CQM1-CPU41 DIP Switch Write-Protect: Resolving Program Lock

Resolving CQM1-CPU41 Program Lock: DIP Switch Write-Protect and Operating Mode

The Omron CQM1-CPU41 CPU Unit occasionally presents a frustrating condition: the controller appears online in CX-Programmer, the user can read (upload) the program, but every write, edit, or clear (unload) operation is rejected. This article isolates the two underlying causes that produce this exact symptom on the CQM1-CPU41 (and the closely related CQM1-CPU11/21/42/43/44-EV1 variants): the user-memory (UM) write-protect DIP switch on the front of the CPU, and the controller's operating mode (RUN vs. MONITOR vs. PROGRAM). Every diagnostic step, the relevant AR-area status flags, the required cable configuration, and the CX-Programmer v4.x / v9.x settings are documented below for an engineer working against a CQM1-CPU41 with a Windows XP (or later) host running the CX-One or CX-Programmer suite.

1. Problem Summary

When connected to a CQM1-CPU41 with an RS-232C peripheral or host-link cable, the engineer observes one or more of the following:

• CX-Programmer reports the PLC is online.
• Program upload (PLC -> PC) succeeds.
• Program download (PC -> PLC) is rejected with messages such as "Write protected," "Cannot write to UM," or "Protected by DIP switch."
• Online edit attempts in MONITOR mode return "Write disabled" or produce a red error indicator.
• Program clear / memory clear (unload) is blocked.

The reported DIP switch state on the unit is:

SW7 in the ON position is the most common cause of the locked-out condition described in the source case. SW3/SW4 govern RS-232C communications parameters, and SW8 controls the expansion-instruction set (must be ON to use advanced instructions such as PID, MSG, SCL, etc.).

2. CQM1-CPU41 Hardware Reference

The CQM1-CPU41 is a compact modular CPU Unit in the CQM1 series introduced in the 1990s and superseded by the CJ1M. Key specifications relevant to this troubleshooting case:

For complete programming and installation details consult the CQM1/CPM1/CPM1A/SRM1 Programming Manual (W228-E1-08) hosted by Omron eData.

3. Front-Panel DIP Switch Reference (SW1-SW8)

The CQM1-CPU41 front-panel 8-position DIP switch (often labelled "DIP" or "SET" on the housing) controls default peripheral-port and RS-232C communications, write protection, and expansion-instruction set availability. Pin numbering is left-to-right with switch 1 nearest the LED array.

Confirm SW7 against the actual plastic silkscreen on the specific CPU. On CQM1-CPU41-EV1 units the legend is usually printed above the switch block; on CQM1H-CPUxx units the mapping differs (SW5 = expansion set, SW6 = write protect), so always cross-check the variant marking on the side label.

4. Operating Mode and AR Status Flags

The CQM1-CPU41 has a physical three-position key switch on the front panel:

• RUN - program executes; online edits disabled.
• MONITOR - program executes; online edits, force-set/reset, and online program modification are permitted (subject to UM write protect).
• PROGRAM - program execution stopped; full download, program clear, and UM transfer permitted (subject to UM write protect).

The current operating mode and the write-protect state are reflected in the Auxiliary Relay (AR) area:

To verify the write-protect state without altering the program, open the CX-Programmer Watch window, set AR 00 as the data type, and read the value. Bit 15 high (1) confirms UM is hardware-locked. With AR 00.15 = 1 there is no software workaround that overrides the SW7 = ON condition.

5. Root Cause Analysis

The reported condition (program uploads, edits and program clears rejected; CX-Programmer reports a write-protect error) maps to two distinct but additive conditions, both of which the source case exhibits:

1. Hardware UM write-protect active. SW7 = ON disables all writes to the user-memory (UM) area. The CX-Programmer transfer dialog will report "Could not transfer to PLC: write-protect switch is ON" (the exact wording varies with CX-Programmer version: v3.x, v4.x, v9.x differ slightly).
2. Operating mode mismatch. If the controller key switch is in RUN, even with SW7 = OFF the program cannot be edited online (download and program clear are rejected). The engineer must move the key to PROGRAM for a full transfer or clear, or to MONITOR for online edits.

Both conditions must be addressed before the program can be written, edited, or cleared. In the source case the DIP switch word 1=OFF, 2=OFF, 3=ON, 4=ON, 5=OFF, 6=OFF, 7=ON, 8=OFF clearly indicates SW7 = ON, so the UM is hardware-locked. The first corrective action is to set SW7 = OFF.

6. Resolution Procedure

6.1 Prerequisites

• Small flat-blade screwdriver or non-conductive tool for DIP switch actuation.
• Valid RS-232C host-link cable (Omron XW2Z-200S-CV, 2 m, or compatible) or CQM1-CIF01 / CQM1-CIF02 peripheral-to-serial adapter.
• CX-Programmer v4.x or v9.x installed and licensed. (CX-One v1.x / v4.x bundles CX-Programmer; the CQM1-CPU41 is supported in CX-Programmer v3 and later. CX-Programmer v9.7x is the last version supporting Windows 10 / 11 for legacy C/CQM1/CV-series devices.)
• Administrative rights on the host PC to install or update the Omron USB serial driver (DRV-USB-SERIAL / VCP bridge).

6.2 Step-by-Step

1. Stop the controlled process. Switching the operating-mode key from RUN to PROGRAM halts program execution. The user is responsible for machine safety; coordinate with operations and apply lock-out/tag-out if the I/O drives machinery.
2. Set the operating mode key to PROGRAM. This unlocks the transfer / clear commands in CX-Programmer.
3. Toggle SW7 from ON to OFF. Use a small screwdriver to slide the switch position. The change takes effect immediately; no power cycle is required. SW7 must be set to OFF before initiating any download or program clear.
4. (Optional) Toggle SW8 to ON if the program uses expansion instructions (PID, MSG, RXD/TXD, SCL, AVG, PID, TKY, HKY, etc.). Leaving SW8 = OFF with a program that contains expansion instructions will produce "instruction not recognized" errors at execution.
5. Verify AR 00.15 is 0. In CX-Programmer, go online and open the Watch window. Add AR 00.15 as a binary address. The bit must read 0 for UM to be writable.
6. Establish / verify the host-link connection. In CX-Programmer, select PLC > Auto Online > Direct Online, or manually configure the C-series driver. Recommended settings: 9 600 bps, 7 data bits, even parity, 2 stop bits, host link unit number 00.
7. Perform the desired write operation. Choose PLC > Transfer > To PLC (download), PLC > Transfer > Program Clear, or right-click a rung and select Online Edit while in MONITOR mode.
8. Return the key switch to RUN or MONITOR once the transfer completes. Re-arm the safety circuit before restarting the machine.

7. Cable and Adapter Reference

USB-to-serial converters using FTDI FT232 or Prolific PL2303 chipsets generally work, but CX-Programmer v3 / v4 was originally developed for native COM ports. On Windows XP, install the legacy Omron VCP driver before plugging in the converter, and assign the resulting COM port a number between 1 and 16 (CX-Programmer v3 has trouble with COM > 16). On Windows 7 / 10 / 11 use the certified FTDI driver and CX-Programmer v9.7x for reliable operation.

8. CX-Programmer Configuration

8.1 Driver and Network Setup

1. Open CX-Programmer and select File > New or open the existing .cxp project.
2. From the project tree, right-click the PLC icon and select Change PLC Type > CQM1-CPU41.
3. Double-click the icon to open the Change PLC dialog. Click Settings next to the Driver and choose C-series Host Link (peripheral or RS-232C port).
4. Configure the COM port number, baud rate (9 600 default), data bits (7), parity (Even), stop bits (2), and unit number (0). These must match the SW1-SW6 setting on the CPU.
5. Click Test to verify communication. A green OK confirms the host link is alive.

8.2 Transfer and Online-Edit Procedure

1. Set the controller key to PROGRAM (for full download) or MONITOR (for online edit / rung overwrite).
2. Toggle SW7 to OFF.
3. Select PLC > Transfer > To PLC... in CX-Programmer. The dialog should report "UM write protect: OFF"; if it reports "ON" the DIP switch has not been correctly actuated.
4. Confirm the transfer. A progress bar indicates the write; on success the project memory equals the controller memory.
5. To perform a program clear / unload, select PLC > Clear > All Memory Areas or PLC > Transfer > Program Clear (CX-Programmer v4 and later). Confirm the prompt.

9. Verification

After completing the resolution, perform the following verification sequence:

1. Re-read the AR area: confirm AR 00.15 = 0 (write protect cleared) and the operating-mode bits in AR 00.13 / AR 00.14 match the key-switch position.
2. Compare the project memory with the controller memory: PLC > Compare > With PLC. The dialog should report "No differences."
3. Cycle the power (off, wait 5 seconds, on) and re-online. The program should load and execute without errors.
4. Check the ERR LED on the front of the CPU. It must be OFF. If it lights red, cycle the key switch to PROGRAM, look for the first error in the ladder, and resolve before returning to RUN.
5. Check the battery status (AR 00.12). If low, replace the C500-BAT08 / 3G2A9-BAT08 backup battery before relying on RAM-resident program storage.

10. Related Error Conditions

11. Best Practices and Field Notes

• Document the DIP switch settings. Every CQM1-CPU41 commissioning should record SW1-SW8 on the controller label, the electrical drawing, and the project submittal. SW7 and SW8 are the most frequently changed after commissioning.
• Disable write-protect after commissioning only when actively editing. Leaving SW7 = ON during production prevents accidental overwrites from a misrouted host-link session. Toggle it to OFF only during scheduled program changes.
• Use the 1:1 host link rather than 1:N when the project is being actively developed. 1:N host link on a multi-drop RS-485 line is convenient in production but slows transfers and complicates unit-number management.
• Always go online and verify AR 00.15 before declaring a write failure. This bit is the unambiguous indicator of the SW7 state, regardless of what CX-Programmer says in the error dialog.
• Prefer CX-Programmer v9.7x on Windows 10/11. CX-Programmer v3 and v4 run on Windows XP / 7 but are not officially supported on Windows 10. If a Windows 10+ host is required for the legacy CQM1-CPU41, install CX-One v4.x (which bundles CX-Programmer v9.7x) and confirm the C-series driver is licensed.
• Replace the battery every 5 years as preventive maintenance, or sooner if AR 00.12 indicates a low-battery state. A dead battery erases the program on the next power-down.
• Avoid mixing expansion instructions across the SW8 boundary. If the program must remain SW8 = OFF, replace PID, MSG, RXD/TXD, SCL, and similar instructions with their basic-instruction equivalents where possible.

12. Frequently Asked Questions