Allen-Bradley PowerFlex 753/755 Control Sync Fault Fix

Quick answer

On PowerFlex 753 and 755 drives, the Control Sync Fault (fault code F70, depending on firmware version sometimes paired with F71 Drive OL Lvl 1 or F72 Drive OL Lvl 2) means the control board lost time-synchronized communication with the gate-fire logic on the power module. By far the most common field cause I see is a loose or corroded ribbon cable between the main control cassette (the 20-750-S1 board) and the power module backplane — typically after someone has slotted an option module in or out and didn’t reseat the control cassette properly. A close second is a 24VDC auxiliary control supply that’s sagging under load from too many option cards installed without a 20-750-APS power supply.

What PowerFlex 753/755 Control Sync Fault means

The PowerFlex 750-class architecture splits control intelligence between three boards: the Main Control Board (20-750-S1 standard control or 20-750-S3 advanced control), the Power Module gate-drive logic, and any option modules in slots 4, 5, and 6. These boards talk to each other through a high-speed serial bus on the cassette backplane, with deterministic timing — every IGBT switching event is scheduled by the control board and acknowledged by the gate-drive logic within a fixed window (typically under 50 microseconds).

Control Sync Fault latches when the main control board issues a switching command and does not receive the expected acknowledgement from the gate-drive logic within the timing window. This can happen for three reasons: a physical connection problem (loose cassette, dirty pins, broken trace), a control-side firmware issue (corrupted parameter set, mismatched firmware between cassette and power module), or insufficient auxiliary power on the 24VDC control bus (too many option cards, failing 20-750-APS auxiliary power supply).

On a 753 (single-frame design) the cassette is more integrated and Control Sync is rarer. On a 755 (frame 1 through frame 8, modular design with separate cassette and power module) Control Sync is more common because the cassette can physically loosen over time from thermal cycling and vibration. The 755 also supports much larger option-card counts which increases the chance of an auxiliary power problem.

Read the fault history first

The 750-class drives keep a detailed fault history. Do not press Clear Faults until you have read the log. Drive log entries persist through fault clear but the live snapshot of operating parameters does not.

Two paths to read it:

Via Studio 5000 Logix Designer (PowerFlex 755 on EtherNet/IP):

1. Open your project, expand the I/O tree
2. Right-click the PowerFlex 755 drive, select Properties
3. Navigate to Drive → Faults tab
4. The fault history shows the last 32 faults with code, descriptor, and timestamp
5. Click on the most recent Control Sync entry to see the snapshot — output frequency, motor current, DC bus voltage at the moment of trip
6. Also check the Drive → Diagnostics tab for live parameter readings — particularly parameter 935 Drive Status 1 and 936 Drive Status 2

Via Connected Components Workbench (PowerFlex 753 via USB or 22-SCM-232):

1. Connect via the 1203-USB cable or the 22-SCM-232 serial converter
2. Open the project, double-click the drive icon
3. Navigate to Diagnostics → Faults & Alarms
4. The dialog shows fault code, descriptor, timestamp (from drive’s internal hour counter), and snapshot data
5. Read parameter 951 Last Fault, 952 Fault 2, 953 Fault 3, through 958 Fault 8 for the rolling history

For either drive, also pull these snapshot parameters:

• N002 — Last Stop Source
• N003 — Last Fault Code
• N005 — Fault 1 Bus Voltage (the DC bus reading at the most recent trip)
• N006 — Fault 1 Current (motor current at the most recent trip)
• N011 — Fault 1 Drive Status

Rockwell documents this in PowerFlex 750-Series AC Drives Programming Manual publication 750-PM001 chapter 4 (Diagnostics) and the Hardware Service Manual 750-TG001.

Field insight — the Control Sync history trap: when you read N003 (Last Fault Code) you may see Control Sync paired with a preceding fault from a few seconds earlier — often F12 HW Overcurrent or F4 Undervoltage. Don’t assume Control Sync is the root cause. The control board can lose sync because the gate-drive logic asserted an emergency stop on a hardware-detected overcurrent and the cassette interpreted that as a missing acknowledgement. Check N002 and the preceding fault in the history — if there’s a precedent fault, diagnose that one first.

Common causes (ranked by frequency)

1. Loose control cassette — cassette retaining screws not fully torqued, or cassette wasn’t fully seated after the last option card swap; vibration over time loosens it further
2. Dirty cassette-to-backplane pin connector — oxidation on the gold-plated pins from cabinet humidity, oils from fingerprints during installation, or industrial atmosphere contamination
3. Mismatched firmware between cassette and power module — someone updated the cassette firmware via DriveExplorer or Studio 5000 but didn’t update the power module firmware (or vice versa); the two pieces must be at compatible revision levels
4. 24VDC auxiliary supply sagging — too many option cards (DeviceLogix, EtherNet/IP, encoder feedback, additional digital I/O) drawing more than the integrated supply can deliver; need a 20-750-APS auxiliary power supply module
5. Failed control cassette — control board damage from a power surge, lightning strike, or thermal cycling failure of a surface-mount component
6. Failed power module backplane — physical damage to the backplane traces from vibration or impact, less common but does happen in retrofit installations where the drive was struck during transit
7. Ground loop / EMC interference — high-frequency noise on the chassis ground from improperly bonded motor cable shields induces sync timing errors

Step-by-step diagnosis

Before opening the drive cabinet: lock and tag the upstream disconnect. Wait the rated capacitor discharge time — PowerFlex 753 frames 0–3 specify 5 minutes, frames 4–6 specify 10 minutes. PowerFlex 755 frames 1–4 specify 5 minutes, frames 5–8 specify 15 minutes. Verify zero DC bus voltage at terminals DC+ and DC- with a CAT-IV meter rated 1000 VDC. Respect the NFPA 70E arc-flash boundary marked on the cabinet label until zero energy is confirmed and you are in the proper PPE.

1. Read the fault log before clearing. Pull N002, N003, N005, N006, N011 plus the full fault history (951–958 or via Studio 5000 / CCW). Note any preceding faults — Control Sync often shadows an earlier overcurrent or undervoltage event that’s the real culprit.

2. Identify your firmware revisions. With control power available (drive can be in faulted state, just need control to read parameters), navigate to:

   • Parameter 17 — Main Control Firmware Version
   • Parameter 19 — Power Module Firmware Version (on 755 cassetted drives)
   • Parameter 21 — Option module firmware (per slot) Compare against Rockwell’s compatibility chart in publication 750-RN001. If main control is newer than power module by more than one major version, you have a compatibility issue.

3. De-energize, verify zero, and reseat the control cassette. Loosen the four cassette retaining screws (Phillips head, M4) and gently rock the cassette to free it from the backplane. Inspect the gold-plated pins on the cassette and the receiving sockets in the power module. Oxidation appears as a dull yellow-brown discoloration. Clean with DeoxIT Gold (G100L) on a lint-free swab — never use abrasive contact cleaner on gold pins. Reseat firmly, then torque the four screws to 0.9 Nm (8 in-lb) using a Wera or Wiha torque screwdriver.

4. Inspect option cards in slots 4, 5, 6. Same procedure: loosen retaining screws, pull each card, inspect pins, clean if needed, reseat and torque. Even an unrelated option card with a marginal connection can dump enough noise onto the 24V bus to trigger Control Sync.

5. Verify auxiliary power capacity. Read parameter 180 Aux Power Status (on drives with the 20-750-APS installed). If you have three or more option cards but no 20-750-APS, the integrated 24V supply is likely overloaded. Spec: integrated supply provides 1.0 A at 24 VDC. Each communication card draws 100–200 mA, each encoder card 250 mA. Add up your option-card current — if you’re above 800 mA you need the APS module.

6. Power up with minimum configuration. With drive de-energized, remove every option card you can spare (leave only what’s required for control — typically the EtherNet/IP card and any safety modules). Re-energize and try to start the drive. If Control Sync clears, you have an option-card or auxiliary-power problem and you reinstall cards one at a time until the fault returns.

7. Verify firmware compatibility and update if needed. Use DriveTools SP or Studio 5000 Logix Designer to read all firmware revisions. If they don’t match Rockwell’s compatibility chart, flash the appropriate components. Always update the power module first, then the main control cassette, then the option cards — never reverse the order.

8. Check chassis grounding and motor cable shielding. Inspect the motor cable shield termination at the drive end — the shield should be 360° bonded to a properly grounded cable gland or terminal block, not just twisted into a pigtail. A poor shield termination is a frequent source of high-frequency noise that triggers sync errors.

9. If everything checks and Control Sync still trips: swap the control cassette into an identical drive in your shop (a 20-750-S1 from a non-running drive). If the fault follows the cassette, the cassette is bad. If the fault stays with the original drive, the power module backplane is damaged and the drive is replaced.

Parts that may need replacement

When to call a controls engineer

Bring in senior support when: Control Sync trips happen only at specific operating frequencies (rare, but suggests EMI from a coincident plant load that needs power quality investigation); firmware updates have been attempted but the drive will not enter normal flash mode (TechConnect support needed); the cassette appears mechanically damaged from rough handling during a panel rebuild; or when you’re managing a fleet of 50+ PowerFlex drives and Control Sync is appearing across multiple drives suggesting a systemic install or environment problem.

FAQs

Can I swap the cassette from a different size drive? No. The 20-750-S1 cassette firmware is tagged to the power module rating and will fault on mismatched horsepower. You need a cassette specifically from a matching-frame drive — or order a new 20-750-S1 (or S3) from Rockwell, which ships as a generic part and learns the host drive’s power-module identity on first power-up.

Does Control Sync mean the drive is dead? Almost never on the first occurrence. Reseat the cassette, clean pins, verify firmware compatibility, and inspect auxiliary power. Eight out of ten Control Sync calls I’ve responded to were resolved by reseating the cassette.

Why does my drive get Control Sync only after running for an hour? Thermal expansion of the cassette and backplane creates a marginal connection at room temperature that gets worse as the enclosure heats up. Reseating with proper torque is the fix. If the problem persists after reseating, look for inadequate cabinet cooling — drive ambient temperature should not exceed 50°C per spec sheet 750-TD001.

Difference between Control Sync and Drive OL Lvl 1? Drive OL Lvl 1 (F71) is an inverter overload warning — the drive is approaching its thermal limit. Control Sync (F70) is a communication-internal fault between control and power module. They can co-occur because thermal stress on the cassette can cause both issues, but they have different root causes.

Should I leave Studio 5000 connected continuously to monitor the drive? Not necessary, but a useful diagnostic. Drive Composer Pro (for ABB) and Studio 5000 Trends (for Rockwell) can capture pre-fault data — set up a trend on the relevant parameters and you’ll have rich diagnostic data the next time Control Sync trips.

Related guides

• Allen-Bradley PowerFlex 755 Power Loss Fault Fix
• Allen-Bradley PowerFlex 755 Aux Input Fault Fix
• Allen-Bradley PowerFlex F004 Fault — Undervoltage Fix
• Allen-Bradley PowerFlex F081 Fault — Communication Loss Fix
• Allen-Bradley PowerFlex F122 Fault — I/O Card Fix

See Also

• Allen Bradley PowerFlex 753 F12 Fault — Causes & Fix
• Allen-Bradley PowerFlex 755 Fault Codes — Complete Troubleshooting Guide
• Allen Bradley PowerFlex 525 F7 Fault — Causes & Fix
• Allen-Bradley PowerFlex F122 Fault — I/O Board Failure Fix