S7-1200 CPU 1212C Resolving PWM 1 kHz Limit Using PTO Pulse

Problem Details

A plant engineer commissioning a SANMOTION SH1603 two-phase bipolar stepper motor on a Siemens S7-1200 CPU 1212DC/DC/DC (firmware V4.x) using TIA Portal V10.5 cannot push the on-board pulse output above 1 kHz. The stepper driver requires a 24 V DC pulse-and-direction command stream of 0 Hz to 10 kHz. With the standard CTRL_PWM instruction the engineer measures a hard ceiling near 1 kHz regardless of the cycle-time parameter entered in the PWM configuration dialog. A second subsystem, a Keyence SR-610 barcode reader connected through an N-R2 RS-232C module, must also be integrated on the same PLC program.

The engineer attempted to follow Siemens Entry ID 38391227 but the example project supplied was created for TIA Portal V11 (Step 7 Basic V11) and would not import cleanly into the V10.5 SP2 environment available on the engineering workstation. The local search was performed against the legacy URL support.automation.siemens.com, which has since been redirected to support.industry.siemens.com.

Root Cause

The S7-1200 PWM function is being used where PTO (Pulse Train Output) is the correct technology. CTRL_PWM is a variable duty-cycle block intended for slow process control (heating elements, proportional valves, LED dimming). Its internal minimum pulse width (the smallest ON time the output hardware can resolve) is enforced by the configuration dialog and by the output-stage timing. When the requested cycle time forces the ON time below this minimum, the firmware holds the previous valid value and the apparent maximum frequency collapses to 1/(2 × t_min_pulse).

For the standard CPU 1212C outputs running the default PWM profile, this minimum pulse width is approximately 1 ms, which limits usable PWM to roughly 500 Hz at 50% duty and to 1 kHz absolute at very small duty cycles once you account for the half-period resolution step. That is exactly the symptom reported.

For stepper pulse-and-direction control the correct primitive is CTRL_PTO, which is a high-speed hardware counter engine that uses the output hardware in PTO mode (also referred to as FM (Frequency Modulation) in older Siemens documentation). PTO bypasses the PWM minimum-pulse-width enforcement and exposes a frequency range of 0.5 Hz to 100 kHz on the CPU 1212C on-board outputs, with separate pulse-count and integrated ramp parameters.

CPU 1212C DC/DC/DC Hardware Reference

All output specifications below are taken from the SIMATIC S7-1200 manual collection hosted on TIA Siemens Cloud – CPU 1212C technical specifications and the System Manual entry S7-1200 Programmable Controller (109741593).

The CPU 1212DC/DC/DC variant is the transistor-output model. The relay-output variant (DC/DC/RLY) cannot be used for PTO/PWM because the electromechanical relay cannot switch at the required rates. Confirm the part number suffix is -1AE40- or -1BE40- before proceeding.

PWM vs PTO on the S7-1200

Solution: Migrate from CTRL_PWM to CTRL_PTO

The fix is to delete the PWM configuration on the output channel and reconfigure the same physical output as a PTO. Both functions live on the same hardware channel of the CPU 1212C, so no wiring changes are required; only the device configuration and the program logic change.

Prerequisites

• SIMATIC S7-1200 CPU 1212DC/DC/DC, firmware V3.0 or later (V4.x recommended for full feature set). Firmware can be verified in Online & Diagnostics → Module Information → Firmware.
• STEP 7 Basic V10.5 SP2 (or migrate the project to V13/V14/V15/V16/V17 to access newer PTO features; the program logic described is portable across versions).
• SANMOTION SH1603 bipolar stepper with compatible 24 V DC pulse-and-direction driver (e.g., Sanmotion Q series, Oriental Motor CVD, or equivalent).
• 24 V DC logic supply with adequate inrush margin. CPU 1212C draws 1.0 A typical from the 24 V DC sensor supply; the driver electronics typically require an additional 0.5 A to 2 A.
• TIA Portal project with the CPU 1212C added to the device tree and the standard PLC tags created.

Step-by-Step Configuration in TIA Portal V10.5

1. Open the device configuration of the CPU 1212C. In the project tree, expand PLC_1 → Device configuration.
2. Select the high-speed output channel. Click on the digital output DQa you intend to use (Q0.0 is the canonical PTO0 channel on the CPU 1212C). Q0.1 becomes the direction output automatically.
3. Open the Properties pane for the channel. Navigate to Properties → Outputs → Pulse generator (PTO/PWM).
4. Switch from PWM to PTO. Set the mode selection to PTO (Pulse Train Output). The dialog expands to reveal the PTO0 sub-properties.
5. Configure PTO0 pulse output. Set Pulse output = DQa (Q0.0). Set Direction output = DQb (Q0.1).
6. Set the high-speed counter resource. TIA assigns HSC 1 to PTO0 by default. Confirm this is not conflicting with another HSC; if it is, choose HSC 2 (PTO1) or HSC 3 (PTO2).
7. Confirm the I/O filter on the direction output. Set the direction output filter to Off or 0.1 µs; never leave the 6.4 ms default filter in place, otherwise the direction changeover will lag the pulse and the motor will stall at high speeds.
8. Compile and download the hardware configuration. The output channel is now reserved for PTO and cannot be written with a normal Q-assignment.
9. Insert the CTRL_PTO instruction in the OB1 (or OB35 cyclic interrupt at 10 ms). Drag Instructions → Motion Control → Pulse → CTRL_PTO into the network.
10. Wire the CTRL_PTO inputs. Connect the parameters as shown in the table below.

CTRL_PTO Interface Wiring

Sample Ladder Snippet

Network 1: PTO0 Enable and Speed Command
      |    M10.0       M10.1        DB1.DBX0.0      DB1.DBD4
   --| |-----------| |------------( )-----------|  |
   |   RUN         EN_OK                        CTRL_PTO_DB
   |
   |  DB1.DBD4    =  target frequency in Hz (DINT)
   |  DB1.DBD8    =  pulse count (0 = continuous)
   |  DB1.DBX12.0 =  direction bit

Network 2: Set frequency from integer tag
      |    M10.2
   --|/|-----------[MOVE EN]
   |                IN  := MW20  // scaled 0–10000 Hz
   |                OUT := DB1.DBD4

Mapping the 0–10 kHz Speed Reference

The driver accepts a pulse frequency between 0 Hz and 10 kHz. To run the stepper at its rated speed you should derive the target frequency from a per-second count of step pulses required to advance one revolution. If the SH1603 is set to 1000 step/rev (full step, no microstep), then 10 kHz equals 600 RPM. If the driver is set to 8000 step/rev (typical 1/8 microstep), then 10 kHz equals 75 RPM. The PLC program should:

1. Convert the HMI RPM command to Hz by Hz = RPM × steps_per_rev / 60.
2. Clamp the result to 0 Hz to 10000 Hz (or 100000 Hz if the mechanical envelope allows).
3. Pass the clamped value to the FREQ input of CTRL_PTO.
4. Use NUM_PULSES = 0 for continuous run, or pre-compute the pulse count for indexed moves.

SANMOTION SH1603 Wiring

The SH1603 is a 1.8° two-phase bipolar stepper motor from Sanyo Denki's SANMOTION series. Its datasheet specifies a holding torque of approximately 0.95 N·m and a rated phase current of 1.5 A. Pair it with a 24 V DC pulse-and-direction driver such as the Sanmotion Q Step driver or a comparable Oriental Motor, AMC, or Leadshine unit. Verify the driver's opto-isolator input current (typically 5 mA to 15 mA) is within the S7-1200 output rating (0.5 A continuous per channel).

Keyence SR-610 Barcode Reader Integration

The Keyence SR-610 is a 1D/2D industrial barcode reader. The N-R2 module is the RS-232C communication option, providing DB-9-style serial at 9600 to 115200 baud. The SR-610 supports two trigger modes, and the difference between them is the root of the integrator's follow-up question.

For a high-throughput packaging line the hard trigger is preferred because it removes the serial round-trip latency. For a slower automation cell the command trigger is faster to integrate because no additional wiring or PLC output is required. In the global program the trigger source can be unified by a single boolean tag Trigger_SR610:

• If hard trigger is wired: route Trigger_SR610 to a free DQ output wired to the N-R2 trigger terminal.
• If command trigger is used: when Trigger_SR610 := TRUE, transmit LON\r\n over the RS-232C port using the S7-1200 Send_P2P instruction on the CM 1241 RS-232C module (not on the on-board port of the CPU 1212C, which is RS-485 only).

S7-1200 RS-232C Send/Receive Wiring with CM 1241

Commissioning Verification

1. Online watch table on the PTO output. Force ENABLE = TRUE, FREQ = 10000, NUM_PULSES = 0. Force the same and observe BUSY go TRUE.
2. Scope on Q0.0 and Q0.1. Confirm a 24 V pulse train at the commanded frequency (within ±1% due to HSC quantization). Confirm Q0.1 (direction) is stable and not chattering; any chatter means the I/O filter on the direction output is set too high.
3. Stepper response. Command a slow 100 Hz for one rotation, then 1 kHz, 5 kHz, and finally 10 kHz. The motor must run smoothly without stalling or audible cogging at each step.
4. Direction reversal. Toggle DIR. The motor must reverse within one pulse. If it stalls during reversal, increase the deceleration ramp in the CTRL_PTO ACCEL/DECEL parameters.
5. Stop behaviour. Drop ENABLE to FALSE. The output must stop within one pulse cycle. The Q0.0 line must return to 0 V within 100 µs.
6. Barcode trigger round-trip. Send LON, wait for the RDY reply, present a known-good code, and verify the decoded string is returned within 200 ms.

Troubleshooting Matrix

Frequently Asked Questions