SYST-020

Verify PNSTROBE signal duration

Navigate to MENU > I/O > UOP Signals and monitor signal UI[17] PNSTROBE in real-time. The strobe must remain ON for minimum 100ms plus program verification time (typically 200-500ms total depending on program size). Use a PLC timer or oscilloscope to measure actual pulse width from the PLC output.

For large programs or slow memory access, increase strobe hold time to 1 second minimum. The controller will assert PNUMOK (UO[19]) when verification is complete—this is your confirmation signal.

Check PNUMOK feedback signal timing

Monitor MENU > I/O > UOP Signals > UO[19] PNUMOK. This signal goes HIGH when program verification succeeds. Modify PLC logic to hold PNSTROBE until PNUMOK goes active, then release strobe. This handshaking prevents premature strobe release.

Implement a watchdog timer in PLC (3-5 seconds) for PNUMOK response. If timeout occurs, log the PNS inputs being sent and check if that program number exists in controller memory.

Review PLC ladder logic timing

Examine the PLC program controlling PNSTROBE. Look for edge-triggered outputs, scan-time dependent logic, or one-shot pulses that may create insufficient pulse width. Replace with latched output that holds until PNUMOK confirms, or use fixed timer (minimum 500ms).

Do not use PLC one-shot or rising-edge instructions for PNSTROBE—these typically create pulses too short for controller verification.

Verify system configuration parameter

Check MENU > SETUP > System Config > PNS Check ($SCR.$PNS_CHECK). This should be enabled (value = 1). Also verify $SCR.$PNS_MODE matches your application: 0=No PNS, 1=Standard PNS, 2=Extended PNS. Incorrect mode prevents proper verification.

Extended PNS mode (value 2) supports up to 1000 programs but requires different input wiring. Confirm your hardware matches the configured mode.

Monitor PNS input signals for stability

Navigate to MENU > I/O > UOP Signals and observe UI[9] through UI[16] (PNS1-PNS8). While PNSTROBE is active, these signals must not change. Use Force Display mode (press and hold UI signal) or Status > Digital I/O with Fast Refresh (SHIFT+DISP) to catch intermittent changes.

Record several program selection cycles with a data logger or scope on physical inputs. Look for noise spikes or glitches especially on longer cable runs (>15m) where capacitance and noise pickup increase.

Verify PLC output latch logic

Ensure PLC program latches all PNS binary outputs before asserting PNSTROBE. The sequence must be: 1) Update all PNS outputs with new program number, 2) Wait minimum 10ms for signals to settle, 3) Assert PNSTROBE, 4) Hold all signals until verification complete. Check for race conditions in PLC scan cycle.

Never change PNS input values while PNSTROBE is active. This will always cause SYST-020 alarm.

Check I/O circuit and wiring integrity

Inspect physical wiring from PLC outputs to robot controller UI terminals. Look for loose connections, damaged insulation, or shared conduit with high-power cables. Verify proper shielding and grounding on signal cables. Check for proper voltage levels: UI signals should be 24VDC nominal (18-30VDC acceptable range).

Add RC noise suppression filters (0.1μF capacitor + 1KΩ resistor) across each PNS input at the controller terminal block if noise is suspected. This is especially important in welding or high-EMI environments.

Test with manual PNS selection

Temporarily disconnect PLC and manually force PNS inputs from teach pendant: MENU > I/O > UOP Signals > Force UI[9]-UI[16] to binary value for a known program number. Then force UI[17] PNSTROBE ON and hold for 2 seconds. If verification succeeds, issue is in PLC/wiring; if it fails, issue is in controller configuration.

Use program number 1 for initial test (binary 00000001, only UI[9] forced ON). This eliminates multiple-bit decoding issues and confirms basic PNS functionality.

Verify program exists with correct number

Press SELECT on teach pendant and review program list. Confirm the program number being requested via PNS inputs actually exists. Cross-reference PNS binary input value: calculate decimal value from binary inputs (UI[9]=bit 0, UI[10]=bit 1, etc.) and verify matching program number in list.

Common mistake: PNS sends program number 100 but program is actually numbered 0100 or saved with leading zeros. Use MENU > FILE > Program to view exact program numbers including leading characters.