Resolving SP16 Red Error LED Lockout: MiniLSS LR Size Timeout

Problem Overview: SP16 Red Error LED Lockout

The Omron Sysmac Mini SP16 enters a fatal error state when its CPU LED is latched solid red and the host programming tool — typically MiniLSS — returns the dialog Can't read LR size, communication time out during the initial link handshake. This state is not the same as a routine communications fault: the SP16 has refused to enter RUN/MONITOR mode and will not service host-link frames until the underlying cause is cleared. A power cycle alone does not always extinguish the LED; in many field cases the PLC requires an explicit memory clear, a programmed error-reset sequence, or a hard reset with battery removal.

The reported symptom set:

• CPU/PWR LED latched solid red; ERR indicator may be latched or flashing depending on fault class.
• No program execution — output LEDs remain in their initial-state values, contactor logic does not advance.
• MiniLSS terminates the connect dialog before the PLC model string (e.g. SP16) and the LR-area size word can be read.
• Subsequent Online > Connect attempts all return the same time-out; Transfer > From PLC aborts immediately.

This reference walks through the indicator semantics, the link-layer handshake that fails, and the supported recovery paths so a maintenance engineer can restore the unit to service without resorting to a destructive firmware reload.

SP16 Hardware and Indicator Architecture

The SP16 belongs to the Sysmac Mini SP-series — a compact brick PLC family that includes the SP10 (10 I/O), SP16 (16 I/O), and SP20 (20 I/O). The CPU unit houses an integrated RS-232C peripheral port, a removable terminal strip for I/O, and on later units a DIP-switch bank for selecting host-link mode, baud rate, and unit number.

Front-panel indicators on a typical SP16 CPU module:

A solid red ERR LED is the diagnostic anchor for this article. It maps to a CPU state in which the firmware has executed FALS(nn) with nn != 07, has detected a watchdog timeout, or has encountered a memory or scan integrity fault at boot. The PLC continues to respond to NAK frames and may echo its model string, but will not honor a memory read or write until the latch is cleared.

Decoding the "Can't Read LR Size, Communication Time Out" Fault

MiniLSS performs a fixed sequence when connecting to an SP-series CPU over the peripheral (RS-232C) port:

1. Open COM port at the configured baud rate, data bits, parity, and stop bits.
2. Send @00FA04000000FA*<FCS><CR> — a host-link FA (Status Read) command at unit number 00.
3. Await @00FA....<FCS><CR> within ~3 s.
4. Issue a model/area read (RD or QQ command depending on MiniLSS version) to obtain DM, LR, HR, AR, TC, and IR capacities.
5. Populate the LSS project view with the returned capacity values.

When step 3 times out, MiniLSS raises Can't read LR size, communication time out and aborts. The PLC did not reply to the FA frame within the link-layer retry window. Three root causes dominate:

If the ERR LED is solid red before any host software is launched, the firmware is signaling that the program area or system area is corrupted. A MiniLSS-only reset cannot bypass this — the CPU must first be persuaded to release the latch, then the host link can complete.

Root Cause Matrix: Fatal vs. Non-Fatal States

Omron classifies PLC faults as fatal (stop the CPU, latch the error) and non-fatal (log the error and continue execution). For the SP16 the classes are:

The locked-LED scenario in this article is overwhelmingly a fatal-class condition. The path to a green RUN LED therefore starts by identifying the class, then applying the matching clear method.

Pre-Reset Safety and Isolation Procedure

Before clearing a program on a running machine, isolate the system to prevent unexpected outputs.

1. Place the controlled machine in a safe state (e-stops engaged, motion axes braked and locked).
2. Open the main disconnect and lock out / tag out (LOTO) per OSHA 1910.147.
3. Verify zero energy at the PLC's 24 VDC supply terminals with a calibrated meter.
4. Document the current DIP-switch positions and the existing program revision (label the SP16 if multiple units are present).
5. Confirm that any expansion I/O or HMI that depends on shared LR/TIM/CNT data has been noted — a memory clear will zero the user program and most data areas.

MiniLSS Setup and Configuration

MiniLSS is the legacy ladder programming tool for the C-series and SP-series PLCs. It runs on Windows 9x / NT 4.0 / 2000 / XP. Newer 32-bit Windows installs frequently require compatibility mode and a real or virtual COM port.

Install and Configure the COM Port

1. Install MiniLSS from original media (or copy the install directory; no activation is required for SP-series support).
2. Launch MiniLSS. From the main menu open Setup > Communication.
3. Select the COM port mapped to your USB-to-serial adapter or native COM1/COM2.
4. Configure the link parameters to match the SP16 DIP-switch-selected settings. Defaults for most SP16 units:

Save the configuration. Cycle MiniLSS if the COM port was opened by another application.

Primary Reset Path: MiniLSS Online Operations

If the SP16 ERR LED is solid red and the unit does not respond to a normal Online > Connect, perform the following sequence. Each step has a verification check that confirms whether the fault is cleared or merely masked.

Step 1 — Force PROGRAM Mode via Host Link

1. From MiniLSS main menu, choose PLC > Operating Mode > PROGRAM.
2. When the dialog returns Communication time out, switch to Setup > Communication and verify baud, parity, and unit number against the DIP-switch table on the CPU.
3. Retry the mode change. If still unsuccessful, the host link is not yet established — proceed to Step 2 without exiting MiniLSS.

Step 2 — Attempt a Forced Memory Clear

1. From the MiniLSS menu bar, choose PLC > Memory Clear > All Clear.
2. Confirm the warning dialog. MiniLSS issues a host-link EM (Erase Memory) command with all-areas selection.
3. If the command succeeds, the SP16 reboots into PROGRAM mode with empty user memory. The ERR LED extinguishes; PWR and RUN indicator behavior returns to default (RUN off, PWR on).

Step 3 — Reboot and Reconnect

1. Power-cycle the SP16 (24 VDC off for 5 s, then on).
2. From MiniLSS, choose Online > Connect. The dialog should now report SP16 and the LR size (typically LR 0..63 — confirm in the response box).
3. Open File > New, select SP16 as the device type, and the freshly cleared program area will accept a new ladder upload.

Alternate Reset Path: SYSMAC Support Software (SSS) and CX-Programmer

On a Windows 9x / 2000 / XP system with a working COM port, the SYSMAC Support Software (SSS) — Omron's mid-generation tool that succeeded MiniLSS — provides a more robust All Clear dialog with explicit confirmation of each cleared area. CX-Programmer (≥ v2.0) supports SP-series on legacy device drivers with the C-mode compatibility layer.

1. Install SSS or CX-Programmer (the SSS installer is the recommended path for SP-series CPUs).
2. In CX-Programmer, choose File > New, select Device Type: SP-series > SP16, and set Network Type: SYSMAC WAY Host Link.
3. Right-click the project node and choose Transfer > To PLC. The dialog includes a Clear PLC Memory checkbox; enable it.
4. Click Execute. The PLC reboots with cleared user memory. Reconnect with the corrected COM settings.

For reference, see the Omron Industrial Automation product portal for current CX-One tool documentation and the legacy C-series / SP-series support notes.

Handheld Programming Console Recovery

If a host PC is unavailable, a C-series-compatible handheld programming console (e.g., Omron C200H-PR027 or the older C500-PRW01) can clear the SP16 directly through its peripheral port. The console uses a four-key sequence:

1. Power the console and connect it to the SP16 peripheral port with the appropriate Omron console cable.
2. Press FUN, then 0, 7, then MON. The console displays the last fatal FALS number, e.g. FALS 90.
3. Press CLR, then MON. This issues the equivalent of an All Clear on the FALS latch.
4. Press PLAY (or SET on older consoles) to return to PROGRAM mode.
5. Verify by pressing FUN, 0, 7, MON again — the display should return 0000, indicating the fatal error latch is released.

After the console clear, attach the host PC and use MiniLSS to upload a known-good program.

Manual Hardware Reset Sequence (Power Cycle + Battery)

When the firmware refuses to release the fatal latch via host link or console, the next escalation is a hardware-level reset:

1. Remove 24 VDC supply. Wait 10 s for internal rails to decay.
2. Locate the battery compartment on the SP16 CPU. On most SP-series units this is a CR2032 (3 VDC) holder accessible after removing the terminal cover.
3. Disconnect the battery clip and wait at least 60 s. This drains the user-memory retention capacitor and forces a cold-start integrity check on next boot.
4. Reconnect the battery (or fit a fresh CR2032 if voltage measured below 2.7 V). Observe polarity — reversed insertion will not damage modern units but will prevent memory retention.
5. Reapply 24 VDC. The PLC boots; observe the ERR LED behavior:

• Solid red within 500 ms of power-on: firmware-level memory parity fault persists. The user memory must be cleared.
• ERR LED extinguishes after 1–2 s: boot completed normally; the prior latch has been released.

6. Attach MiniLSS and perform the PLC > Memory Clear > All Clear sequence. The CPU should now accept host-link commands.

Communication Parameter Validation (Host Link Defaults)

The SP-series peripheral port uses the Omron Host Link protocol. The four link parameters must agree between the host PC and the PLC DIP-switch selection.

DIP Switch Layout (typical SP16)

SW1-1 is the most frequently mis-set switch. When OFF, the port expects a different framing convention; MiniLSS will report time-out even though the cable is correct. Verify the actual switch position by removing the terminal cover and reading the silk-screen labels on the CPU body.

Frame Check Sequence (FCS)

Host-link commands carry a two-character ASCII FCS computed as the XOR-8 of all characters between the @ and the *. A wrong FCS causes the PLC to silently drop the frame. Tools such as the HostLinkTester utility or a terminal in raw mode can be used to manually verify FCS calculations:

The expected response on a healthy PLC in PROGRAM mode is @00FA00000400<FCS><CR> (status word with CPU in PROGRAM).

Cable and Port Verification

The SP16 peripheral port is a female 9-pin D-sub. The recommended Omron cable is the XW2Z-200S (2 m) or XW2Z-500S (5 m). Pinout for direct PC connection:

Loopback test: short pin 2 to pin 3 on the PC end and run a terminal at the configured baud rate. Typed characters should echo. Failure of the loopback indicates a USB-to-serial adapter or driver issue — not a PLC fault.

Software/OS Compatibility Notes

MiniLSS is a 16-bit Windows application. On 32-bit Windows 7 and later it requires:

• Administrator rights during install.
• Compatibility mode set to Windows 98 / Windows ME.
• Real COM port mapping (native DB-9 or USB-to-serial with vendor driver that exposes a legacy COM number; some virtual COM drivers do not expose the API calls MiniLSS expects).

Field reports of the communication time out error on Windows 95 / 2000 systems almost always trace to USB-to-serial adapters that lose frames under load. Try a PC with a native DB-9 COM port before concluding the SP16 has an internal fault.

When the Reset Fails: Hardware Diagnostics

If all software- and console-based resets fail, escalate to bench diagnostics:

1. Measure the 24 VDC supply at the SP16 input terminals under load. Sag below 21 V for > 40 ms will latch a fatal power fault.
2. With the SP16 isolated from field wiring, attempt the All Clear again. A shorted output (transistor SP16-DT units) can pull the internal rail down.
3. Inspect the PCB for electrolytic capacitor bulging or PCB discoloration around the voltage regulator.
4. Check the lithium battery voltage. CR2032 cells below 2.7 V produce memory parity errors at next power-on.
5. Verify that no expansion I/O rack is causing a bus fault — disconnect expansions one at a time and retry the boot.

Verification Procedure

After a successful reset, validate the SP16 is fully operational before returning the machine to service:

1. Power-cycle the CPU. Confirm PWR green, ERR off, RUN off (PROGRAM mode default).
2. From MiniLSS, perform Online > Connect. Confirm the dialog box reports SP16 and a valid LR size (typically 64 words).
3. Upload PLC > Transfer > From PLC to verify the program matches the archived source.
4. Switch to MONITOR mode. Verify scan execution (COMM activity, RUN LED solid green).
5. Force each output via Force Set / Reset from MiniLSS and confirm field device response.
6. Switch to RUN mode. Monitor scan time on the programming console — should remain well below the 130 ms watchdog.
7. Power-cycle one final time. Confirm the program retains without ERR latch and RUN mode auto-starts if so configured.

Field-Engineer Checklist

• ERR LED state confirmed solid red before any host attempt.
• 24 VDC supply measured within tolerance at the SP16 terminals.
• DIP switches verified against the host PC link settings.
• Battery voltage > 2.9 V on a healthy CPU.
• MiniLSS All Clear executed; CPU rebooted to PROGRAM mode.
• Program reloaded from verified archive.
• MONITOR and RUN modes validated with output forcing.