Problem Overview
The Siemens SINUMERIK 810D CNC reports "663 missing" (alarm 663 or status indication that pulse enable I_R is not present) while a voltmeter at the physical terminal block still shows +24 V DC. This contradiction — NCK software says the enable is absent, but the terminal is clearly energized — is a well-known failure mode on the SIMODRIVE 611D / 611U digital drive family, and it has a very specific root cause: the pulse-enable signal is hardware-only. It cannot be de-bounced, re-mapped, masked, or substituted from inside the NCU, the HMI, or the PLC. Any attempt to work around the missing 663 by editing PLC logic, MD parameters, or drive firmware will fail by design. The fault therefore has to be repaired at the electrical/electronic level on either the CCU3 module 6FC5410-0AY03-0AA1, the wiring harness to the SIMODRIVE 611 drive, or the drive's own terminal interface.
Terminal 663 Signal Path and Function
On every SIMODRIVE 611D / 611U drive module, terminal 663 is the dedicated input for Impulse Release (German: Impulsfreigabe, symbol I_R). It is physically wired to the drive's two-channel enable gate and is required in addition to the controller-enable signals on AS1 / AS2 (Terminal 9, controller enable / Servo Enable) and Terminal 65 (drive enable / Reglerfreigabe).
| Terminal | Designation | Symbol | Function | Source |
|---|---|---|---|---|
| 9 (pin pair) | AS1 / AS2 | I/F (RF) | Controller enable (Servo Enable) | NC/PLC output, fed back to drive |
| 65 | Enable 1 | RF1 | Drive enable channel 1 | NC/PLC output |
| 663 | Impulse Release | I_R | Pulse enable to IGBT gate drivers | CCU3 motherboard relay contact (hardware) |
| X431 / 48 | 24 V supply | P24 | Auxiliary 24 V for enable inputs | External PSU |
The drive will only generate the gate pulses for the IGBT power stage when all three enables are simultaneously high (24 V) and the drive has no internal fault. Removing 663 is the safest, fastest way to de-energize the motor torque — it is the hardware equivalent of STO.
For an 810D CCU3 system the wiring topology is:
- External 24 V supply → 663 input pin on the SIMODRIVE 611 drive terminal block (X431 or X432 depending on drive type).
- From the drive, the 663 line routes back to the CCU3 module (article number 6FC5410-0AY03-0AA1) via the backplane ribbon cable and a connector on the CCU3 motherboard.
- On the CCU3 PCB, the 663 line is fed through an optocoupler for galvanic isolation, then drives a relay contact that re-asserts 663 (or AS1/AS2 enable feedback) back to the drive.
- When all conditions are valid, the optocoupler LED conducts, the relay pulls in, and the drive sees a closed 663 path.
Why Software Override Is Impossible
Many engineers familiar with modern SINAMICS S120 drives assume that any enable can be controlled by PLC flag, drive parameter, or NCK variable. On the SIMODRIVE 611D/611U generation this is not the case for 663. The pulse-enable path runs through discrete optocouplers and an electro-mechanical relay that live on the CCU3 motherboard. There is no NCK variable, no MD (machine data) bit, and no PLC flag that can substitute the contact closure — the drive's hardware enable gate is literally a wire that must see 24 V at the right time.
The same restriction applies to the AS1/AS2 pair (Terminal 9) on the 611D/611U family: although the NC sets the command to enable the drive, the actual hardware loop is closed by the CCU3 relay contact. If the relay is open, the drive will not pulse even if the PLC says "controller enable = TRUE". This is by design — it is the safety architecture of the legacy 611 platform.
CCU3 6FC5410-0AY03-0AA1 Hardware Architecture
The 810D CCU3 is a 3-axis (or 4-axis with options) compact controller that integrates the NCK, PLC, HMI, and axis modules onto a single module plus a backplane. The article number 6FC5410-0AY03-0AA1 identifies a specific export / hardware revision: the suffix 0AA1 is the release key, and the -0AY03- mid-section indicates the SW version of the firmware shipped from the factory (typically SW 4.x or SW 5.x depending on the production year).
| Attribute | Value |
|---|---|
| Module type | CCU3 (Compact Control Unit 3) |
| Article No. | 6FC5410-0AY03-0AA1 |
| Associated drives | SIMODRIVE 611D / 611U |
| Backplane | 810D 6FC5510-0AA00-0AA0 or compatible |
| Optocoupler isolation (enable path) | ≈ 2.5 kV galvanic isolation |
| Enable relay | 24 V coil, 1× changeover contact, mechanical life ~10⁵ cycles |
| Firmware range | SW 3.x – SW 5.x (810D powerline) |
On the CCU3 PCB, the 663 path is implemented with a through-hole optocoupler (typical Siemens part on the BOM: a 4N35-class DIP-6 device or the Siemens-branded equivalent) feeding a 24 V miniature relay (commonly a Siemens V23057 or Schrack RT-series relay, 1 changeover, 5 A contact rating). The relay's NO contact is the actual hardware line that re-asserts the enable to the drive.
Common Failure Points
The phrase "663 missing with 24 V at the terminal" is the diagnostic fingerprint of one of the following four hardware faults, in order of frequency on legacy 810D systems:
| # | Failure point | Symptom | Verification | Repair |
|---|---|---|---|---|
| 1 | Optocoupler input LED degraded (CTR drop) | 663 signal present at terminal but never reaches the relay coil; relay stays dropped | Measure optocoupler pins 1–2 with clamp-on or solder test leads; LED Vf should be 1.1–1.4 V at ~10 mA; CTR originally ≥ 20 % | Replace optocoupler (DIP-6, 4N35 / PC817 class) or replace the whole CCU3 module |
| 2 | Relay coil open or contact welded / dirty | Coil energizes but contact does not close (or is stuck closed) | Listen for click; measure continuity across NO–COM while PLC requests enable | Replace miniature relay (V23057 series, 24 V DC coil) |
| 3 | Broken backplane / ribbon cable to drive | 24 V at the drive terminal but never seen by CCU3; or the CCU3 sees 24 V but cannot echo it back | Continuity test on the 663 conductor end-to-end; inspect IDC insulation-displacement connectors for oxidation | Replace ribbon cable, re-crimp IDC, or re-pin the connector |
| 4 | Drive-side optocoupler on the 611 module input | Drive itself is not reading the 663 line despite CCU3 echoing 24 V | Open the 611 drive, locate the X431 / pulse-enable optocoupler, measure Vf at its input | Send the 611 power module / control card to a Siemens-certified repair center |
The first item — optocoupler CTR (current-transfer ratio) degradation — is by far the most common. The optocouplers in the 611U era are known to fail after 15–25 years of continuous 24 V operation; the LED inside the package slowly loses output, and the phototransistor side can no longer source enough current to pull the relay coil. This is the same "classic issue" that affects Terminal 65a / 663 input channels on 611U cards. A degraded optocoupler will still pass enough leakage current to confuse a casual voltmeter reading at the terminal while failing to pull in the relay downstream.
Diagnostic Procedure
- Confirm the alarm is genuine. Read the active alarm list on the HMI. The exact text should reference 663 / I_R / Impulsfreigabe. Note the drive number (e.g. drive 1, drive 2) and the axis it is mapped to.
- Measure at the drive terminal. With the machine in a safe state (ESTOP active, drive de-energized), measure DC voltage between terminal 663 and terminal 48 (24 V reference) on the SIMODRIVE 611 module. You should see +24 V DC ±10 %. If 0 V, the wiring or the CCU3 relay is the upstream fault; the drive is innocent.
- Measure at the CCU3 end. Open the CCU3 housing and locate the enable optocoupler (usually near the backplane connector). With the PLC commanding enable, measure the LED side of the optocoupler: anode-to-cathode should be ~1.1–1.4 V DC at 5–15 mA forward current. If Vf is high (>2 V) or the current is below 1 mA, the upstream driving transistor is the fault.
- Measure the phototransistor side. With the LED forward-biased, measure Vce across the phototransistor. It should saturate below 0.4 V when the relay is pulling in. A Vce of 5 V or more with no relay click means CTR has collapsed.
- Bypass-test the relay only. Withdraw the relay from its socket and bridge the NO contact with a jumper. If the drive now reports 663 present, the relay is the fault. If the drive still reports 663 missing, the drive's own optocoupler is the fault.
- Inspect the ribbon / backplane cable. Look for green corrosion on IDC contacts, broken conductors at the strain-relief, and oxidized pins on the CCU3 connector. Flex the cable gently and watch the alarm status on the HMI — flickering between "663 missing" and "663 OK" while flexing confirms a broken conductor.
- Cross-check AS1/AS2 and Terminal 65. If those enables are also missing in the same pattern, the common-mode failure is usually a blown 24 V fuse on the CCU3 or a dead auxiliary PSU — not the optocoupler itself.
Repair Options
Three repair tiers are available, ordered by cost and downtime.
| Option | Cost (relative) | Downtime | Skill required | When to choose |
|---|---|---|---|---|
| Component-level: replace optocoupler or relay on the CCU3 PCB | Low (parts only) | 1–3 h | IPC-7711/7721 re-work certified technician | Single optocoupler or single relay failure, no other CCU3 symptoms, machine age > 15 years |
| Module-level: send CCU3 to certified repair center | Medium | 3–7 days plus ship time | None for the customer; RMA process | Multiple optocouplers failed, firmware corruption discovered, or BGA / QFP rework needed |
| Replacement: order a refurbished or new CCU3 (6FC5410-0AY03-0AA1) | High | Same day if stocked | None; module swap | Production critical, multiple PCB faults, or end-of-life migration planned |
For a 15- to 25-year-old 810D system the component-level repair is almost always the correct economic choice. The optocoupler is a sub-€1 part, the relay is a sub-€5 part, and the labor is typically 60–90 minutes for a trained technician with hot-air or focused-IR re-work equipment. The original Siemens spare-part catalog lists equivalent optocouplers (e.g. Siemens BPW17N, Vishay 4N35, or LiteOn LTV-817) that drop in as pin-compatible replacements; the relay cross-reference is V23057-B0006-A101 (24 V DC, 1 changeover) or any Schrack RT-24V equivalent with the same footprint.
Verification and Commissioning
After the repair, run a structured re-commissioning sequence to confirm that the 663 path is healthy under all operating conditions — not just the static "terminal has 24 V" check.
- Static test: Power the cabinet with the drives de-energized. Request "enable" from the PLC. Confirm the CCU3 optocoupler LED is lit, the relay clicks, and the drive terminal 663 shows +24 V.
- Dynamic enable/disable test: Cycle the enable line (PLC flag, HMI key, ESTOP chain) 20–30 times. The drive should respond within ≤ 50 ms every time. A missed response indicates a marginal optocoupler that needs replacement, not just a relay swap.
- ESTOP test: Press the physical emergency-stop. The 663 line must drop to 0 V within ≤ 20 ms. Measure with an oscilloscope, not a multimeter — a slow drop is a sign of relay contact bounce or weak optocoupler CTR.
- Axis-jog test: With safety circuits restored, jog each axis at 5 %, 25 %, 50 %, and 100 % of rapid. Monitor the drive's status word (ZSW1) for any flicker of bit 6 (drive enable) or pulse-inhibit bits. The alarm list must remain clear of any 663-related entry.
- 24-hour burn-in: Leave the machine in production mode for 24 hours, monitoring alarm history. A marginal optocoupler will often fail within the first thermal cycle after repair.
Related SIMODRIVE 611 Enable Signals
Although the article focuses on 663, the 810D / 611D platform uses a coordinated set of enable signals. Misreading the chain can cost hours of unnecessary diagnosis. The following table consolidates the practical mapping; refer to the Siemens Industry Online Support portal for the 810D / 611D commissioning manual to verify the wiring on your specific revision.
| NC/PLC source | Destination terminal | Signal | Hardware path |
|---|---|---|---|
| DB31..DB61, DBX21.7 (or equiv.) | AS1 / AS2 (X431 pin) | Controller enable (Servo Enable) | NCK → CCU3 driver → 24 V output → drive optocoupler |
| DB31..DB61, DBX21.6 (or equiv.) | 65 (X431 pin) | Drive enable (Reglerfreigabe) | NCK → CCU3 driver → 24 V output → drive optocoupler |
| Hardware relay on CCU3 | 663 (X431 pin) | Pulse enable (I_R) | CCU3 motherboard relay NO contact (NOT NC-controllable) |
| External 24 V PSU | 9 / 48 | Auxiliary supply | Direct wire, fused |
Note the asymmetry: AS1/AS2 and Terminal 65 are essentially static 24 V outputs commanded by the NCK/PLC; their absence usually points to a missing enable bit in the PLC program. Terminal 663, by contrast, is asserted by a relay contact on the CCU3 motherboard, and its absence is always a hardware problem — wiring, optocoupler, relay contact, or drive-side input stage.
Preventive Maintenance Recommendations
On legacy 810D systems, optocoupler aging is inevitable but predictable. The following PM schedule keeps 663 faults from becoming unscheduled downtime:
- Annual: measure the CCU3 optocoupler CTR with a curve tracer or by sourcing a known 10 mA forward current and measuring the phototransistor collector current. Replace when CTR falls below 50 % of the datasheet minimum (typically < 10 % absolute).
- Every 5 years: replace the enable relay proactively. Mechanical life of a 24 V miniature relay is finite; field data shows contact resistance drift after ~80,000 enable cycles.
- Every 10 years: replace the entire CCU3 module with a refurbished unit, or migrate to a SINUMERIK 840D sl with SINAMICS S120 — at this point the 810D spare-part pool is increasingly restricted.
- Always: keep the cabinet free of conductive dust and coolant vapor. Conductive contamination on the CCU3 PCB is the second leading cause of optocoupler failure after natural aging.
Can I deactivate or override Terminal 663 from inside the NCU on a Siemens 810D?
No. Terminal 663 (Pulse Enable / I_R) on the SIMODRIVE 611D / 611U family is a pure hardware path: external 24 V → drive terminal → CCU3 optocoupler → CCU3 relay contact → back to the drive. There is no NCK variable, machine data, or PLC flag that can close the relay or substitute the contact. The fault must be repaired on the CCU3 PCB (typically a degraded optocoupler or failed relay), in the wiring, or in the drive's input stage.
I have +24 V at the drive terminal, but the alarm still says "663 missing". Where is the fault?
The most likely cause is a degraded optocoupler on the CCU3 motherboard 6FC5410-0AY03-0AA1: the LED is still partially conducting, so a voltmeter sees voltage, but the phototransistor can no longer source enough current to pull in the enable relay. Measure Vce across the phototransistor side of the optocoupler; values above ~0.4 V with the relay coil connected indicate CTR collapse. Replace the optocoupler (4N35 / PC817 class) and re-test.
Is it safe to jumper Terminal 663 to 24 V as a temporary workaround?
No. Terminal 663 is part of the STO (Safe Torque Off) chain required by EN ISO 13849-1 and EN 61800-5-2. Permanently jumpering 663 to 24 V removes the hardware stop path and can result in uncontrolled axis motion if a downstream fault occurs. The only acceptable use of a jumper is on the bench, with the drive disconnected from mains, as a 30-second diagnostic test to localize the fault between CCU3 and the drive.
How is Terminal 663 different from AS1/AS2 (Terminal 9) and Terminal 65 on a 611D drive?
AS1/AS2 (Terminal 9) and Terminal 65 are controller-enable and drive-enable signals commanded from the NCK / PLC as 24 V outputs; they are software-controllable. Terminal 663 is pulse-enable (I_R) and is gated by a discrete relay on the CCU3 motherboard, which is why it cannot be set or cleared by software. All three must be present simultaneously for the drive to produce gate pulses.
What optocoupler part number can I use to repair the CCU3 6FC5410-0AY03-0AA1 663 path?
The original Siemens-bill optocoupler in the 611D/611U-era enable path is functionally equivalent to a 4N35, PC817, LTV-817, or BPW17N — all DIP-6 packages with a CTR of at least 20 % at If = 10 mA. Use a through-hole part to keep the original PCB footprint; SMD adapters introduce mechanical-stress risks on a backplane-mounted module. After replacement, run the dynamic enable / ESTOP verification sequence described in the article before returning the machine to production.