Problem Details
A retrofitted Heidenhain TS 111 touch probe connected through an existing APE 110 probe interface on a TNC 407 control reports the stylus as deflected the instant the probe is plugged in. The machine was originally prepared for TS 111 operation, so the wiring harness and interface are nominally correct.
Observed symptom set:
- With the APE polarity switches
S1/S2in their as-found position, the control declares the stylus deflected immediately and refuses to start a probing cycle. - Inverting the switching signal with
S1/S2lets a probing cycle start, but the switching signal then stays permanently high — the probe never reports a valid trigger transition. - Resistance measured between the probe signal line and 0 V: 1.8 MΩ at rest, dropping to 1.2 MΩ when the stylus is deflected.
- Output signal from the APE toward the control stays fixed at approximately 0.3 V (a valid low level) in both probe states.
Root Cause Analysis
The evidence supports several competing hypotheses. Work them in order of cost, not in order of suspicion.
| Hypothesis | Supporting evidence | Contradicting evidence | Discriminating test |
|---|---|---|---|
| Open circuit inside the probe (broken trigger contact chain, cracked PCB joint, corroded contact ring) | Megohm reading in both states; APE output never toggles | Slight resistance change on deflection suggests the probe is not fully dead | Ohm reading directly at the probe connector pins with the probe fully disconnected from the APE |
| Cable / connector fault (broken conductor, bent or recessed pin, shield-to-signal leakage) | Megohm-range readings are typical of a partially broken conductor or moisture ingress | Machine was prepared for TS 111 use | Wiggle test with continuity meter on each conductor, connector-to-connector |
| Measurement artifact — ohmmeter reading an energized/active circuit | Resistance measurement was taken between signal line and 0 V, which is only valid on a de-energized, disconnected circuit | — | Repeat all resistance checks with the APE de-energized and the probe cable unplugged from the interface |
Wrong probe variant or wrong signal convention for this APE / S1/S2 setting |
Inverting S1/S2 changes the fault presentation from "deflected" to "permanently high" |
Neither switch position produces a working trigger | Compare the APE switch setting against the mounting instructions for the exact probe model in hand |
| Missing or incorrect probe supply / ready signal from the APE | Output pinned at 0.3 V regardless of probe state | — | Measure the supply voltage delivered to the probe connector while the APE is powered |
The fact that flipping S1/S2 changes the control’s behavior confirms the APE 110 output stage and the link to the TNC 407 are alive and that the control is reading the interface. It does not prove the APE input stage is healthy — the APE can invert a stuck input just as faithfully as a live one. So the "APE is good, probe is bad" conclusion is plausible but not yet proven.
Structured Diagnostic Procedure
- Power down and isolate. Switch off the machine control and remove the probe cable from the APE 110. Every resistance measurement below is only meaningful on a de-energized, disconnected circuit.
- Ohm the probe at its own connector. Measure between the switching-signal pin and the 0 V pin directly at the probe body connector, with the extension cable removed from the path. Record the value with the stylus at rest and with the stylus deflected by hand in +X, −X, +Y, −Y and +Z. A functional contact-type trigger chain shows a clear two-state result — low ohms in one state and open in the other — not a megohm value in both. If both states remain in the megohm range at the probe connector, the fault is inside the probe.
- Ohm the cable separately. With the probe removed, check conductor-to-conductor continuity end to end and insulation resistance from each conductor to shield and to 0 V. Flex the cable at both strain reliefs while watching the meter. Intermittent megohm readings that move under flexing indicate a broken strand.
- Verify pin assignment. Cross-check the pinout of the probe connector, the extension cable, and the APE 110 input terminal against the mounting instructions for the specific probe and interface part numbers. A retrofit that is "prepared for the TS 111" can still carry a harness built for a different probe generation or a different connector variant.
- Measure the probe supply. Re-energize the APE with the probe connected and measure the DC supply present at the probe connector. Zero or out-of-tolerance supply explains a permanently non-switching output regardless of probe health.
- Simulate the probe at the APE input. With the probe disconnected, replace it with a hand-operated normally-closed test switch and short link across the APE input terminals corresponding to the trigger signal and 0 V. Toggle it and watch the APE output toward the control. If the APE output now switches cleanly between low and high with the switch, the APE input stage, output stage and control link are all proven good, and the fault is confined to the probe or the cable.
- Confirm the 0.3 V level. The measured 0.3 V is a valid logic low; do not chase it as a fault. What matters is whether it changes state when the simulated trigger is toggled in step 6.
Switch S1 / S2 Configuration Logic
The S1/S2 switches in the APE 110 set the polarity of the trigger signal presented to the control. They exist to match probes and controls with opposite signal conventions — they are not a troubleshooting workaround.
| Setting | Observed behavior | What it tells you |
|---|---|---|
| As-found position | Control reports stylus deflected immediately; cycle blocked | Control sees the "triggered" state permanently — input stuck in one state |
| Inverted position | Cycle starts; switching signal stays high, no trigger detected | Same stuck input, presented with opposite polarity |
Because both positions produce a static result, the input to the APE is not changing state at all. That is the single most diagnostic observation available here, and it points away from a configuration error and toward an open signal path.
Return S1/S2 to the position specified for this probe/control combination once the hardware fault is cleared. Leaving inverted polarity in place will make a repaired probe report inverted trigger logic and can cause the control to run the stylus into the workpiece instead of retracting.
Verification
After any repair or component swap, confirm the following in sequence before running a live probing cycle:
- Probe connector resistance shows a clean two-state transition on deflection in all axes.
- APE output toward the control switches between the low level (measured around 0.3 V) and the high level when the stylus is deflected by hand.
- With
S1/S2in the specified position, the TNC 407 reports the stylus as not deflected at rest and reports deflection only when the stylus is physically moved. - Run a manual probing cycle against a known reference surface with the feed override reduced. Confirm the control stops on contact and retracts, rather than overtravelling.
- Repeat a single-axis probe five times on the same reference face and compare the recorded values. Scatter beyond the probe’s rated repeatability indicates a marginal contact chain or a loose stylus — check stylus torque and any stylus extension before accepting the probe.
When to Escalate
If step 6 of the diagnostic procedure proves the APE input stage switches correctly with a simulated contact, the fault is inside the probe or its cable and no field adjustment will fix it. The TS 111 trigger mechanism is a sealed precision assembly; opening it destroys the mechanical reference and the sealing. Route the unit through Heidenhain’s official service channel for test and repair rather than attempting an internal repair, and quote the exact probe and interface part numbers plus the measurement set above — the two-state resistance results and the APE simulation result are exactly what a service department needs to triage the unit.
Why does my Heidenhain TS 111 show "stylus deflected" as soon as it is plugged in?
The control is seeing the triggered state permanently, which almost always means the trigger signal path is open — a broken contact chain inside the probe, a broken conductor in the cable, or a wrong pin assignment. Flipping the APE polarity switches only changes which static state the control sees; it does not fix an open path.
What resistance should I measure across a TS 111 trigger contact?
A healthy contact-type trigger chain gives a clear two-state result — low ohms in one state and effectively open in the other — measured at the probe connector with the probe disconnected and de-energized. Readings of 1.8 MΩ at rest and 1.2 MΩ when deflected are both open circuits electrically and indicate a leakage path, not a working switch.
What do the S1 and S2 switches in the APE 110 do?
They set the polarity of the trigger signal sent to the control so the probe convention matches the control convention. Use them only to match documented probe/control requirements — never leave them inverted as a workaround, because inverted trigger logic can cause the control to drive the stylus into the workpiece instead of retracting.
How do I prove the APE 110 interface is good before replacing the probe?
Disconnect the probe and substitute a hand-operated test switch across the APE trigger input and 0 V, after confirming terminal functions from the interface documentation. If the APE output toward the control toggles cleanly between the low level and high level as you operate the switch, the interface, output stage and control link are proven good and the fault lies in the probe or cable.
Is 0.3 V on the APE output a fault?
No — 0.3 V is a valid logic low. The fault is that the level never changes when the stylus is deflected. Focus diagnosis on whether the output transitions, not on the absolute low-level voltage.
