Mori Seiki MV-40 EX 293 Magazine Index Error Yasnac

Problem Overview

The EX 293 Magazine Index Error is a Mori Seiki-generated alarm on the MV-40 machining center equipped with a Yasnac CNC control. The fault presents when the automatic tool changer (ATC) attempts a tool change: the tool magazine carousel begins to rotate, decelerates progressively during the index cycle, then comes to a stop before announcing EX 293 on the operator panel. The on-screen "change tool" parameter flashes and no longer agrees with the physical pocket position the carousel has actually reached. Once tripped, the controller refuses subsequent M06 tool-change requests until the alarm is cleared and the magazine is re-referenced.

Unlike position or servo alarms generated natively by Yasnac firmware, EX-class alarms on the MV-40 are written by the Mori Seiki ladder program that runs on top of the Yasnac PMC. This is a critical distinction for diagnostics: the alarm message itself is a Mori macro that watches a specific set of input conditions in the ladder and throws EX 293 when those conditions are violated.

Affected Equipment and Control Generations

The MV-40 vertical machining center was produced by Mori Seiki across roughly 25 years of production with multiple Yasnac control iterations. The EX 293 magazine index fault is documented across several of these generations:

Identifying the exact control and machine year is the first step. The serial tag on the MV-40 electrical cabinet door and the Yasnac control software version page (typically SYSTEM > PARAM > PMC) will identify the generation. Cross-reference this against the Yaskawa Machine Controls product archive for firmware identification.

How the MV-40 Magazine Indexing Logic Works

The MV-40 tool magazine is a 20-pocket (some variants 16 or 24) horizontal chain-type carousel. Position feedback to the Yasnac PMC depends on machine vintage:

1. Prox-only vintage (early MX-1/MX-2): The magazine rotates freely until it passes pot 1 (home proximity switch). The PMC latches that transition and then counts pulses from a second cam/prox arrangement to derive the current pocket. EX 293 trips if the magazine stops before completing the count expected for the called pocket, or if it overshoots and the count never settles.
2. Encoder/resolver vintage (MX-3 and later): A multi-turn absolute encoder or resolver on the magazine shaft reports the current pocket directly. The PMC compares this against the requested pocket. EX 293 trips when the reported pocket does not stabilize within a time window, or when a parity check on the encoder data fails.

In both architectures, the Mori ladder program is the only authority that raises EX 293. This is why standard Yasnac alarm manuals do not list the code — you must consult the Mori Seiki MV-40 maintenance manual that ships with the machine (or its successor DMG MORI service documentation portal for registered owners).

Root Cause Matrix

Step-by-Step Diagnostic Procedure

Step 1 — Capture the alarm and parameter state

Before resetting, navigate to the Yasnac alarm history and the PMC input/output monitor:

1. Press SYSTEM → ALARM → HISTORY. Record every EX-class alarm, not just the active 293.
2. Press SYSTEM → PMC → I/O MON and screenshot the magazine-related inputs (typically addresses X0040–X005F on MX-1, X0080–X009F on MX-3, though exact maps vary — confirm against your ladder print).
3. Note the change tool parameter value being displayed. This is the pocket the PMC believes it is pointing at.

Step 2 — Manual magazine jog test

Place the control in MDI/JOG with magazine override:

1. Select ATC MENU on the operator panel (older MX-1 firmware hides this under JOG > ATC).
2. Jog the magazine forward and reverse in single-step pocket mode.
3. Listen for the cam/prox pulses. On a healthy machine you hear a clean click per pocket; a missed or double-pulse indicates the prox is failing or the cam is worn.

Step 3 — Prox switch inspection (prox-only vintage)

With the magazine jogged one full revolution:

• Measure each prox switch output with a meter: it must switch cleanly between 0 V and 24 V DC with a < 5 ms bounce.
• Check the air gap: typical spec is 1.5–2.5 mm for the inductive prox used on the MV-40. A worn cam lobe can push the gap outside this window.
• Inspect the home pot prox specifically. The PMC requires this signal to latch before counting pocket transitions. If the magazine starts rotating from any pocket other than #1, the home reference is lost and EX 293 will eventually fire.

Step 4 — Encoder/resolver test (absolute feedback vintage)

For MX-3 and later, verify encoder health:

1. Locate the magazine encoder (typically a Heidenhain ROD or Yaskawa resolver on the magazine motor end bell).
2. Check the backup battery voltage: 3.6 V lithium cells must read > 3.3 V. Below this, the absolute position is lost at power-down.
3. On the PMC monitor, watch the encoder input bits cycle as the magazine rotates one full turn. Every bit must toggle consistently. A stuck bit indicates contamination or a failing encoder element.

Step 5 — Ladder review

Because EX 293 originates in the Mori ladder, the actual condition that raises it must be traced:

1. Open the Mori-supplied ladder print (paper or PDF supplied with the machine).
2. Locate the rung where the EX 293 coil is energized. The rung will reference inputs such as MAG_AT_HOME, COUNT_COMPLETE, ENCODER_PARITY_OK, and a timer (typically 5–8 seconds of expected index time).
3. Walk the rung with the PMC monitor live. Identify which upstream condition is false — that is your true root cause.

Common Repairs

Verification Procedure

After any repair, perform a controlled index test before returning the machine to production:

1. Single-pocket test: From MDI, call T02, T05, T10, T15 in sequence. Each must complete in < 3 seconds with no alarm.
2. Full-revolution test: Call T01 → T20 → T01. Watch for any pocket where the magazine hesitates.
3. Power-cycle test: Power down the control for 60 seconds, then power up. Confirm the tool-change parameter is retained and matches the physical pocket (encoder-vintage only — prox-only machines must re-home on every power-up, which is normal).
4. Production test: Run a 20-tool program with random tool changes. Run at least three full iterations of the program.

Preventive Maintenance Recommendations

• Quarterly: Inspect prox switches and cams; clean magnetic debris from inductive prox faces.
• Annually: Replace encoder backup batteries on MX-3 and later controls — do not wait for the low-battery alarm.
• Annually: Verify magazine motor contactor tips; pitted contacts cause voltage drop that mimics a prox or encoder fault.
• At every major service: Print and archive the current Mori ladder revision. DMG MORI has issued revised ladder blocks for the MV-40 family to add debounce filtering and extended index timers — owners should confirm they are running the latest revision.

Notes on Cross-Platform Similarity

The EX 293 class of magazine index error is conceptually identical to ATC alarms found on Fanuc-controlled Mori Seiki machines (e.g., EX alarms on a Fanuc OM-C control with an MV-40 conversion) and on similar Yasnac-controlled machines from the same era. The diagnostic workflow — prox/encoder health, cam timing, ladder input truth table — applies broadly. When migrating diagnostics to a Fanuc-controlled variant, the alarm name changes (often PS0300 / OT0508 range) but the underlying Mori ladder logic still owns the trip condition. Always defer to the Mori maintenance manual, not the control vendor's generic alarm list.

Frequently Asked Questions