S7-1200 Servo Positioning: MC_MoveAbsolute vs MC_MoveRelative

Overview

Positioning a servo drive from a Siemens SIMATIC S7-1200 controller requires three things: a wired pulse/direction (PTO) or analog interface to the drive, a configured technology object "TO_Axis_PTO" in TIA Portal, and the correct PLCopen motion function block (FB) in the user program. The most common commissioning mistake is selecting the wrong motion FB: engineers load MC_MoveRelative when they actually need MC_MoveAbsolute, or vice versa. Both FBs accept a Position and Velocity input, but their semantic meaning is fundamentally different.

This reference covers the correct function-block selection for absolute and incremental moves, the wiring of a S7-1200 PTO channel to a servo drive pulse/direction input, the parameter set of each MC FB, and the verification steps required after download.

Prerequisites

• SIMATIC S7-1200 CPU with firmware 4.2 or higher (CPU 1211C, 1212C, 1214C, 1215C, or 1217C). Motion control was substantially expanded in V4.x; refer to the S7-1200 Motion Control V4.0 Function Manual.
• TIA Portal V15.1 or higher (V17/V18 recommended for current S7-1200 firmware V4.5+).
• A servo drive that accepts step/direction (pulse train) input, or analog ±10 V with digital enable. Common S7-1200-compatible families include Siemens SINAMICS V90 (with pulse/direction variant), Festo EMMS-AS, Yaskawa Sigma-7 in pulse-train mode, and Delta ASDA-A2.
• Encoder feedback wired back to the drive (the S7-1200 PTO is open-loop from the PLC's perspective; closed-loop control happens inside the drive).
• Axis mechanics: leadscrew pitch, gearbox ratio, and load inertia should be known so that user units (mm, deg, or revolutions) can be derived.

MC_Power: Enabling the Drive

Before any motion FB can issue setpoints, the axis must be enabled. MC_Power is the mandatory first block in every motion sequence. It energizes the drive's power stage and clears the controller's enable interlock.

Standard usage pattern in ladder or structured text:

// Enable axis continuously while bEnable is true
MC_Power_Instance(Axis := MyAxis, Enable := bEnable, StopMode := 0,
                  Status => bDriveReady, Busy => , Error => bErr,
                  ErrorID => wErrID);

MC_MoveAbsolute vs MC_MoveRelative: The Core Difference

Both FBs accept a target Position in the technology object's configured user units, and both accept a Velocity in user units per second. The difference is the coordinate reference:

Worked Example: Selecting the Right FB

Assume the axis is currently sitting at user-unit position 1000 (e.g., 1000 mm on a linear slide) and the technology object "MyAxis" has been homed.

If your application requires the drive to go to a fixed station number regardless of the axis's current position, use MC_MoveAbsolute. If the application only ever needs to move a fixed step in one direction (e.g., a feeder advancing a part), MC_MoveRelative is appropriate.

MC_MoveAbsolute Parameter Reference

MC_MoveRelative Parameter Reference

Note that the parameter is named Distance on MC_MoveRelative, not Position. A common TIA Portal edit error is to wire a tag called Position to a Distance input; the compiler will accept it but the semantic is wrong. Always rename the tag to Distance for clarity.

Pulse/Direction (PTO) Wiring Reference

On a CPU 1214C, the first PTO channel is on output Q0.0 (pulse) and Q0.1 (direction). On CPU 1215C and 1217C, the second channel is Q0.2/Q0.3, and the third is Q0.4/Q0.5. The fourth PTO (only on 1217C from V4.4) is Q0.6/Q0.7. These outputs are differential 5 V TTL or 24 V push-pull depending on the wiring module used.

For high-frequency servo drives, use a Siemens signal board (SB 1222 DQ 200 kHz, 6ES7 222-1AD30-0XB0) or the SB 1223 high-speed counter board. Wire the differential outputs through a 24 V line-driver adapter (e.g., Siemens Sirius 3TX7002) if the drive expects 24 V open-collector or 5 V differential.

Configuring the Technology Object

1. In TIA Portal, expand the S7-1200 station in the project tree and right-click Technology Objects → Add new object → TO_Axis_PTO.
2. Select the pulse generator (PTO) channel that matches the wired output pair (Q0.0/Q0.1 for PTO1).
3. Set the drive type to "Servo" (vs. stepper) so the technology object exposes a closed-loop-ready error window.
4. Configure user units: choose mm, inches, degrees, or revolutions, then enter the load distance per motor revolution (e.g., 10 mm/rev for a 10 mm-pitch ballscrew).
5. Set the maximum velocity, acceleration, and jerk to values supported by the mechanics. A safe commissioning starting point is 20% of the drive's rated speed.
6. Define software limit switches (positive and negative) in the configuration to protect the mechanics.
7. Configure the homing mode: active homing with the home-switch input (DI of the drive or DI of the S7-1200) is the standard.
8. Download the project. The technology object is now accessible as <TO_name> in the user program.

Sample Program in Structured Text

The following ST snippet implements the typical sequence: enable the drive, home, then move to absolute position 500 mm at 5 000 mm/s. This is the corrected version of the typical "MC_Power + MC_MoveRelative" pattern that operators new to PLCopen motion often attempt.

// FB instances (declare as STATIC in FB or in global DB)
InstMC_Power : MC_Power;
InstMC_Home  : MC_Home;
InstMC_MoveAbs : MC_MoveAbsolute;
InstMC_MoveRel : MC_MoveRelative;
InstMC_Reset  : MC_Reset;

// Tag declarations
bEnable       : BOOL;     // Start enable
bStartHome    : BOOL;     // Start homing
bStartMoveAbs : BOOL;     // Start absolute move
bStartMoveRel : BOOL;     // Start relative move
lrAbsTarget   : LREAL := 500.0;   // mm
lrRelDist     : LREAL := 50.0;    // mm
lrVelocity    : LREAL := 5000.0;  // mm/s
bMoveDone     : BOOL;
bDriveReady   : BOOL;
bErr          : BOOL;
wErrID        : WORD;

// 1) Enable the drive continuously
InstMC_Power(Axis := MyAxis,
             Enable := bEnable,
             StopMode := 0,
             Status => bDriveReady,
             Busy => , Error => bErr, ErrorID => wErrID);

// 2) Run homing (call once, on first start, or after power cycle)
InstMC_Home(Axis := MyAxis,
            Execute := bStartHome AND bDriveReady,
            Position := 0.0,
            Mode := 0,           // 0 = passive, 1 = active
            Done => , Busy => ,
            Error => bErr, ErrorID => wErrID);

// 3) Absolute move - correct FB for "go to position X"
InstMC_MoveAbs(Axis := MyAxis,
               Execute := bStartMoveAbs AND bDriveReady,
               Position := lrAbsTarget,
               Velocity := lrVelocity,
               Acceleration := 10000.0,
               Deceleration := 10000.0,
               Direction := 0,    // shortest path
               Done => bMoveDone,
               Busy => ,
               CommandAborted => ,
               Error => bErr, ErrorID => wErrID);

// 4) Optional relative move - correct FB for "step N units from current pos"
InstMC_MoveRel(Axis := MyAxis,
               Execute := bStartMoveRel AND bDriveReady,
               Distance := lrRelDist,
               Velocity := lrVelocity,
               Done => ,
               Error => bErr, ErrorID => wErrID);

// 5) Error reset on rising edge
InstMC_Reset(Axis := MyAxis, Execute := bReset, Error => , ErrorID => );

Common Error Codes and Root Causes

The complete list is published in the S7-1200 Motion Control V4.0 function manual. For firmware-specific additions (V4.4 and V4.5), refer to the release notes in the same Knowledge Base entry.

Verification Procedure

1. After download, open the S7-1200 online view in TIA Portal and navigate to Commissioning → Axis control panel.
2. Click Enable in the axis control panel. The MC_Power Status output must turn TRUE within 2 seconds.
3. Click Home. The drive must travel to the home position and stop. MC_Home.Done must rise TRUE.
4. Use the Jog buttons to verify direction matches the physical axis movement. If positive jog moves the axis in the negative direction, invert the direction in the technology object configuration (Basic parameters → Direction).
5. Trigger the Position to position test from the axis control panel with a small target (e.g., 10 mm) and low velocity (50 mm/s) to confirm the wiring, direction, and limits are correct.
6. Switch to the user program and trigger the MC_MoveAbsolute block. Monitor the axis's actual position on the HMI or in the watch table; it must reach the target within the configured positioning window.
7. Test MC_MoveRelative by issuing a +10 mm incremental move, then a -10 mm move. The axis must return to its previous position.

Troubleshooting Matrix

Notes on the Pulse/Direction (CE-X7) Profile

The S7-1200 PTO emits a step/direction signal, which is the most common interface on small-format servo drives. The CE-X7 profile referenced in older Festo / Siemens documentation describes a 5 V differential (RS-422) pulse output with 24 V opto-isolated direction. Most current drives (Siemens V90 pulse variant, Yaskawa Sigma-7, Delta ASDA-A2, Mitsubishi MR-J4 in pulse-train mode) accept this directly. Always wire the drive's PULS+ / PULS- and SIGN+ / SIGN- to the S7-1200 differential outputs through a shielded twisted pair; ground the shield on the drive end only.

Field-Proven Best Practices

• Always call MC_Power in a cyclic task, not in a startup OB. Cyclic invocation lets the axis re-enable automatically after MC_Reset.
• Interlock MC_MoveAbsolute.Execute with MC_Power.Status AND MC_Home.Done. Issuing motion before homing is the single most common cause of ErrorID 16#800B.
• Use MC_Reset to clear a drive-side error and motion-side error in one operation; a rising edge on its Execute input performs the reset and re-arms the axis.
• For pick-and-place routines with multiple stations, store each target as a constant in a global DB and call MC_MoveAbsolute with the constant. Avoid dynamic distance calculations; they invite logic errors.
• Limit Jerk to 80-100 % of maximum during commissioning. Allowable jerk protects the mechanics; a jerk-limited (S-curve) profile is dramatically smoother than a trapezoidal one.
• For applications that must survive a power cycle without re-homing (vertical axes, large rotary tables), use a multi-turn absolute encoder and switch to a closed-loop position interface such as PROFINET IRT with a Siemens V90 PN or a Festo CMMT-AS. Pulse/direction is inherently incremental.

FAQ