SIMOREG 6RA70 o8.0 Status: Troubleshooting OFF2 Signal Loss

SIMOREG 6RA70 o8.0 Status: Troubleshooting OFF2 Signal Loss on Profibus-Controlled DC Drives

On a two-drive rewinder machine governed by a SIMOREG DC Master 6RA70 via PROFIBUS-DP PPO5, both converters present status o7.1 (switch-on inhibit due to missing OFF2 enable) before the start command. After the operator commands run, one drive transitions cleanly to RUN (o0.x) while the second drive settles at o8.0. The visible signature, captured with DriveMonitor, is that the OFF2 bit in the drive's control word is briefly reset to 0 during the start sequence even though parameters P655, P656, and P657 are statically bound to binector B0001 (logic 1). This article isolates the root cause, walks through a methodical diagnostic procedure, and documents the corrective actions that restore reliable start-up on both drives.

1. Problem Description

The lack of any F-fault code eliminates the usual suspects (F004 line failure, F008 DC-link undervoltage, F005 armature overcurrent). What is left is the operating-state graph in the SIMOREG firmware, which is driven entirely by control-word bits, terminal inputs, and binectors.

2. SIMOREG 6RA70 Operating Status Codes

The 6RA70 reports its state through a two-character operating code beginning with the letter o. Understanding the difference between o7.x and o8.x is the key to this fault.

The transition o7.1 → o8.0 is significant. It indicates that OFF2 has been released (bit 1 of the control word is now = 1) but the drive has not yet received operation enable (bit 3). If the PLC is asserting the entire control word at once and bit 1 is momentarily low while bit 3 is high, the drive can land on o8.0 and remain there until the next control-word update re-asserts bit 1. See the operating-state flowchart in function diagram G175 of the SIMOREG 6RA70 parameter list.

3. PPO5 Communication Structure

PPO5 (Process Parameter Object, type 5) is the standard PROFIdrive profile used when the master needs both cyclic setpoints and a fixed map of parameter access words. PPO5 carries ten words of useful data in each direction on a single PROFIBUS-DP slot:

The control word is the only mechanism by which the master commands the drive. Any glitch — drop-out, intermittent connector, scan-cycle mismatch, or GSD mis-alignment — that resets one of its bits forces the drive into a different state.

4. Control Word Bit Mapping (STW1 per PROFIdrive)

5. Parameter Investigation: P648, P654–P675

5.1 Control-word source selection (P648)

Parameter P648 selects the source of the active control word used by the drive's sequencer. With P648 = 9 (the only setting that lets the PROFIBUS word take effect), the PZD1 telegram bit-by-bit routes through connectors K3001 (control word from PROFIBUS) and K8001 (status word to PROFIBUS). Other P648 settings force a fixed pattern or a terminal-based source.

Confirm that both drives have P648 = 9. A drive with P648 = 0 or with a terminal-based source will ignore PROFIBUS OFF2 commands entirely; if Drive B had reverted to one of those values after a parameter save, the symptom would be different (it would never enter o7.1 at all). Therefore the parameter set is almost certainly intact.

5.2 Control-word bit source parameters (P654 to P675)

Every bit of the control word has its own connector selection. The values reported in the source (P655 = B0001, P656 = B0001, P657 = B0001) are correct: B0001 is the always-high binector that represents "permanently 1". When the PROFIBUS telegram delivers STW1 = 0x047E (the standard "RUN request": bit 0 = 1, bit 1 = 1, bit 2 = 1, bit 3 = 0, bit 4 = 1, bit 5 = 1, bit 6 = 1, bit 7 = 0, bit 10 = 1), the OR combination of the PROFIBUS bit and the binector overrides B0001, so the bit follows PROFIBUS. As long as the telegram is consistent, the result is identical to having B0001 wired.

The trap is the OR-logic during a bus glitch: if PROFIBUS fails to update (telegram freeze, watchdog trip, connector unplug), the previous telegram value is retained in K3001 only if the DP master is configured for "output freeze on failure"; otherwise K3001 drops to 0 and bit 1 follows it down. The drop-out observed on Drive B's K3001 bit 1 — visible in DriveMonitor's time chart — is therefore a Profibus-side disturbance, not a parameter-side disturbance.

5.3 Function diagram references

• G117 — assembly of control word 1 from bit sources P654..P675, OR-combined with binectors. Shows how P648 routes K3001 into STW1.
• G180 — "OFF1/OFF2/OFF3" logic. Shows the OFF2 path including terminal 107, bit 1 of STW1, and the resulting pulse-inhibit latch.
• G175 — overall operating-state machine that maps the four OFF commands and the enable signals to the o-codes listed in section 2.

Print these three sheets from the SIMOREG 6RA70 parameter list (Siemens order number 6RX1700-0AD76) and mark up the K3001 → STW1.bit1 → OFF2 path on every discussion.

6. E-STOP Wiring at Terminals 105 and 107

A bounce or open-circuit on terminal 107 of Drive B alone would explain the partial OFF2 drop-out without affecting Drive A. If terminal 107 is shared between the two drives through a single contact, that contact's bounce affects both — and that scenario is excluded by the data (only Drive B is disturbed). If the contact is separate, inspect:

1. The screw torque on terminal 107 of Drive B (recommended 0.5 to 0.6 N·m for Phoenix UK-4 or comparable cage-clamp terminals; over-loose leads to intermittent 24 V).
2. The cable screen termination at the drive end only (do not bond at both ends — circulating currents can pull the 0 V reference).
3. The E-STOP push-button NC contact wired to terminal 107 — verify the NC contact opens cleanly (no chatter on release).
4. Any intermediate relay in the E-STOP chain — a 24 V DC relay with weak coil voltage on dropout will produce exactly this signature.

Bridge terminal 107 to terminal 109 (24 V out) with a short jumper as a temporary test. If the symptom disappears, the external wiring (E-STOP chain or relay) is guilty. Remove the jumper after the test and route the real signal back.

7. Root Cause Analysis

Combining the DriveMonitor trace (OFF2 dips for several scan cycles on Drive B only), the static parameter binding (P655 = B0001), and the lack of any F-fault, the plausible root causes rank as follows:

The most common field-proven cause is hypothesis #2 or #3, often both at once: a slow PLC scan that holds OFF2 = 0 for one extra cycle, combined with the DP watchdog reacting to that telegram as if the connection were lost. Both are software-side, and both are corrected without touching the drive hardware.

8. Step-by-Step Diagnostic Procedure

8.1 Prerequisites

• SIMOREG DriveMonitor installed (version 6.x or later), RS-232 cable to the drive's X300 service port.
• PROFIBUS trace tool (Siemens BT200 / DP/PA diagnostics or third-party class-2 master with capture).
• PLC programming software (STEP 7 V5.5 / TIA Portal) with the drive's GSD file SIEM8115.GSD loaded.
• Digital storage oscilloscope with isolated inputs.
• Drive parameter backup file (per-drive, dated) for rollback.

8.2 Procedure

1. Backup both drives. On Drive A and Drive B, use DriveMonitor File → Upload all parameters (UP). Save each file with the converter serial number and date in the filename.
2. Confirm parameter identity. Use DriveMonitor Compare parameter sets on the two backup files. Expect differences only in the PROFIBUS node address (P790 or P918 depending on firmware), the motor data block, and possibly the fieldbus diagnostic mask.
3. Capture the start sequence. In DriveMonitor, open the Trace function. Record connectors K3001 (PROFIBUS STW1 in), K0101 (terminal OR with K3001), and connector K0001 (final STW1 to sequencer). Trigger on the rising edge of bit 0 (ON). Sample at 10 ms for 5 s.
4. Inspect bit 1 over time. Verify whether the dip in bit 1 aligns with the bus-telegram interval or with the E-STOP chain voltage on terminal 107.
5. Scope terminal 107. Connect the isolated scope probe across terminal 107 to terminal 109. Trigger on a single sweep during the start sequence. Look for < 10 ms dips below 17 V (the PLC-output / drive-input 24 V threshold is typically 17 V minimum high).
6. Inspect the PLC program. Locate the FC/FB that builds the STW1 word for each drive. Confirm that OFF2 (bit 1) is always set true except in the explicit OFF2 fault path. Confirm the order: bit 1 = 1, bit 0 = 1, bit 3 = 1 (or follow PROFIdrive transition sequence 0x047E → 0x047F). Cross-check the cycle time of the OB1 / OB35 in which the FC runs.
7. Check the DP watchdog configuration. Parameter P785 on the 6RA70 sets the reaction to bus failure. Common values: 0 = no reaction, 1 = warning, 2 = fault F080. Drive B and Drive A may have different settings — align them.
8. Cross-check firmware. Read parameters r060 (firmware version) and r065 (firmware build). If Drive B is on firmware older than 2.4 and Drive A on 2.5 or later, the OFF2 debounce time may differ. Plan a firmware upgrade to a common version through Siemens SiePortal.
9. Apply the corrective action (see section 9) based on the strongest evidence.
10. Repeat the trace from step 3 and confirm that bit 1 stays at 1 for the entire start sequence on both drives.

9. Solutions and Workarounds

10. Verification

After applying the corrective action, perform all of the following checks on both drives:

1. Static check. Power the cabinet, do not issue a run command. Both drives should report o7.x (switch-on inhibit, expected before start).
2. OFF2 path integrity. DriveMonitor live monitor of connector K3001: while the PLC is in "ready but not running" state, bit 1 = 1. No dips.
3. Scope terminal 107. Verify 24 V ±10 % during start and run. No dropouts > 1 ms.
4. Start command trace. Repeat the DriveMonitor trace from section 8 step 3. Confirm that bit 1 stays at 1 from before the rising edge of bit 0 through the run command and steady-state operation.
5. Operating-state sequence. Issue the run command ten times in quick succession. Every time, Drive B should transition cleanly o7.x → o8.x → o0.x. No intermediate o8.0 hang, no F-faults.
6. E-STOP functional test. Press the E-STOP push-button with the machine running. Both drives should transition to o7.1 immediately. Release and acknowledge; both drives should return to o7.0 / o8.x as per OFF command logic.
7. Bus failure simulation. Unplug the PROFIBUS connector with the machine running. Both drives should respond per P785 setting (or hold last value if P785 = 0). Reconnect and verify recovery to RUN.

11. Related Fault Codes and States

12. Preventive Maintenance Recommendations

• Add OFF2 monitoring to the HMI/SCADA. Surface a warning whenever any drive reports o7.1 longer than 1 s without an explicit operator stop.
• Log PROFIBUS quality counters (PRM, CFG, SSR) in the PLC for trending. A rising CFG counter predicts connector problems.
• Schedule annual re-torque of all control terminal blocks on SIMOREG cabinets (Phoenix cage-clamp terminals in particular relax over thermal cycles).
• Maintain two firmware versions per drive family in the engineering library. Standardize new installs to the highest common version.
• Periodically (quarterly) capture a DriveMonitor trace of K3001 during a normal start. Compare against the baseline file. Drifting bit timing is an early indicator of a degraded bus.

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