When a rotating machine trips on vibration, the first question is always the same: was the signal real, or was the chain broken? This article explains one of the most common VM600 measurement paths used in machinery protection. The chain starts at the TQ402 eddy current proximity sensor, passes through the IQS450 signal conditioner, moves into the IOC4T communication and I/O card, and ends at the CPUM processor. A clear understanding of this path helps maintenance teams avoid false trips, reduce commissioning time, and source the right spare parts before downtime becomes expensive.
Signal Chain Overview
Stage | Component | Primary Function | Key Output |
|---|
Sensor | TQ402 | Eddy current proximity probe measures shaft displacement and vibration | Negative DC gap voltage plus dynamic vibration signal |
Conditioner | IQS450 | Converts probe signal to standard machine protection outputs | 0–10 V, 4–20 mA, gap voltage, alarm/trip thresholds |
I/O and Communication | IOC4T | Reads conditioner signals and exchanges data with the rack processor | Scaled vibration values, channel status, digital I/O |
Processor | CPUM | Processes protection logic, generates trips, communicates with DCS/PLC | Trip relays, communication status, system health |
Each stage must be configured correctly. A mistake at any link will show up as a bad reading, a communication loss, or a false trip.
Stage 1: TQ402 Sensor and Gap Setting
The TQ402 is an eddy current proximity transducer. Typical model codes are 111-402-000-012 and 111-402-000-013. The suffix determines cable length, thread type, and approval options. For the signal chain to work, the probe must first produce a stable gap voltage.
Gap Voltage and Mounting
Eddy current probes operate on a negative DC gap voltage. The exact value depends on the probe calibration and the installed gap. Most TQ402 configurations use a linear range around 2 mm, with sensitivity defined on the calibration sheet. A healthy chain should show a smooth negative DC voltage when the shaft is static.
Common design checks:
Set the physical gap according to the calibration curve, not by eye.
Confirm the target surface diameter is larger than the probe tip.
Mount the probe rigidly. Shaft movement or bracket vibration will create false dynamic signals.
Keep the EA402 extension cable routing away from power cables and high-current conductors.
Ground the cable shield at the IQS450 end only. Grounding both ends creates ground loops and noise.
If the TQ402 gap voltage drifts, check EA402 913-402-000-012 or 913-402-000-013 cable aging. Long cables on steam turbines are common, and shield damage can produce slow signal changes that look like real vibration drift.
Stage 2: IQS450 Signal Conditioner
The IQS450 204-450-000-001 and 204-450-000-002 are not drop-in interchangeable without parameter reconfiguration. The 001 and 002 versions have different output options and suffix configurations. Before replacing one with the other, verify the output type, power rating, and terminal assignment.
Power and Signal Wiring
Use a stable 24 V DC supply. The IQS450 output can be 0–10 V, 4–20 mA, or both depending on model suffix. For current outputs, the loop load must match the conditioner specification. An open or shorted loop will cause an output fault.
Typical installation checks:
24 V power supply negative must be referenced correctly. Floating grounds create offset errors.
Signal terminal shields must be connected only at the conditioner.
Buffer gap voltage output should match the TQ402 gap voltage. A large difference suggests wiring or calibration mismatch.
Filtering algorithms in IQS450-002 can reduce noise, but they also introduce a slight response delay. Do not use heavy filtering if the machine requires fast trip response.
For new installations, record the IQS450 gain and trip settings before commissioning. This prevents confusion when a spare card is installed later.
Stage 3: IOC4T Communication Card
The IOC4T 200-560-000-016 and its related versions are mixed digital and analog I/O cards for the VM600 rack. Available versions include 200-560-000-016, 019, 014, 013, 011, 113, 111, and 114. The version differences affect channel counts, signal types, and firmware compatibility.
Rack Slot and Addressing
The IOC4T must sit in the correct rack slot and make solid contact with the 204-040-100-012 backplane. A common field issue is a card that appears healthy but loses communication intermittently. The cause is often poor backplane contact or a wrong address DIP switch.
Design checklist for IOC4T:
Set the address DIP switch according to the rack slot map. A duplicate address will block communication.
Inspect the backplane connector for bent pins or dust.
Use a grounded wrist strap when handling the card.
Confirm the CPUM and IOC4T firmware versions are compatible before mixing old and new cards.
For digital outputs, verify the external load does not exceed the relay contact rating.
If the IOC4T reports channel failure, reseat the card first. If the failure follows the card, replace the card rather than repeatedly resetting the system.
Stage 4: CPUM Processor
The CPUM 200-595-067-114 is the processing core for protection logic and communication. It receives scaled data from the I/O cards, evaluates alarm and trip thresholds, and exchanges status with the DCS or PLC. The older 200-595-063-314 can be replaced by the 067-114, but the hardware change requires firmware and configuration verification.
Processing and Bus Checks
The CPUM does not read raw probe signals directly. It depends on accurate data from the IQS450 through the IOC4T. A CPUM fault code is often the end result of a signal chain problem, not the original cause.
Before replacing a CPUM:
Check communication status on the rack bus.
Verify all I/O cards are recognized.
Record fault codes and compare with the reset procedure.
Check power supply voltage and rack grounding.
For legacy firmware upgrades, do not assume new CPUM firmware will work with all older IOC4T versions. Verify the compatibility table before ordering.
Signal Chain Design Checklist
Step | Component | Check |
|---|
1 | TQ402 | Gap voltage stable, probe gap set from calibration, target surface clean |
2 | EA402 cable | Shield grounded at IQS450 only, no damage, correct length suffix |
3 | IQS450 | 24 V supply stable, output load correct, gain and trip settings recorded |
4 | IOC4T | Address DIP correct, backplane contact solid, firmware compatible |
5 | CPUM | All cards recognized, bus communication stable, fault code reset performed |
6 | Full chain | Simulate vibration or use calibrator to verify end-to-end scaling |
Use a 204-215-000-101 calibrator to verify TQ402/IQS450 sensitivity before startup. Calibration confirms that the displayed value at the CPUM matches the physical vibration.
Common Failure Symptoms and Checks
Symptom | Likely Cause | First Check |
|---|
No gap voltage | Open probe or cable | EA402 continuity, TQ402 connector |
Output stuck high | Ground loop or shield fault | Shield grounding at IQS450 only, power supply common |
Communication loss | IOC4T address or seating | DIP switch, backplane contact, CPUM bus status |
False trip | Threshold too low or sensor loop noise | IQS450 trip settings, TQ402 gap voltage, cable routing |
CPUM fault code | Signal chain error or firmware conflict | Check all card status before replacing CPUM |
FAQ
What is the correct gap for a TQ402 probe?
The correct gap is set from the probe calibration sheet, not from a general table. The gap voltage is the primary indicator. Set the probe so the static gap voltage falls within the linear portion of the calibration curve.
Can I replace an IQS450-001 with an IQS450-002?
Only after verifying the output configuration and suffix. The 001 and 002 use different output and filtering options. A direct swap without parameter reconfiguration can cause wrong scaling or no output.
Why does the IOC4T lose communication with the CPUM?
Usually it is a rack address conflict, poor backplane contact, or mismatched firmware. Reseat the card and check the DIP switch before replacing the CPUM.
What replaces a CPUM 200-595-063-314?
The 200-595-067-114 is the common replacement, but the upgrade must be checked for compatibility with the installed I/O cards and rack firmware.
How do I verify the whole signal chain without running the machine?
Use a signal injection or a 204-215-000-101 calibrator to simulate a known vibration. Compare the reading at the IQS450 output and at the CPUM communication point. A mismatch points to a scaling or configuration problem.
Sourcing and Support
For maintenance teams, the most important rule is to keep the signal chain documented and the spare parts ready. Joyoung International Trading Co., Limited supplies industrial automation spare parts for VM600 systems, including CPUM processors, IOC4T cards, IQS450 conditioners, TQ402 sensors, MPC4 protection cards, and backplane components. We support global clients with system matching, technical consultation, and fast delivery.
Contact us for VM600 spare parts, proximity probes, and vibration monitoring modules. Email: chen@htechplc.com. Phone: +86-181-5013-7565.
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