Configuring Equidistant Profibus DP for ADI4 on SINUMERIK 802D sl

A retrofitted CNC milling machine built around a SINUMERIK 802D sl controller and an ADI4 (Analog Drive Interface for 4 axes) module is stopping intermittently with an emergency stop alarm. Both ADI4 status LEDs (RDY/Ready and Exchange) extinguish at the moment of the fault, the alarm stack reports lost drives on every axis, and the line-module ready signal is still physically present at +24 V when the stack is read. The root cause is not the wiring and is not the 24 V supply — it is the PROFIBUS DP cycle running in non-equidistant mode. This article walks through the alarm analysis, the hardware checks that were correctly performed, the equidistant DP configuration that resolves the fault, and the commissioning tooling available for the 802D sl.

1. Problem Overview

The reported symptoms on the SINUMERIK 802D sl / ADI4 retrofit are:

• Random emergency stop faults (alarm 3000) during normal machining — observed within hours of warm-up, sometimes within minutes after the first axis move.
• Both ADI4 LEDs Ready and Exchange switch OFF simultaneously at the moment of the fault.
• Recovery requires a power-on reset of the ADI4 — the fault does not self-clear.
• When the alarm stack is read after the fault, the ready feedback from the line module (T_RDY_LM) is still at +24 V. The CNC has therefore lost the logical view of the drive, not the physical signal.
• The fault is observed on every axis (X1, Y1, Z1), not on a single servo amplifier.

2. Captured Alarm Stack

The alarm stack read from the 802D sl HMI after the fault is the entry point for the diagnosis.

The chain is consistent: a single event on the ADI4 PROFIBUS link is being interpreted by the NCK as simultaneous drive-not-ready on every axis, which the PLC then converts into an emergency stop.

3. Why Alarms 25202 / 26101 / 3000 Stack This Way

Alarms 25202 ("Axis %1 drive %2") and 26101 ("Axis %1 drive %2 monitoring") on a 802D sl / ADI4 retrofit are not generated by the analog amplifier itself — they are generated by the NCK when the PROFIBUS telegram from the ADI4 either stops arriving, arrives out of the expected equidistant window, or reports status bits that the NCK decodes as "drive not ready".

User alarm 700016 is the PLC program's mirror of the same event: it is raised from the OEM block SR_EMG_STOP when either of the discrete NCK interface signals drops:

• T_RDY_LM — Ready feedback from the line module / infeed.
• T_OFF3m — OFF3 monitoring (ramp-down monitoring) signal.

Because the fault is captured after the event with T_RDY_LM still at +24 V, the most plausible explanation is that the NCK and the PLC have already reacted to a transient PROFIBUS loss long enough to latch the alarm bits, while the discrete wiring on the cabinet floor has had time to recover. That is the signature of a bus-cycle timing fault, not a wiring fault.

4. Root Cause: Non-Equidistant PROFIBUS DP

The Siemens field engineer's diagnosis — that PROFIBUS must be configured as equidistant DP (also called isochronous DP) — is the correct root cause for this stack on an ADI4 retrofit. The reasoning is mechanical:

1. The 802D sl NCK drives the ADI4 with a fixed setpoint cycle (Ti/To) defined by the IPO cycle and the position-controller cycle.
2. The ADI4 must sample the encoder actual values and latch the analog setpoints inside a deterministic time window within each DP cycle.
3. If PROFIBUS DP is configured in standard (non-equidistant) mode, the master broadcasts the new setpoint telegram and polls the actual-value telegram at whatever point in the cycle the bus arbiter allows. Jitter on the order of 1–3 DP cycles is normal.
4. The ADI4 (and the NCK) interpret a missed or late setpoint window as a drive-not-ready condition. The NCK latches 25202/26101, the PLC raises 700016 and triggers /ES, and the ADI4 status LEDs go dark because the slave watchdog has timed out.

The physical fact that T_RDY_LM is still high after the fault is irrelevant — the NCK already decided that the drive was unreachable and latched the alarm. Equidistant DP eliminates the jitter, which removes the false drive-not-ready events entirely.

5. Hardware Verification (No Fix Found Here, But Must Be Done First)

Before changing PROFIBUS configuration, several hardware checks were performed on the affected machine. All of them must still be completed on any new retrofit — they are prerequisites, not the fix.

5.1 ADI4 Housing Bonding

The Siemens installation note for the ADI4 module states that, for high-frequency interference currents, the housing of the ADI4 module must be connected through a low-ohmic resistance to the rear panel of the cabinet. SINUMERIK 840D sl / 840Di sl / 802D sl ADI4 — Installation. Without that bond, common-mode currents on the PROFIBUS shield can drift the bus reference and trigger exactly the kind of intermittent slave-loss observed on this machine.

5.2 PROFIBUS Connector Pinout

5.3 Cable Topology

The reported topology is a standard PROFIBUS cable (green and red twisted pair, overall foil + braid shield) with the shield bonded only at the connectors. Confirm that:

• Both end nodes carry a bus terminator (ON at the first and last slave only).
• Stubs are < 1 m per segment, total stub length < 6.6 m for 1.5 Mbps / 3 m for 12 Mbps — see PROFIBUS cable guidelines.
• The shield is bonded to the cabinet ground bar at every cabinet entry.

5.4 ADI4 24 V Supply Isolation

The field engineer moved the ADI4 to a separate regulated 24 VDC power supply during diagnosis. This is a sound isolation step — the ADI4 24 V must come from a regulated, low-noise source, ideally on its own branch of the cabinet DC bus, decoupled from the relay / solenoid DC bus. On the 802D sl retrofit the ADI4 PLC interface signals (T_RDY_LM, T_OFF3m, axis enables) draw their 24 V from the same regulated rail; cross-talk from an unfiltered 24 V rail can corrupt the readiness sampling on its own.

6. The Fix: Equidistant PROFIBUS DP Configuration

The fix is to enable equidistant (isochronous) PROFIBUS DP on the segment that carries the ADI4. With equidistant mode, the master holds a constant DP cycle (Tdp), and each slave is told exactly when in that cycle to update its outputs and sample its inputs.

6.1 Parameters Involved

6.2 Step-by-Step on the PROFIBUS Master Tool

1. Open the PROFIBUS DP master configuration (the 802D sl integrated PLC project in STEP 7 / the 802D sl commissioning tool that targets the integrated S7 PLC).
2. Right-click the ADI4 slave in HW Config and open Properties > DP Slave Properties > Equidistance.
3. Tick "Activate equidistant bus cycle".
4. Confirm that the master cycle Tdp is set equal to the NC position-controller cycle. On 802D sl this is read from the SINUMERIK machine data — do not hard-code a value that is not a divisor of the IPO cycle.
5. Accept the Ti and To values proposed by the ADI4 GSD file. They are calculated from the slave's processing time and the bus baud rate.
6. Disable the slave watchdog or raise it to at least 3× Tdp to absorb the first few cycles after power-up.
7. Save, recompile, and download to the PLC.

6.3 Expected Effect

With equidistant DP active:

• The ADI4 telegram always arrives inside the same NCK setpoint window.
• The NCK no longer misinterprets jitter as "drive not ready".
• The 25202 / 26101 alarms stop being raised on bus jitter events.
• The ADI4 Exchange LED stays green throughout normal operation; only a real bus break will extinguish it.

7. Commissioning Tool Selection for 802D sl

A frequently raised follow-up question on this retrofit is which commissioning tool to use, because the field engineer did not have the 802D sl toolchain. The boundary is:

The 802D sl PROFIBUS topology — including the equidistant-DP enable on the ADI4 slave — must therefore be edited from the 802D sl toolbox or from STEP 7 with the 802D sl device catalog loaded. STARTER must be excluded from the candidate list: it is for SINAMICS drives, and the ADI4 is an analog interface, not a digital drive.

8. Verification Procedure

1. Power-cycle the ADI4, then download the new equidistant configuration to the 802D sl integrated PLC.
2. From the HMI, open the PROFIBUS diagnostics screen and confirm that the ADI4 slave reports Equidistant: active.
3. Confirm that the ADI4 Ready and Exchange LEDs remain green in idle.
4. Run a full warm-up cycle: traverse all three axes (X1, Y1, Z1) at rapid, run a typical contouring program, leave the machine idle for at least 30 minutes, then repeat the traverse. Capture the cycle-time jitter from the PROFIBUS diagnostics — jitter must be < 1 ms (typically < 0.1 ms) on an equidistant segment.
5. Run the machine for an extended shift (5–8 hours) and check the alarm history for 25202 / 26101 / 3000 / 700016 / 700001. Any recurrence during this window indicates either a residual hardware issue or that Tdp has been set to a non-divisor of the IPO cycle.
6. Capture a PROFIBUS trace if any recurrence happens — the trace will show whether the late telegram is a master-side issue (Tdp wrong) or a slave-side issue (ADI4 supply droop, shield bond lost).

9. Field-Commissioning Matrix

10. Summary

The combination of 25202 / 26101 across every axis, 700016 raised from SR_EMG_STOP, 3000 emergency stop, and ADI4 RDY + Exchange LEDs going dark is the textbook fingerprint of an ADI4 PROFIBUS cycle problem, not a wiring problem. The physical readiness signals (T_RDY_LM, T_OFF3m) can be measured at +24 V after the fault because the fault originates in the timing of the PROFIBUS telegram, not in the discrete wiring. The fix is to enable equidistant PROFIBUS DP on the segment that carries the ADI4 and to set Tdp equal to the NC position-controller cycle, using the 802D sl commissioning toolchain (not STARTER, which only addresses SINAMICS drives). The hardware checks — ADI4 housing bond, shield handling, pin 1 not used as a signal return, regulated 24 V for the ADI4 — are prerequisites and must still be in place for the equidistant configuration to be stable.