Sinamics G120 CU240E-2 Two-Speed Reverse Direction

Problem Overview

A common field issue with the SINAMICS G120 modular drive occurs when a two-speed (or bi-directional fixed setpoint) application is wired to the CU240E-2 Control Unit (article number 6SL3244-0BB12-1BA1) and the PM240 power module. The forward command, analog setpoint, and inching-forward command operate correctly, but the reverse command via digital input and the inching-reverse command do not produce any motor motion. Both negative setpoints appear to be ignored by the inverter, although no fault is raised.

The root cause is rarely a hardware failure. It is almost always a combination of (a) the default direction-inhibit parameter blocking the negative direction, (b) the fixed setpoint not being routed through a BICO source that is allowed to take a negative value, and (c) the command data set switching not being enabled. This article explains the parameter set, the wiring, and the verification sequence required to make both reverse and inch-reverse functional on a CU240E-2 with firmware V4.4 through V4.7 (the parameter numbers used here are valid across the V4.x line).

System Identification and Pre-Checks

Confirm the hardware revision and firmware before changing parameters. Wrong firmware assumptions lead to parameter-not-found errors and incomplete commissioning.

Read the firmware version with a BOP-2, IOP, STARTER, or TIA Portal (Startdrive). Older firmware (V4.3 and earlier) does not support all the index structures described here. If the firmware is below V4.4, upgrade first using a Siemens S7 memory card or via SINAMICS firmware V4.7 SP3 release notes.

Terminal and Signal Mapping

The CU240E-2 variant 6SL3244-0BB12-1BA1 exposes the following default-relevant terminals on the customer terminal block. The numbers below refer to the factory-printed labels on the CU cover.

For the reported two-speed application, the wiring is correct as described in the field report: DI0 = Forward command, DI1 = Inch forward, DI2 = Inch reverse, DI3 = Reverse command source. The problem lies entirely in the parameter layer that converts these digital signals into negative setpoints and a reverse ON command.

Parameter-by-Parameter Analysis

The working channels and the failing channels are configured almost identically. The difference between the working and failing paths is the sign of the setpoint value and the index on P0840 (which carries two command data sets: P0840[0] for CDS0 forward and P0840[1] for CDS1 reverse).

2.1 Forward Channel (Working)

Because the jog setpoint is positive, the drive simply rotates the motor in the positive direction as soon as DI1 is high. No special configuration is needed.

2.2 Reverse Channel (Failing)

The single most common reason for the reverse path to be dead-quiet on a CU240E-2 is the default value of P1110 = 1. With P1110 = 1, any negative setpoint request from the setpoint channel is interpreted as zero speed. The drive will accept the ON command via P0840[1], it will accept the DI3 trigger via P1020[0], but the speed setpoint is forced to 0 rpm, and the motor does not rotate. No alarm is raised because the command was honored; only the setpoint was clamped.

2.3 Inch Reverse Channel (Failing)

Jogging with P1059 negative is the classic symptom of P1110 left at default. Both reverse paths fail for the same reason: the inverter refuses to command a negative speed.

Root Cause: Direction-Inhibit Parameters

Parameters P1110 and P1111 are global direction-inhibit bits that clamp the speed setpoint at the very last stage of the setpoint channel, after the ramp generator and before the speed controller. They are evaluated on every scan, regardless of which command data set is active, regardless of whether the source is the analog input or a fixed setpoint, and regardless of whether the speed came from P1058/P1059 (jog) or P1001-P1015 (fixed).

Siemens ships every G120 with P1110 = 1 and P1111 = 0 as a factory safety default. This protects machines that cannot tolerate reverse rotation (fans, pumps, single-direction conveyors). For a two-speed bi-directional application the values must be flipped.

Step-by-Step Solution

Apply the following sequence. The order matters because the drive will reject writes to some parameters if the inverter is in the RUN state, and will reject parameter saves if the safety password has not been cleared.

1. Stop the drive. Open the ON/OFF1 command by forcing DI0 low (or remove the enable from the safety chain). Confirm the drive is in state S1 (Ready for switching on) or S2 (Ready for operation). Check r0002 = 1.
2. Clear the safety password. From STARTER or TIA Portal, navigate to Drive > Parameters > Safety Integrated and enter the old password. If the password is unknown, perform a factory reset using P0970 = 30 (note: this resets all parameters to factory defaults, including motor data, and will require a re-commissioning).
3. Allow negative direction. Set P1110[0] = 0. This is the primary fix. Save with P0971 = 1 or click the RAM-to-ROM button in STARTER.
4. Allow positive direction. Verify P1111[0] = 0 (default).
5. Enable fixed setpoint routing. Set P1000[0] = 3 (fixed setpoint as primary source for CDS0). The value 3 means "fixed setpoint, no additional main setpoint." If the application needs both an analog main setpoint and a fixed reverse setpoint, use P1000[1] = 3 and switch CDS via P0810.
6. Configure the fixed setpoint value. Set P1001[0] = -500.000 (rpm). The CU240E-2 accepts signed fixed setpoints only when P1110 = 0. Save to ROM.
7. Connect the fixed setpoint selection bit. Set P1020[0] = 722.3 (DI3). With this single bit, P1001 is selected whenever DI3 is high.
8. Verify the reverse ON command. The factory default already provides P0840[1] = 722.3, so DI3 acts as the reverse ON/OFF1 for CDS1. If CDS1 needs to be the active data set for reverse, set the CDS switchover so that DI3 high selects CDS1. The simplest approach is to leave P0840[0] = 722.0 and P0840[1] = 722.3 in the same CDS; the index [0] vs [1] is the data set identifier.
9. Configure inch reverse. Set P1056[0] = 722.2 (already correct), P1059[0] = -100 (already correct). Verify that P1055[0] and P1056[0] are not interlocked by an active jog enable that requires P1051 or P1052.
10. Save to ROM. Write P0971 = 1. Cycle power to make the new values the boot values.

Complete Parameter Set for the Reported Application

The following is a complete, working parameter set for a CU240E-2 + PM240 used as a two-speed bi-directional drive with jog. All indexed parameters show CDS0 / DDS0 in the index field unless otherwise noted.

Command Data Set Switching

If the application requires that DI3 simultaneously select a different setpoint source and a different ON/OFF1 command, the cleanest engineering approach is to use the two built-in CDS:

• CDS0: DI0 is the ON/OFF1 forward command, analog input is the setpoint source.
• CDS1: DI3 is the ON/OFF1 reverse command, fixed setpoint is the setpoint source.

To switch between CDS0 and CDS1 automatically based on which input is high, use the BICO selection bits P0810 and P0811. The simplest deterministic scheme is to wire DI3 directly to the CDS selection: set P0810 = 722.3. With this configuration, a high on DI3 selects CDS1 (the reverse profile); a low on DI3 returns the drive to CDS0. Because both DI0 and DI3 are gated by the same inverter enable logic, only one direction can be commanded at a time, which is the desired interlock for a two-speed drive.

Manual CDS Switching Example

If the application requires two physically separate "speed select" push-buttons rather than one toggle, leave P0810 at default and use the BOP-2 arrow keys, the IOP, or a PLC-driven HMI tag to switch CDS. The drive reports the active data set in r0050 (active CDS) and r0051 (active DDS).

Pre-Assignment Reference (Manual Section 3.4.6.2)

The SINAMICS G120 CU240E-2 manual (entry ID 61618946) chapter 3.4.6.2 documents the factory wiring pre-assignments of every digital and analog terminal. The following mapping is the relevant subset for the reported fault:

Because all four DIs ship as stand-alone, the user must explicitly route each one to the desired function via BICO. The reported configuration (DI0 to P0840[0], DI1 to P1055[0], DI2 to P1056[0], DI3 to P1020[0]) is consistent with the manual. There is no missing wiring - only missing direction-block reset.

Verification Procedure

After saving and cycling power, perform the following functional checks in this order:

1. Power-up state. Read r0002; it should be 1 (Ready for switching on). Read r0722; all four DIs should be 0 (no command active). Read r0020; it should be 0.000 rpm (no setpoint).
2. Forward test. Apply DI0 = high. Drive should accelerate to the analog setpoint value. Read r0021 = commanded rpm, r0023 = output frequency (Hz). Remove DI0; drive should ramp down.
3. Reverse test. Apply DI3 = high. Drive should accelerate to -500 rpm. Read r0020; it should report a negative value. Read r0023; the sign should be negative. If r0020 stays at 0.000 while DI3 is high, P1110 is still 1. Check with r0722.3 (must be 1) and r0050 (CDS index).
4. Inch forward test. With DI0 low, apply DI1. Drive should run at +100 rpm. Release DI1; drive should ramp to zero.
5. Inch reverse test. With DI0 low, apply DI2. Drive should run at -100 rpm. Release DI2; drive should ramp to zero.
6. Interlock test. Apply DI0 and DI3 simultaneously. Drive should latch an F07220 (or A07801 - drive: power unit overload warning, depending on firmware) or simply hold the last command. Verify with the machine schematic whether the application requires the drive to fault or hold.

Fault and Warning Reference

The following drive-side diagnostics are useful when reverse still does not respond after applying the fix:

For a complete fault list, refer to the SINAMICS G120 fault and warning list (entry ID 109751049) and the SINAMICS G120 parameter manual (entry ID 109782835).

Alternate Commissioning Tools

The parameter set above can be entered through any of the following interfaces:

• STARTER (V5.x): PC tool, USB-to-RS485 on the CU240E-2 front service port. Project tree > drive > Parameters > expert list. Use the search field "P1110" for fastest navigation.
• Startdrive (TIA Portal V15.1 and later): integrated in TIA Portal. Project > Devices > G120 > Parameters. Right-click the parameter to write the new value. Save the project to the PLC's memory card.
• IOP (Intelligent Operator Panel): handheld panel that plugs into the CU front. Navigate to Parameters > Search > P1110. Use the rotary knob to set the value and press OK.
• BOP-2 (Basic Operator Panel): two-line LCD panel. Press the FN key, use the up/down arrows to scroll to P1110, press P (parameter), edit, press P to confirm.

After any write, cycle power (off, wait for DC bus discharge, on) so the boot values reflect the new parameters. BOP-2 also provides a "SAVE" parameter, P0971, which writes RAM-to-ROM without requiring a power cycle.

Edge Cases and Field-Proven Caveats

1. P1110 takes effect only after an OFF1 transition. If you change P1110 while the drive is running, the new value is latched at the next OFF1 edge. After the change, perform one stop/start cycle.
2. Ramp generator direction is independent. P1110 and P1111 clamp the setpoint, not the ramp output. So even with P1120 = 0 (no ramp), a negative fixed setpoint is honored once P1110 = 0.
3. Analog input polarity. If the analog input is configured for 0-10 V (P0756 = 0, P0757 = 0, P0758 = 0, P0759 = 10, P0760 = 100%), it cannot produce a negative value. The negative setpoint must come from a fixed setpoint (P1001-P1015) or from the jog setpoints (P1058/P1059). For negative analog setpoints, configure AI0 for -10 V to +10 V (P0756 = 4, P0757 = -10, P0758 = -100, P0759 = 10, P0760 = 100) and use that analog input for reverse.
4. Jog has priority over fixed setpoint. If both DI1 (jog forward) and DI3 (fixed setpoint reverse) are high at the same time, the drive will execute the jog command at +100 rpm, not the reverse fixed setpoint. This is by design: jog overrides the normal setpoint channel.
5. Safe Torque Off (STO). If the CU240E-2 variant has STO enabled (F variant 6SL3244-0BB12-1FA1), the STO inputs must be high for the drive to leave the safe state. With STO active, DI0 or DI3 will not start the motor and no fault is raised - the drive simply remains in state S4 (Safe standstill). Check r9772.0 for STO state.
6. Encoder feedback (V/f with encoder). If the application uses an HTL or TTL encoder on the CU240E-2 encoder interface, the encoder direction must match the commanded direction. A reversed encoder (A and B swapped) will cause the drive to report speed deviation warning A07901 and may prevent reverse rotation.
7. Motor direction convention. The "positive" direction is defined by the order of motor phases. If the physical wiring of U-V-W is reversed relative to the motor rating plate, the drive will rotate in the opposite direction. Swap two of the three motor leads to fix the convention.

Reverse Engineering the Application

If the application schematic is unavailable, the following read-only parameters can be used to reconstruct the configuration from a connected drive:

For a live trace, use STARTER's trace function or Startdrive's trace to record r0722, r0020, r0023, and r0050 over a 10-second window while toggling DI3 and DI2. The captured trace will show whether the setpoint ever goes negative or whether P1110 is clamping it to zero.

Related Documentation

The following Siemens Knowledge Base and manual entries support this configuration:

• SINAMICS G120 with CU240E-2 Control Units - Operating Instructions (entry ID 61618946)
• SINAMICS G120 List Manual - Faults and Warnings (entry ID 109751049)
• SINAMICS G120 Parameter Manual (entry ID 109782835)
• SINAMICS G120 Firmware V4.7 SP3 Release Notes (entry ID 67364620)

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