifies selected wiring and supply faults.

Working Principle

The 6ES7551-1AB00-0AB0 receives position and pulse information from connected encoders and processes these signals directly within the technology module. The resulting count values, measured positions, and diagnostic information are transferred to the S7-1500 CPU through the backplane bus.

The basic signal arrangement is:

Incremental / Absolute Encoder → TM PosInput 2 → S7-1500 Backplane Bus → CPU → Machine Control Program

The module supports two different approaches to position acquisition.

Incremental Encoder Operation

When an incremental encoder is connected, the module evaluates the A and B signals. These signals are phase-shifted by 90 degrees, allowing the module to determine both the number of pulses and the direction of movement. An additional N signal can be used as a reference signal where required.

For example, a conveyor equipped with an incremental encoder generates pulses as its drive roller rotates. The TM PosInput 2 evaluates these signals and updates the position value. The PLC can use the accumulated count to determine conveyor movement, track product location, or coordinate downstream operations.

The module also supports pulse encoders with direction signals and up/down counting signals. This allows it to handle different pulse-generation arrangements without requiring every application to use a conventional A/B incremental encoder.

SSI Absolute Encoder Operation

When an SSI absolute encoder is connected, the module reads position information through the synchronous serial interface. The encoder transmits its position value as a serial data frame, which the module evaluates and transfers to the controller.

The SSI frame length can be configured from 10 to 40 bits, with a maximum 32-bit position value transferred to the controller. Gray-code and dual-code SSI encoders are supported.

This arrangement is useful in positioning systems where the controller needs an absolute position measurement, such as a lifting mechanism, linear positioning axis, or rotary indexing table.

Capture and Synchronization

The capture function records the current position when a configured input edge occurs. For example, a reference sensor can trigger a capture event as a moving component passes a defined location. The recorded position can then be used by the PLC for measurement, inspection, or synchronization.

Synchronization allows the counter to align its counting operation with an external signal. This is useful when a machine needs to establish a reference position before beginning a production cycle.

Position-Based Output Switching

Each channel has two digital outputs and two comparators. The comparators evaluate the current position or count value against configured comparison values. When the specified condition is reached, the module can trigger the corresponding output response.

In a cutting or indexing machine, this function can be used to switch an output at a predefined position. The response takes place within the technology module, avoiding the need to rely exclusively on the CPU program to evaluate every position transition.

The module also provides frequency, period, and velocity measurement. These functions allow the same encoder interface to support both movement tracking and machine-speed monitoring.

Role in the SIMATIC S7-1500 Control Architecture

The 6ES7551-1AB00-0AB0 operates as a dedicated encoder and position acquisition interface within the S7-1500 automation architecture. It handles high-speed encoder signals and provides processed position and measurement values to the CPU.

A typical system arrangement follows this structure:

Encoder / Position Sensor → TM PosInput 2 → S7-1500 Backplane Bus → CPU → Motion Control / Machine Logic

The module can be configured to operate with the S7-1500 Counting and Measurement technology object or the Motion Control technology object. It also supports manual operation through its control and feedback interfaces.

In a positioning application, the encoder provides movement information while the technology module handles signal evaluation. The CPU then uses the resulting position values to coordinate drives, machine sequences, and other control functions.

For applications requiring synchronized acquisition, the module supports isochronous operation. This allows encoder measurements and controller processing to be coordinated within a configured system cycle.

The two-channel structure is particularly useful when separate position measurements are required at different points in a machine. For example, one channel can monitor a feed roller while the second monitors a cutting or indexing mechanism.

Industrial Applications

Installation and Maintenance

1. Verify the Module Type: Confirm that Siemens 6ES7551-1AB00-0AB0 is the required TM PosInput 2 version and that its encoder interfaces match the application.
2. Check CPU Compatibility: Verify that the S7-1500 CPU and engineering software support the intended counting, measurement, and Motion Control functions.
3. Inspect the Module: Examine the housing, mounting contacts, and front connector interface for physical damage or contamination.
4. Prepare the Mounting Rail: Ensure that the S7-1500 mounting rail is correctly installed and that the designated technology module position is available.
5. Isolate the System: Follow electrical safety procedures and disconnect the relevant power supply before installing or removing the module.
6. Install the Module: Mount the TM PosInput 2 in the designated S7-1500 configuration and verify that it is correctly seated.
7. Select the Encoder Interface: Identify whether the application uses RS422, 5 V TTL, or SSI signals before preparing the field wiring.
8. Connect Encoder Signals: Wire the A/B/N signals or SSI data and clock signals according to the encoder type and channel assignment.
9. Check Encoder Supply: Verify the required 5 V or 24 V supply and confirm that the connected encoder is compatible with the selected supply arrangement.
10. Configure Channel Parameters: Set the encoder type, signal evaluation, counting range, measurement functions, and input filters in STEP 7 or TIA Portal.
11. Test Position Acquisition: Monitor the channel diagnostics and verify that the measured position, counting direction, capture events, and comparison functions operate correctly.
12. Document the Configuration: Record encoder wiring, channel assignments, supply voltage, SSI frame parameters, counting settings, and Motion Control configuration for future maintenance.

Compatible System Components

Recommended Related Models

Key Advantages

• Processes two independent encoder and position acquisition channels.
• Supports RS422, 5 V TTL, and SSI encoder interfaces.
• Handles incremental and absolute position detection.
• Supports counting frequencies up to 4 MHz with quadruple evaluation.
• Provides frequency, period, and velocity measurement.
• Includes capture and synchronization functions.
• Supports position-dependent output switching through integrated comparators.
• Integrates with S7-1500 Motion Control technology objects.
• Provides channel-level encoder diagnostics and configurable signal filtering.

Technical FAQs

What is the difference between TM PosInput 2 and TM Count 2x24V?

The TM PosInput 2 supports a wider range of encoder interfaces and position acquisition functions. The 6ES7551-1AB00-0AB0 accepts RS422 and 5 V TTL incremental encoder signals as well as SSI absolute encoder signals. The TM Count 2x24V, on the other hand, is primarily intended for 24 V incremental and pulse encoder applications. The PosInput 2 is therefore more suitable when the machine requires absolute position acquisition or differential encoder signals.

Can the 6ES7551-1AB00-0AB0 read SSI absolute encoders?

Yes. The module supports SSI absolute encoders using a synchronous serial interface. It can read configurable SSI frames from 10 to 40 bits and transfer position values of up to 32 bits to the controller. Gray-code and dual-code SSI formats are supported.

What is the maximum counting frequency of the TM PosInput 2?

The maximum counting frequency is 4 MHz with quadruple evaluation. This capability is particularly useful for applications involving high-resolution incremental encoders and fast-moving mechanical components. The actual achievable frequency depends on the encoder interface, signal characteristics, and configured evaluation method.

Can the module be used directly with S7-1500 Motion Control?

Yes. The TM PosInput 2 supports position input for compatible S7-1500 Motion Control technology objects. Encoder information can be acquired by the module and transferred to the controller for positioning and movement-related tasks. The selected encoder type, hardware configuration, and engineering software version must support the intended function.