Sending speed references over Profibus
Fundamentals of Sending Speed References over PROFIBUS DP
PROFIBUS DP (Decentralized Peripherals) is a deterministic, cyclic communications protocol widely used in industrial automation to connect PLC masters with field devices such as variable frequency drives (VFDs) and servo controllers. When transmitting speed references over PROFIBUS DP, the master PLC cyclically sends output data—including speed references over PROFIBUS DP and control commands—to slave drives. In return, the drives provide feedback data such as actual motor speed and status words, facilitating closed-loop control and monitoring.
Each communication cycle initiated by the master typically occurs within a predefined cycle time (e.g., 50 to 100ms). This cyclical exchange ensures synchronized operation across multiple drives, enabling centralized speed control, ramping, and coordinated motion sequences. An alternative acyclic communication model supports parameterization or diagnostic operations but is not preferred for time-critical speed setpoint transmission due to higher latency.
Understanding how cyclic and acyclic communication differ is essential for automation engineers. Cyclic exchange is optimized for fast, repeatable data transfer during runtime, whereas acyclic exchange handles non-time-critical tasks such as configuration and alarms. Selecting the appropriate communication mode affects performance and response characteristics of speed control over PROFIBUS DP.
Calculating and Mapping Speed Values for PROFIBUS Transmission
The speed reference value sent over PROFIBUS DP is commonly represented as a 16-bit unsigned integer. Typically, this 16-bit word ranges from 0000h (0 decimal) to 4000h (16384 decimal), representing the full scale of speed from zero up to the motor's maximum RPM, defined by a parameter such as P0134 in many drives.
The scaling formula to convert a desired speed setpoint to the PROFIBUS value is:
Profibus Value = Desired RPM × 16384 / Maximum RPM (P0134)
For example, if P0134 = 1800 RPM and the PLC needs to send a speed reference corresponding to 225 RPM, the calculation is:
2048 = 225 × 16384 / 1800
This mapping allows flexible and precise speed control over the network. Handling negative speed values to indicate reverse direction depends on the manufacturer's protocol. Some drives use the sign bit within a two's complement format, while others apply custom manufacturer-specific conventions. It is always critical to verify the drive’s documentation to ensure proper interpretation of directional speed commands.
Selecting Between PROFIdrive and Manufacturer-Specific Profiles
PROFIBUS DP drives can be configured to use either the PROFIdrive profile or a manufacturer-specific profile for data exchange. The PROFIdrive profile standardizes control and status word formats (parameters P0967 and P0968) and defines the speed reference word to improve interoperability between devices from different vendors.
Manufacturer-specific profiles may allow faster implementation or extended features tailored to proprietary drives but reduce cross-vendor compatibility. Parameter P0741 in the drive selects the active profile and can only be changed when cyclic communication is halted to avoid data mismatches. After switching profiles, drives may require a communication timeout window with no cyclic data exchange to properly apply new settings.
It is advisable to choose the PROFIdrive profile in multi-vendor installations or when maximum standardization is desired. For strictly homogeneous systems, manufacturer-specific profiles may optimize performance and additional features.
Configuring Baud Rate and Transmission Parameters for Reliable Operation
PROFIBUS DP supports multiple baud rates ranging from 9.6 kbit/s up to 12 Mbit/s. Choosing the appropriate bit rate involves a tradeoff between communication speed and maximum cable lengths. At lower baud rates such as 9.6 kbit/s, the maximum segment length can reach up to 1200 meters, while at 12 Mbit/s this decreases to about 100 meters due to signal attenuation and distortion effects.
All devices on a PROFIBUS DP segment must be configured to the exact same baud rate to communicate correctly. Some modern devices and modules, such as the WEG SCA06, support automatic baud rate detection, simplifying network setup. When designing a PROFIBUS network, engineers must consider cable length, number of slaves, and cycle time demands to select a suitable baud rate and avoid communication errors caused by overlength cables or improperly terminated segments.
Network termination resistors (typically 120 ohm, 0.25 W) must be applied at the first and last nodes of the PROFIBUS bus to prevent signal reflections that can corrupt data. Incorrect termination or damaged cable shielding frequently causes CRC errors, leading to lost or delayed speed setpoint updates.
Addressing and Node Configuration Essentials
Each PROFIBUS DP node has a unique 7-bit address ranging from 0 to 127. Both master and slave devices must have distinct addresses to prevent communication conflicts. Duplicate addresses cause the bus to become unstable or inoperative.
Slave drive addressing is typically configured using parameter P0918 in the drive settings. Before commissioning, verify slave addresses for uniqueness and proper assignment. The master device’s configuration database—usually loaded via a .GSD files containing slave parameters—must list all slave addresses accurately.
Drive parameters such as P0963 can be used to display detected configured baud rate and confirm successful communication during startup. Proper addressing lays the foundation for robust cyclic I/O data exchange and seamless speed reference transmission.
Drive Parameter Setup for Speed Reference Control
To accept speed references from the PLC master over PROFIBUS DP, drive parameters must be configured correctly based on drive make and model. For example, Yaskawa A1000 drives use parameter b1-01 (hex 180) to select the frequency reference source:
- 0 = Operator presets (local control)
- 1 = Analog input terminals
- 2 = Modbus communication
- 3 = PROFIBUS DP network
Additionally, parameter b1-02 enables start/stop control via PROFIBUS by setting it to 3. WEG SCA06 drives require operating mode P0202 to be set to 6 for PROFIBUS DP speed control mode, or 4 for positioning mode.
Enable the control word bit 6 (P0967) to permit the drive to accept new speed references transmitted via the network. Neglecting proper parameter configuration prevents the drive from executing remote speed commands and impedes networked speed control implementation. Modern variable frequency drives (VFDs) often provide dedicated PROFIBUS parameter groups to make this configuration more straightforward.
Master Setup: Configuring Cyclic I/O and Cycle Time
The PLC master must be configured to cyclically exchange input and output data with slave drives. Output words from the master comprise speed reference values and control command bits, while input words from the drive include status returns and actual speed feedback.
Input word mapping, controlled by parameter P0742 and subsequent settings, allows selection of which drive internal parameters are read back and reported to the master. For example, motor speed feedback and drive status bits are often mapped to specific input word positions.
The communication cycle time influences control resolution and system responsiveness. Typical cycle times of 50 ms to 100 ms are used in speed control applications; servo applications with tighter motion requirements may require faster cycle times.
A watchdog timeout should be configured in the master logic to set drives to a safe state, usually zero speed, if communication is lost or cyclic data stops. This protects equipment from uncontrolled operation. Legacy platforms like the S7-300 PLC frequently use hardware watchdogs in combination with software logic for this purpose.
Understanding Control Word Bit Functions
The control word sent from the master to the drive contains several bits that command drive operation. Typical control bits include:
- Bit 0: Enable or disable drive operation
- Bit 1: Ramp selection (up or down)
- Bit 2: Fault reset command
- Bit 6: Speed reference write enable (PROFIdrive profile); speed value is accepted only when bit 6 transitions from 0 to 1
- Bits 7 and above: Manufacturer-specific commands such as jog control, preset speed selection, or multi-step enable
It is important to document the exact bit definitions for your specific drive model to avoid command conflicts and ensure proper remote control behavior when sending speed references over PROFIBUS DP. Many Siemens-oriented resources on control and status words provide standardized layouts and examples.
Handling Reverse Speeds and Direction Control
Sending negative speed references to indicate reverse motor direction depends on the drive’s protocol implementation. Some drives use two’s complement representation for the 16-bit speed word; others use a sign-magnitude format with a dedicated sign bit.
For two's complement, negative values range from FFFF8000h (-32768 in decimal) as maximum reverse speed, up to FFFF9999h for smaller reverse values. It is advisable to test reverse direction control using small-scale negative values to avoid sudden load reversals and mechanical stress.
Before applying full reverse speed commands, verify direction logic and drive response with the motor unloaded or in safe conditions. Some applications implement manual verification steps or require additional safety interlocks for reverse motion.
Verifying Cyclic Communication and Speed Feedback
Testing the cyclic I/O data exchange is critical to ensure reliable speed control over PROFIBUS DP. Using PROFIBUS analyzers or protocol sniffers, engineers can capture network traffic and validate cycle times, data content, and consistency.
Measurement should confirm that the configured cycle time matches observed communication timing, no cycles are missed, and retransmissions are low to none. High retransmission rates indicate physical layer issues such as cable quality or grounding problems.
Monitoring parameters such as P0963 (baud rate setting) and P0918 (address configuration) in drives validates that slave parameters have initialized successfully. Comparing actual speed feedback words returned by the drive against the transmitted setpoints helps detect scaling or mapping errors early on. Strong communication diagnostics in the PLC and engineering software can greatly accelerate this process.
Common Troubleshooting Measures for Communication Issues
If speed updates are slow, missing, or erroneous, inspection of the physical and configuration layers is essential. Common causes include damaged cable shielding causing CRC errors, improper bus termination, or mismatched baud rates across devices.
Ensuring all devices share a common ground minimizes bit errors caused by floating potentials. If communication remains unreliable at high baud rates, reducing to lower speeds such as 6 Mbit/s or 3 Mbit/s can improve stability. Verifying termination resistors are only applied at the first and last nodes prevents signal reflections that degrade data integrity.
These troubleshooting steps help maintain robust PROFIBUS DP communication for reliable speed reference transmission in demanding industrial environments.
Advanced Topics: Data Compression and Mapping Schemes
Some modern drives support cyclic data compression, where only changed parameters are transmitted instead of full data sets. This optimization reduces network load and cycle times but requires compatible master configuration and firmware support.
Data block mapping organizes output and input data into fixed telegram structures containing speed reference words, control words, and status words. Alternatively, direct data mapping sends raw parameter values without predefined blocks, providing flexibility at the cost of slower communication.
Before deploying these advanced features, verify drive and master firmware versions support the selected mapping and compression methods. Proper documentation and testing are vital to avoid unexpected behavior during network operation.
Standards Compliance and Safety Considerations
PROFIBUS DP communication follows IEC 61158-2 for the electrical layer and EN 61158-3 for protocol messaging, ensuring interoperability and industrial-grade robustness. Drives implementing PROFIBUS DP variants often comply with these standards along with manufacturer-specific extensions.
When safety-rated speed transmission is required, additional certification such as Pilz-certified firmware or Safe Torque Off integration is essential. These safety features ensure that speed commands and drive enable signals conform to functional safety requirements and fault tolerance.
Documenting compliance certifications and verifying cable characteristics and segment lengths relative to selected baud rates is mandatory for both industrial communication standards and quality standards adherence.
Integrating Speed Control with HMI and SCADA Systems
Human-machine interfaces (HMIs) and SCADA systems commonly visualize speed feedback and allow operator input for speed setpoint adjustments over PROFIBUS DP. Popular HMI and SCADA systems include Siemens WinCC, Wonderware, and Ignition.
Mapping HMI slider controls calibrated from 0 to 100% into 16-bit speed reference values (0000h to 4000h) enables intuitive operator control over drive speed. Master PLC logic should implement smooth ramping of setpoints—generally between 0.5 and 2 seconds—to reduce mechanical shock and wear on motors and driven equipment.
Continuous monitoring of status words in HMI tags allows prompt alerting of drive faults, communication losses, or parameter limit violations. Integrating robust diagnostics improves operational safety and minimizes downtime due to unforeseen communication or hardware issues.
Conclusion: Best Practices for Speed Reference Transmission via PROFIBUS DP
Implementing speed reference communication over PROFIBUS DP requires careful attention to data scaling, parameter selection, and network configuration. Automation engineers and system integrators should thoroughly understand drive-specific protocols, profile selections, and size network segments appropriately based on baud rate and node count.
Practical validation steps such as using protocol analyzers and monitoring real-time feedback ensure high quality and deterministic control essential for industrial operations. Proper drive parameterization, addressing, and control bit usage guarantee seamless master-slave communication for efficient centralized speed management.
By adhering to standards and leveraging advanced network features like cyclic data compression and safety-certified speed control, professional automation teams can confidently deploy PROFIBUS DP networks as part of a broader Siemens automation ecosystem for multi-drive coordination and reliable speed ramping in demanding industrial environments.
Relevant Siemens PROFIBUS Speed Control Hardware
- Siemens S7-300 CPU 315-2 DP with integrated PROFIBUS DP master
- Siemens Micromaster 440 Variable Frequency Drive (MM440)