Key Technical Specifications

• Product Model: CQM1H-CPU21
• Manufacturer: OMRON
• Module Type: Compact PLC CPU Unit
• Program Memory: 3.2K Words
• Data Memory: 3K Words
• Max I/O Points: 256 points
• Built-in I/O: 16 points DC Input
• Communication Port: Built-in RS-232C
• Internal Boards: Not supported on this specific CPU model
• Expansion: Requires CQM1H series expansion units
• Power Supply: Requires separate CQM1-PA or CQM1-PD power unit
• Target Applications: Packaging machinery, conveyor systems, and general factory automation

 

Product Introduction

When a 20-year-old packaging machine suddenly loses its brain, finding a replacement CPU that fits the existing backplane and wiring is a massive relief. The OMRON CQM1H-CPU21 is a classic compact PLC CPU that has been the workhorse of factory automation for decades. It provides 3.2K words of program memory, a built-in RS-232C port for programming and HMI communication, and supports up to 256 I/O points through expansion racks.I’ve installed hundreds of these in tight control panels where space was at a premium. The CQM1H-CPU21 is reliable, but it has one critical quirk: it does NOT support inner boards. If your old CPU had a high-speed counter or serial comms board inside it, this CPU21 cannot replace it. You would need the CPU51 or CPU61 instead. Also, remember that this CPU has 16 built-in DC inputs hardwired to the CPU itself—make sure your field wiring matches those terminal locations.

Quality SOP & Tech Pitfalls

We don’t just ship these without verification. Every CQM1H-CPU21 goes through our live test rack. We start with a visual inspection of the DIN rail clips, backplane connector, and terminal blocks. Then, we install it on a CQM1H test chassis with a power supply and I/O expansion unit, download a test program via the RS-232C port, and verify that the CPU enters RUN mode without errors. We also check the built-in 16 DC inputs for continuity.Here is where guys get burned: Forgetting that the CPU21 doesn’t take inner boards. I once had a customer swap a CPU51 (which had an inner serial board) with a CPU21, and he spent four hours wondering why his barcode scanner wasn’t communicating. The inner board slot was physically empty. Also, always check the power supply current budget. The CPU21 draws 820mA from the 5V DC backplane. If you’ve stacked too many I/O modules, you might be overloading the power unit and causing random CPU resets.

 

Installation & Configuration Guide

1. Pre-Installation: ⚠️ SAFETY FIRST. Power down the CQM1H chassis and wait 5 minutes for the power supply capacitors to discharge. Take photos of the existing CPU’s DIP switch settings, inner board configuration, and wiring.
2. Removal: Disconnect the RS-232C programming cable and all field wiring. Release the DIN rail locking tab and slide the old CPU straight off the backplane.
3. Installation: Inspect the backplane connector for bent pins. Align the CQM1H-CPU21 with the DIN rail and backplane, then slide it down firmly. Ensure the DIN rail clip locks securely—an unseated CPU will cause intermittent communication failures.
4. Power-On & Testing: Restore power and verify the POWER LED illuminates. Connect your programming tool via RS-232C, download the program, and confirm the RUN LED stays solid. Test the 16 built-in inputs with a 24V DC source.

Compatible Replacement Models

• ✅ Drop-in Replacement: CQM1H-CPU21 – Exact same CPU. Ensure your application does NOT require inner boards.
• ✅ Drop-in Upgrade: CJ2M-CPU11 – Official OMRON replacement per their discontinuation guide. Requires new wiring, programming migration, and unit configuration. Budget 1-2 days of engineering labor.
• ⚠️ Software Compatible (Hardware Differs): CQM1H-CPU51 / CPU61 – These CPUs support inner boards and have more memory. They are physically compatible with the same backplane but may require program adjustments if memory mapping differs.
• ❌ Hardware Mod Required: CQM1 (non-H) CPUs – The older CQM1 CPUs have different memory capacities and are NOT fully interchangeable with CQM1H series. Do not mix them.

 

Frequently Asked Questions (FAQ)

Can I hot-swap this CPU while the system is running?
Absolutely not. The CQM1H series does not support hot-swapping the CPU. Pulling it while powered will crash the entire system and could damage the backplane. Always schedule a shutdown window.Is this a genuine OMRON part or a refurbished unit?
These are genuine OMRON New Surplus units. We test every CPU on a live CQM1H rack before shipping. Refurbished CPUs from unknown sources often have degraded EEPROM cells that corrupt programs over time.My old CPU had a serial board inside it. Will this CPU21 work?
No. The CQM1H-CPU21 does NOT support inner boards. If your system relies on an inner serial, high-speed counter, or analog board, you need the CQM1H-CPU51 or CPU61 instead.Can I use my old SYSMAC CQM1H programming tool with this CPU?
Yes, via the built-in RS-232C port. However, if you’re migrating to the CJ2M replacement, you’ll need CX-Programmer and a USB programming cable, as the CJ2M doesn’t support the old SYSMAC console.What’s the maximum number of expansion I/O units I can add?
The CQM1H-CPU21 supports up to 256 total I/O points (including the 16 built-in). Check the OMRON CQM1H Operation Manual for the exact expansion unit count limit, as it varies based on the type of I/O modules used.Does this CPU support Controller Link networking?
No. The CQM1H-CPU21 does not have a Controller Link slot. If you need CL networking, you’d need the CQM1H-CLK21 inner board, which this CPU doesn’t support. You’d have to upgrade to the CPU51/CPU61 or migrate to the CJ2M platform.