Resolving EZ Path S Limit Switch and Yaskawa CN-6A Reference

Overview

The 1996 Bridgeport Romi EZ Path S is a CNC slant-bed turning center that uses Yaskawa servo drives and motors on both the X (cross slide) and Z (longitudinal carriage) axes. Many of the field service problems reported on these machines center on two distinct but interlocking faults: a false "limit switch tripped" message at power-on or homing, and a "following error" that appears after a servo drive replacement. Both faults share a common root cause area—drive-side parameter setup and limit/reference input wiring—but require different diagnostic paths.

This technical reference covers the controlled procedure for clearing both classes of faults on the Yaskawa servo package used in the EZ Path S, including the rarely documented CN parameter procedure that only works with the JUSP-OP02A-1 handheld digital operator.

System Topology and Part Identification

Before any troubleshooting begins, identify the exact hardware in the cabinet. The EZ Path S uses:

The reference/home sequence is driven through a hardware reference switch (deceleration dog) wired back to the control and a marker (C) pulse from the encoder. The drive must see the C pulse after the deceleration dog opens, and the drive will only treat it as valid if the pulse width window is wide enough for the motor RPM at the reduced reference speed. That window is set in CN parameters on the drive.

Problem 1: False Limit Switch Trip — X Axis Will Not Jog

Symptom: At control power-on or homing, the control annunciates a limit switch fault on the X axis even though the carriage is clearly inside the travel envelope. Jogs in either direction are accepted by the control but the axis moves only a few thousandths (a couple of "thou") and then disables. The Z axis jogs normally.

Root cause candidates (in order of probability):

1. Open or shorted X-axis overtravel limit switch (NC contact in series, or NO contact driving an input) wired to the control I/O.
2. Limit switch wiring chafed in the cable carrier (common failure on the X axis because the cable flexes every cut).
3. Drive "drive inhibit" input active because of an upstream safety relay or because the drive reports an internal alarm that is being interpreted by the control as a limit trip.
4. Servo following error that masks itself as a limit trip in older Bridgeport Romi control firmware.

Diagnostic procedure for the limit switch itself:

1. Power off. Locate the X-axis reference / overtravel switch on the cross slide. Manually actuate it (toggle the lever) and listen for an audible click.
2. With the switch disconnected from the wiring harness, measure across the contacts with a digital multimeter:

3. Trace the two conductors from the switch back into the cable carrier and into the electrical cabinet. With the switch disconnected at the switch body, measure resistance to cabinet ground on each conductor. Both should read OL. Any low-ohm reading means insulation breakdown—most often inside the cable carrier.

4. Reconnect the switch. With power on and the drive disabled (ESTOP engaged), measure the DC voltage at the control's limit input terminal referenced to 0 V. The reading will tell you whether the input is sourcing or sinking and whether the input is being pulled in by the control's internal bias.

If the switch itself is electrically intact and the wiring passes the ground test, the fault is upstream—most likely the Yaskawa drive is in an alarm state and the alarm contact is being routed through the same interlock chain as the limit. Move to Problem 3 (Following Error) diagnostics.

Problem 2: Yaskawa Drive Replacement — Will Not Reference Home

This is the most common and most field-confused fault on the EZ Path S after a drive swap.

Symptom: After replacing a Yaskawa servo drive, the homing cycle behaves correctly up to the deceleration dog. The drive decelerates, releases from the dog, looks for the encoder marker (C) pulse, and never sees it. The axis continues at creep speed until it physically hits the hard stop and the drive trips on overload.

Root cause: The Yaskawa drive ships from the factory with the reference marker pulse acceptance window sized for the manufacturer's default motor. On the EZ Path S, that default window is too narrow for the marker pulse width the lathe actually produces at reference speed. The drive therefore time-discards valid marker pulses and never latches a home position.

The fix: Modify Yaskawa parameter CN-6A from the factory setting H840A to H842A. This widens the C-pulse acceptance window so the reference marker is reliably latched.

Procedure: Editing Yaskawa CN-6A with the JUSP-OP02A-1

Perform this procedure with the drive enabled but the motor mechanically unclamped if possible; otherwise perform it in parameter-edit-only mode with the motor disconnected at the power leads to eliminate motion risk.

1. Power down the cabinet. Open the Yaskawa drive front panel. Connect the JUSP-OP02A-1 to the drive's CN1 keypad port.
2. Power up the control. The digital operator will boot to the default Monitor mode.
3. Press MODE/Set until the menu displays Parameter Edit, then press DATA/Set to enter.
4. Press the UP/DOWN arrow keys until the parameter display shows CN-00.
5. Press DATA/Set to enter the CN parameter sub-tree.
6. Press UP until the display shows CN-6A.
7. Press DATA/Set. The display will show the current hex value, for example H840A.
8. Press the SHIFT key to position the cursor under the digit to be modified. Use UP/DOWN to change the appropriate nibble so the final value reads H842A.
9. Press DATA/Set to write the value. The display will flash briefly to confirm.
10. Press MODE/Set twice to back out to the parameter root, then once more to return to Monitor.
11. Cycle drive power (not just control power) and verify the new value persists.

Verifying the CN-6A Fix

1. Reset all faults on the drive and clear the control's fault history.
2. Command a return-to-reference cycle from the control.
3. The axis should approach the reference dog at rapid, decelerate when the dog is contacted, back off the dog, and latch the encoder marker pulse within one motor revolution.
4. Confirm that the displayed machine coordinate matches the parameter P0 or equivalent machine-zero value set in the Bridgeport Romi control.
5. If the cycle still overruns to the hard stop, re-enter CN-6A and double-check the second hex digit is 4 and not 0; a single transposed character returns the drive to the original behavior.

Problem 3: Following Error — Drive Swap Induced

Symptom: After swapping a Yaskawa drive between axes (X and Z) as part of diagnosis, the fault moves with the drive or stays with the original axis. The machine may now move a couple of "thou" at slow jog speed and trip on following error when jogged faster.

Definition: A Yaskawa following error means the drive commanded the motor to move a specified distance, the encoder feedback reported that the motor did not reach that distance within the position loop time window, and the drive annunciated the error to latch off.

Root cause candidates (ranked by field frequency on the EZ Path S):

1. Belt slip on the ball-screw belt drive. The EZ Path S uses a belt between the servo motor and the ball screw. A loose, glazed, or broken belt allows motor rotation without carriage motion—producing a clean following error at higher speeds.
2. Mechanical seizure of the ball screw, nut, or linear ways. More likely to produce an overload fault than a following error, but a partial bind under load can look like one.
3. Open phase in the servo motor (shorted winding), measured as a 0 Ω reading across motor leads.
4. Failed servo drive output stage. Swapping axes and observing the fault follow the drive confirms the drive, not the motor.
5. Servo brushes on older brushed servos only. The EZ Path S uses brushless, so this does not apply, but the same diagnostic logic is used on Bridgeport's earlier product lines.

Motor Resistance Checks

With the drive powered off and the motor leads disconnected at the drive terminal block, measure resistance phase-to-phase at the motor leads:

Also measure each motor lead to the motor frame (ground). All three readings should be OL. A low-ohm reading to ground means winding insulation breakdown or contamination inside the motor housing.

Axis Swap Diagnostic Method

The standard field diagnostic for an unknown axis fault is to swap the suspect drive with a known-good axis drive and observe:

Reversed Direction After Drive Swap

Symptom: After swapping drives, an axis moves correctly but in the opposite direction to what the operator expects. For example, turning the jog handle clockwise moves the cross slide outward instead of inward.

Root cause: The Yaskawa drive stores a direction polarity bit (commonly parameter Pn-001 bit 4 or the equivalent CCWLIMIT / direction setting on older Sigma units). When a drive is moved between X and Z, the polarity it was last configured for is still loaded.

Fix: Edit the sign-of-position or rotation direction bit on the donor drive to match the destination axis. Always re-run homing after this change.

Commutation and Encoder Re-Initialization

After any drive swap on a Yaskawa brushless servo, the absolute encoder offset must be re-learned. On incremental encoders (the typical EZ Path S configuration), this is done by:

1. Move the axis to a known mechanical mid-position by hand (drive disabled).
2. Enable the drive with no command pulse input.
3. Trigger the Yaskawa "encoder reset" or "phase-Z search" routine through the digital operator or via the SigmaWin+ setup tool.
4. Confirm the drive display shows a stable READY state before reconnecting to motion commands.

Skipping this step on a brushless system will produce following errors at low speed and torque-ripple faults at higher speeds as the drive attempts to commutate against the wrong electrical angle.

Cable Carrier and Continuous Flex Wiring Inspection

The X-axis limit switch home reference runs through a continuous-flex cable carrier on the cross slide. Failure modes that mimic switch faults:

• Insulation cracking where the cable passes the carrier's minimum bend radius—intermittent shorts to ground.
• Tension relief pulled out—wires breaking at the connector backshell, producing opens only under motion.
• Shield drain wire contacting a signal conductor—producing a half-fault that looks like the switch is "stuck."

Inspect the carrier with the machine at multiple axis positions; open the carrier at its service split if equipped, and visually verify jacket integrity for the full bend.

Safety and Latch-Clear Procedures

The Bridgeport Romi control on a 1996 EZ Path S latches drive alarms in non-volatile memory. After any drive-level fix:

1. Cycle ESTOP.
2. From the control's diagnostic page, clear the alarm log.
3. Power cycle the cabinet (not just the control) so the drives re-read their parameter non-volatile storage.
4. Verify zero following error displayed for ≥ 30 s before commanding motion.

Field Commissioning Checklist After Yaskawa Drive Replacement

1. Match drive part numbers; record serial number of the replacement.
2. Verify CN-6A = H842A before any homing attempt.
3. Match user parameter table to original (user parameters, not CN).
4. Set or verify direction polarity bit for the destination axis.
5. Re-learn encoder offset (brushless) or set Pn-205 absolute encoder initialization.
6. Perform controlled low-speed jog; monitor following error window in the digital operator.
7. Perform full travel in both directions; verify no alarms.
8. Run a full G-code program in single-block with feedhold override at 25%, verifying tool offsets return to expected coordinates.

Frequently Asked Questions