When a PowerFlex 753 accepts Ethernet/IP start and stop commands yet refuses to leave a hard 60 Hz running speed, the instinct is to blame the PLC reference path or the network adapter. That instinct is often wrong. A common field pattern is Param 874 showing a commanded Port 4 reference of 47.5 Hz while the motor shaft continues at line-frequency speed. The drive is not ignoring Ethernet; it is applying a speed floor that many engineers still read as a soft ceiling.

PowerFlex 753 reference arbitration is layered. Ethernet/IP may correctly deliver a speed setpoint into the Port 4 reference register, yet the drive still evaluates that value against Digimotors and parameter limits before the inverter synthesizes stator frequency. If Parameters 522 (Minimum Speed Fwd) and 523 (Minimum Speed Rev) are left at the ±60 Hz defaults commonly inherited from a template or a copy-from-drive operation, any reference below 60 Hz is forced upward to that floor. The result looks like a frozen 60 Hz machine even though Param 874 and the PLC tag both report 47.5 Hz.

Confirm the Ethernet path before changing limits

Begin with the reference source chain so you do not mask a real network fault with a parameter edit. Inspect Parameter 545 (Speed Ref A Sel or equivalent speed reference source selection on your firmware). For Ethernet-commanded speed, Port 4 must be the active reference path. Parameter 874 (Port 4 Reference) should track the PLC speed word after scaling. If 874 is already correct and the shaft is not, the problem is downstream of the adapter.

Next open Parameter 935 (Drive Status 1) and decode bits 10 through 14. Those bits report Auto/Manual mode and which of Ref A through Ref D is currently selected. An Auto mode with Ref A active and Port 4 feeding Ref A is the healthy Ethernet case. If Manual is latched, or if another reference bank is selected by digital inputs or HIM override, Ethernet start/stop can still work while speed remains locally forced.

Also verify Parameter 549 (Speed Ref A Mult). The default multiplier of 1.0 should leave engineering units unchanged. A multiplier left at 0, or a residual scaling factor from a previous OEM program, can zero or distort the reference even when 874 looks plausible in one monitoring screen and wrong in another.

Root cause: minimum speed as a floor, not a ceiling

Parameters 522 and 523 are not maximum-speed clamps. They define the lowest magnitude the drive will accept in forward and reverse. When both are set to ±60.00 Hz, every smaller Ethernet reference is coerced to 60 Hz magnitude. Operators see “Ethernet commands work for run/stop” because those commands do not pass through the same floor logic, while speed appears locked. Setting 522 and 523 toward 0.0 Hz restores the intended reference range and allows 47.5 Hz (or any other intermediate setpoint) to pass through to the inverter.

After lowering the floors, cycle the speed reference from the PLC across several points—0, 20, 47.5, and 55 Hz—and confirm Param 874, the HIM output frequency display, and a handheld tachometer move together. If 874 tracks but mechanical speed does not, revisit encoder feedback, slip compensation, or mechanical binding; those are separate from the floor issue.

Related parameters worth documenting

Commissioning procedure that avoids repeats

1. Place the drive in a safe state with the mechanical load free to rotate or locked out per site procedure.
2. From the PLC, assert stop, then verify Ethernet I/O connection and adapter status LEDs.
3. Write a known intermediate speed (for example 47.5 Hz) and start the drive in Auto with Port 4 selected.
4. Record 874, 935 bit field, 545, 549, 522, and 523 in the as-found sheet.
5. If 874 is correct and 522/523 are ±60 Hz, set both minima near 0.0 Hz, download, and retest the same reference ladder.
6. Validate reverse direction separately if the process uses bipolar speed commands.
7. Save the parameter set to the HIM or EEPROM backup and update the OEM template so the next clone does not restore the 60 Hz floors.

Confirm speed follows the Ethernet reference

Teams often change maximum speed parameters when the symptom is “stuck at 60,” which does nothing to a floor problem. Others scale the Ethernet speed word twice—once in the PLC and again in 549—producing confusion when 874 looks “almost right.” Document units explicitly (Hz versus percent of base speed) before blaming the network. On multi-drive lines, copy-parameter tools frequently transplant 522/523 from a constant-speed fan application into a variable-speed process train; treat those two parameters as mandatory review items on every Ethernet-commissioned PLC & PAC systems integration.

Finally, remember that start/stop over Ethernet only proves the adapter and logic class are alive. Speed following requires an unbroken chain from PLC word through Port 4, status bit selection, multiplier, and the forward/reverse minimum floors. When Param 874 already shows 47.5 Hz and the shaft sits at 60 Hz, inspect 522 and 523 before opening a Wireshark capture.