S7-1512C PTO Pulse Train Output: Resolving Drive Fault Errors

Problem Overview

The SIMATIC CPU 1512C-1 PN is a compact controller with integrated digital and analog I/O and four onboard PTO (Pulse Train Output) channels. When commissioning a pulse-direction servo drive that accepts only PULSE and DIR signals (no PROFINET, no encoder, no PROFIdrive interface), engineers frequently encounter a persistent "Fault in the drive" indication in the TIA Portal commissioning panel. The fault cannot be acknowledged, no telegram is being exchanged, and the physical drive reports no error. The same configuration that works flawlessly on an S7-1200 PTO channel (1211C through 1217C) yields a stuck fault on the 1512C.

The root cause is not a hardware defect, a wiring error, or a faulty drive. It is a configuration mismatch: the technology object (TO) is set up as a PROFIdrive axis when it should be set up as a PTO axis. This article documents the affected hardware, the exact mechanism that produces the spurious "Fault in the drive" error, the correct configuration sequence, the verification procedure, and the engineering parameters required to operate a pulse-direction servo on the 1512C-1 PN.

Affected Hardware, Firmware, and Software

CPU Catalog Numbers

Firmware Compatibility

The original report uses TIA Portal V15.1. The configuration procedure described in this article applies to V15.1 and later versions with minor dialog changes.

Onboard I/O Resources of CPU 1512C-1 PN

The four PTO-capable DQ channels form two PTO channel pairs. The default address assignment depends on the slot configuration; in a standalone CPU, PTO Channel 0 typically maps to the first two PTO-capable outputs of the onboard DQ area.

Reference Documentation

• S7-1500 Motion Control Function Manual
• S7-1500 Compact CPU System Manual
• STEP 7 V15.1 System Manual
• S7-1500C CPU 1512C-1 PN Equipment Manual

Root Cause: PROFIdrive Status Word Bit 3

The "Fault in the drive" indication in an S7-1500 motion control axis is generated from ZSW1 bit 3 (fault present) of the PROFIdrive status word. When the technology object is configured with PROFIdrive as the drive type, the PLC continuously exchanges Standard Telegram 3 (or another configured telegram) with the drive over PROFINET or PROFIBUS. The PLC reads ZSW1 from the drive and monitors bit 3:

• ZSW1 bit 3 = 0 → No fault, axis can be enabled
• ZSW1 bit 3 = 1 → Fault present, axis reports "Fault in the drive"

When the drive is a simple pulse-direction servo with no PROFINET interface, the PLC's PROFIdrive stack cannot establish the communication. The stack interprets the missing telegram response as a permanent fault state, and ZSW1 bit 3 remains effectively asserted from the PLC's perspective. The result is the latched "Fault in the drive" indication that cannot be acknowledged using standard tools.

Three symptoms confirm this root cause:

1. Ready input is disabled (no drive-ready signal wired)
2. No error input from the drive (no drive fault wired to a DI)
3. Telegram 3 is visible in the technology object configuration with no physical PROFINET drive on the network

The fix is to change the drive type from PROFIdrive to PTO (Pulse Train Output) in the technology object. This removes the PROFIdrive communication path entirely, and the PLC no longer polls for a status word. The PTO output channels on the onboard DQ produce the pulse and direction signals directly, with no feedback path required.

Hardware Configuration: PTO Output Wiring

PTO Channel Pair Assignment

Each PTO axis uses one PTO channel pair (two digital outputs). The CPU 1512C-1 PN provides four PTO channel pairs in its onboard DQ area:

where 'a' is the I/O start address of the onboard DQ area (default 0). For PTO Channel 0 in 2-wire pulse and direction mode, DQ a+0 carries the pulse train (CW step signal) and DQ a+2 carries the direction bit.

2-Wire Pulse and Direction Wiring

Verify the drive input type (sink/source/differential) before connecting. Most modern servo drives accept 24 V single-ended inputs. For drives requiring RS-422 differential inputs, install an external line driver (AM26LS31, SN75174, or equivalent) between the PLC and the drive.

Drive Enable Signal

Most servo drives require a dedicated Servo ON (SON) input that must be energized to enable the drive power stage. Wire this input to a free digital output (either onboard DQ not used for PTO, or a DO on an output module). Control the output in the user program before and after calling MC_Power:

// Set drive enable before MC_Power
"DO_ServoEnable" := bAxisEnabled;
MC_Power(Axis := "Axis_1", Enable := bAxisEnabled, EnableMode := TRUE);

Shielded Cable Specification

Technology Object Configuration: Step-by-Step

The following sequence configures a positioning axis for a pulse-direction servo drive. The same procedure applies to a speed axis with the appropriate parameter changes.

Step 1: Enable the PTO Channel in Device Configuration

1. Open the project in TIA Portal.
2. Double-click the CPU 1512C-1 PN in the device view.
3. Navigate to Properties > Digital outputs.
4. Click the first PTO channel (Channel 0).
5. Set Operating mode to "PTO (Pulse Train Output)".
6. Set Output mode to "Pulse and direction" (2-wire).
7. Set the Maximum output frequency (default 100 kHz, range 1 Hz - 1 MHz).
8. Apply the changes.

Step 2: Create the Technology Object

1. In the project tree, expand the CPU folder.
2. Right-click Technology objects > Add new object.
3. Select Positioning axis or Speed axis.
4. Assign a name (e.g., Axis_1, Axis_PulseServo).
5. Click OK.
6. The TO opens in the configuration editor.

Step 3: Configure Basic Parameters

Step 4: Configure the Drive

1. Navigate to Hardware interface > Drive in the TO configuration tree.
2. Set Drive type to PTO (Pulse Train Output).
3. The PROFIdrive-specific fields (telegram, PROFIdrive address) disappear.
4. Select the PTO output channel from the dropdown (e.g., PTO Channel 0).
5. Set the direction logic: Positive direction = HIGH or LOW based on drive requirements.
6. Enter the maximum speed of the drive in pulses/second (or equivalent).
7. Enter the reference speed and maximum velocity in the application unit.

Step 5: Configure the Encoder (Optional)

For a pulse-direction drive without encoder feedback, the encoder connection must be set to "None".

1. Navigate to Hardware interface > Encoder.
2. Set Encoder type to "No encoder" or "None".
3. The axis operates in open-loop mode.

Open-loop positioning relies on the mechanical system to provide repeatable positioning. Acceptable for applications with limited travel, lead screws, and external end-of-travel sensors. Not acceptable for high-precision applications.

Step 6: Configure Limits and Dynamics

Step 7: Configure Drive Enable

1. Navigate to Extended parameters > Drive enable.
2. Map a tag to control the drive enable output (e.g., "Axis_1".DriveEnable).
3. Use the PLC tag table to assign the tag to a DO output (e.g., Q0.0).
4. Or, set the drive enable output directly in the user program before MC_Power.

Step 8: Compile and Download

1. Right-click the CPU > Compile > Hardware and software (rebuild all).
2. Resolve any compilation errors. Common issues: missing PTO channel assignment, mismatched unit systems, encoder configured but no encoder hardware.
3. Connect to the CPU and download the project.
4. Go online and open the TO commissioning panel.

Why Telegram 3 Causes the Fault

Telegram 3 Structure

Telegram 3 is the Siemens Standard Position Telegram used for PROFIdrive positioning. It contains 2 PZD words from PLC to drive and 5 PZD words from drive to PLC:

ZSW1 bit 3 (Fault present) is monitored continuously by the PLC. If the telegram is not received (drive not on PROFINET), the PLC cannot confirm a non-fault state and latches the "Fault in the drive" indication.

Removing Telegram 3 from the Configuration

1. Open the technology object configuration.
2. Navigate to Hardware interface > Drive.
3. Change Drive type from "PROFIdrive" to "PTO (Pulse Train Output)".
4. Compile and download.

After this change, the Telegram field disappears, and the axis no longer polls for a PROFIdrive status word. The "Fault in the drive" error will no longer appear when the drive is correctly configured as a pulse-direction device.

Verification and Commissioning

Online Commissioning Procedure

1. Connect TIA Portal to the CPU via PROFINET.
2. Click Go online.
3. Open the technology object (e.g., Axis_1) in the project tree.
4. Click Commissioning in the toolbar.
5. The commissioning panel opens with the following indicators:
   • Error: should be inactive
   • Warning: may be active for non-critical conditions
   • Axis state: should show "Ready to enable" or similar
   • Drive ready: not applicable for PTO
6. Click Enable to enable the axis.
7. Use the Jog buttons to test motion in positive and negative directions.
8. Monitor the pulse and direction signals with an oscilloscope or logic analyzer at the drive terminals.

Verification Checklist

Status Word Reference (PTO Axis)

Common Error Codes and Resolutions

Motion Control Error IDs

Specific Diagnosis for "Fault in the Drive"

1. Open the technology object configuration.
2. Navigate to Hardware interface > Drive.
3. Confirm Drive type is set to PTO or Stepper.
4. If PROFIdrive is selected, change it to PTO and recompile.
5. Go online and check the commissioning panel for the error.
6. Monitor AxisName.ErrorWord in a watch table to see the specific error code.
7. Use MC_ReadAxisError to retrieve the error code programmatically.

Troubleshooting Matrix

Drive Connection Type Comparison

Sample User Program (SCL)

// Axis control for pulse-direction servo
// Tags: bStart (BOOL), bStop (BOOL), rTargetPos (REAL), bAxisError (BOOL)

// Drive enable and MC_Power
IF "bStart" AND NOT "bAxisError" THEN
    "Axis_1".DriveEnable := TRUE;  // Set drive enable DO
    MC_Power(
        Axis := "Axis_1",
        Enable := TRUE,
        EnableMode := TRUE  // TRUE = enable drive

Migration Notes: S7-1200 to S7-1500C

Engineers familiar with PTO on S7-1200 CPUs (1211C through 1217C) should note these differences when migrating to the 1512C-1 PN:

The most significant difference is the default drive type. On S7-1200 PTO, the axis is implicitly PTO. On S7-1500C, the default drive type when creating a new TO is PROFIdrive, which causes the "Fault in the drive" error for pulse-direction drives. Always verify the drive type after creating a new TO on the 1512C-1 PN.

Best Practices for PTO Operation

Hardware Best Practices

• Use shielded twisted-pair cable for PTO signal wiring
• Keep PTO cables under 3 meters for reliable 1 MHz operation
• Install RS-422 line drivers for cable runs over 10 meters
• Provide a separate 24 V DC supply for the drive to isolate ground loops
• Add pull-down resistors (2.2 kΩ to 4.7 kΩ) on direction output if the drive requires defined logic levels when PLC is in STOP
• Verify drive input voltage levels match the 1512C DQ output (24 V DC, 0.5 A)
• Install flyback diodes on any relay coils connected to the same 24 V supply
• Use proper EMC practices: cable glands, 360-degree shield bonding, separate cable trays for power and signal

Software Best Practices

• Call MC_Power with Enable = TRUE before any motion command
• Use MC_MoveJog for manual testing before automatic sequences
• Implement position verification with external sensors for open-loop applications
• Monitor the axis status word continuously in the user program
• Implement error handling for all motion commands using Error and ErrorID outputs
• Use MC_Reset to acknowledge axis errors before re-enabling
• Limit maximum velocity and acceleration to drive specifications (consult drive datasheet)
• Test motion in jog mode first, then small relative moves, then absolute moves
• Document the axis configuration including all parameter values
• Save the project with a versioned naming scheme

Firmware Update Procedure

If the PTO configuration is not appearing or the behavior is inconsistent, update the CPU firmware:

1. Connect to the CPU online in TIA Portal.
2. Navigate to Online > Accessible devices.
3. Select the CPU 1512C-1 PN in the list.
4. Right-click > Update firmware.
5. Select the firmware file (downloaded from Siemens SiePortal as a *.upd file).
6. Follow the update wizard. The CPU will be in STOP mode during the update.
7. Cycle power to the CPU after the update completes.
8. Re-download the project to the CPU.
9. Go online and verify the configuration.

Diagnostic Resources in TIA Portal

Watch Table for Axis Monitoring

Create a watch table with the following tags for real-time axis monitoring:

Trace Recording

Use the TIA Portal Trace function to record axis behavior over time:

1. Navigate to Traces in the project tree.
2. Create a new trace configuration.
3. Add the following signals:
   • Axis_1.ActualPosition
   • Axis_1.ActualVelocity
   • Axis_1.StatusWord
   • Drive enable DO
4. Set the recording duration (e.g., 10 seconds).
5. Trigger the trace from a user program or manually.
6. Analyze the recorded data to verify motion profile and timing.

FAQ