Omron CQM1H-CPU21 Serial: RS-232 to USB Cable & Host Link

Overview

The Omron CQM1H-CPU21 is a compact modular PLC with two integrated serial interfaces: a peripheral port (round mini-DIN) intended primarily for the Omron Programming Console, and a built-in RS-232C port (DB9 male) that supports Host Link (SYSWAY), peripheral bus, and NT Link protocols. Modern workstations no longer expose a native DB9 COM port, so engineers must bridge the PLC's RS-232C port to USB using an adapter such as the ATC-810. This guide specifies the correct null-modem cable scheme, Host Link parameter set, and CX-Programmer configuration required to establish a stable programming link from a Windows PC through a generic RS-232 to USB converter.

The challenge is that the CQM1H CPU21 RS-232C port implements DTE signalling, while most USB converters also present a DTE-style DB9 (male) on the device side. A null-modem crossover or pre-built cable such as the C200H-CN229-EU resolves the signalling mismatch. Combined with a converter that delivers true ±5 to ±12 V RS-232 levels (not 3.3 V TTL) and an FTDI-based USB chipset, the link can sustain 9600 bps Host Link traffic for program upload, download, and online monitoring without errors.

Prerequisites

• Omron CQM1H-CPU21 CPU module installed in a powered rack with a healthy 24 VDC supply.
• Windows 7 SP1 / Windows 10 / Windows 11 PC with a free USB-A port.
• RS-232 to USB converter (ATC-810 or equivalent using FTDI FT232, SiLabs CP2102, or Prolific PL2303 chipset).
• RS-232 null-modem cable: Omron C200H-CN229-EU (2 m, DB9 female to DB9 female) or a self-built null-modem cable.
• Omron CX-Programmer v9.6x (final release with CQM1H support) installed on the workstation.
• CQM1H Programming Manual (W364) and CQM1H Operation Manual for reference.

CQM1H-CPU21 Hardware Identification

The CPU21 front face exposes two communications channels and four status LEDs. Identifying the correct port is the first step in cabling.

Front-Panel Layout

The COMM LED is the most useful indicator when commissioning a Host Link connection; it should flicker within one second of initiating an online operation from CX-Programmer.

RS-232C DB9 Pinout (CPU21 Port 1)

Cable Selection and Wiring

Option A: Pre-Built Omron Cable (Recommended)

The C200H-CN229-EU is Omron's 2 m pre-wired null-modem cable intended for Host Link connection between C-series / CQM1H CPUs and a PC serial port. Both ends are DB9 female. Internally it cross-wires SD/RD, RTS/CTS, and DTR/DSR. This cable is the lowest-risk option because the wiring is factory-tested and documented in Omron cable diagrams.

Option B: Build or Substitute a Compatible Null-Modem Cable

If a C200H-CN229-EU is unavailable, manufacture a DB9 female to DB9 female null-modem lead with the following pinout. Use shielded cable with overall braid tied to the shell at one end only.

Alternative: Peripheral Port Path (CBL-202 / CS1W-CN626)

If the RS-232C port on the CPU21 is already committed to another device (for example an HMI in NT Link mode), use the peripheral port with an adapter cable:

• CBL-202: Converts the CQM1H peripheral port to a DB9 RS-232C interface. Same wiring logic as the built-in port; pair with C200H-CN229-EU.
• CS1W-CN626: Designed for the CS1 series but electrically compatible with the CQM1H peripheral port for Host Link use.

RS-232 to USB Converter (ATC-810) Considerations

The ATC-810 is a generic RS-232 to USB adapter. For reliable Host Link traffic, the following points are critical.

Chipset and Driver

• FTDI FT232 / FT232R is the most reliable chipset for legacy industrial protocols. Install the latest FTDI VCP driver from the manufacturer before plugging the device in.
• SiLabs CP2102 and Prolific PL2303 are acceptable alternatives. Avoid counterfeit Prolific chips; the Windows 10 / 11 generic driver refuses non-genuine parts.
• After driver installation, verify the COM port appears under Device Manager → Ports (COM & LPT). Note the assigned COM number; values above COM9 work but may require manual reassignment for clarity.

RS-232 Voltage Levels

USB Power Management

• Disable USB selective suspend under Windows Power Options → USB Settings to prevent the converter from sleeping during long upload operations.
• Avoid USB hubs and extension cables beyond 1 m to keep common-mode noise low. If a hub is required, use a powered industrial USB hub.
• Plug the converter directly into a rear-panel USB port when possible; front-panel ports on tower PCs share busses with high-speed devices that can inject jitter.

PLC Host Link Configuration

The CPU21 serial port mode, baud rate, frame format, and unit number are stored in the PLC Setup area within data memory. Refer to the CQM1H Programming Manual (W364) for the exact bit allocations; the typical defaults for Host Link (SYSWAY) operation are listed below.

PLC Setup Word Map (Serial Port 1 / Peripheral Port)

Host Link Frame Format (Default)

• Baud rate: 9600 bps (settings exist for 300 / 600 / 1200 / 2400 / 4800 / 9600 / 19200 in DM6645 bits 0–3).
• Data bits: 7
• Parity: Even
• Stop bits: 2
• Unit number: 00 (range 00–31; must match CX-Programmer setting)
• Protocol: Host Link (SYSWAY) — also called "C-mode" in older literature.

Verifying the Active Settings

1. Connect the cable as described above.
2. Apply power to the PLC and let the CPU complete its 1-second self-test.
3. From CX-Programmer, attempt an online connection (procedure in the next section). If the link succeeds, the parameters in the PLC Setup are already correct.
4. If the link fails, place the CPU in PROGRAM mode and read DM6645 / DM6646 via a Programming Console or pre-existing CX-Programmer project to confirm mode and unit number.

CX-Programmer Setup (Step-by-Step)

CX-Programmer v9.6x supports the CQM1H family natively. The procedure below establishes a Host Link connection via the ATC-810 USB converter.

Project Creation

1. Launch CX-Programmer. If the New Project dialog does not appear, choose File → New Project.
2. In the device tree, expand Omron → CQM1 → CQM1H and select CQM1H-CPU21 as the CPU type. Confirm network type is Host Link.
3. Click Settings next to the network selector. The Host Link settings dialog opens.
4. Choose the COM port assigned to the ATC-810 (e.g., COM3). Set the following parameters: baud 9600, data 7, parity even, stop 2.
5. Enter the Host Link unit number (default 00). Enable Retry on Error if the PC is in a noisy environment.
6. Click OK twice to close the dialogs.

Going Online

1. Ensure the CPU is in PROGRAM or MONITOR mode. In RUN mode, CX-Programmer can read but not write without an explicit mode-change request.
2. Click the Online toolbar button or press Ctrl+W. CX-Programmer issues an RR (Read CIO) and RD (Read DM) Host Link command sequence.
3. The status bar at the bottom of the CX-Programmer window should display Online within two seconds. The yellow COMM LED on the CPU21 front panel should flicker rapidly during the handshake.
4. If Online fails, an error dialog reports the timeout (typically "Host Link Error: No Response"). Re-check the COM port, baud rate, and cable continuity before retrying.

Program Transfer

1. To retrieve the existing program: PLC → Transfer → From PLC. The project workspace populates with the on-line ladder and the PLC Setup words.
2. To write a new program: PLC → Transfer → To PLC. Confirm the dialog that warns about overwriting memory.
3. After transfer, choose PLC → Mode → Run (or Monitor) to place the CPU back into operating mode.

Verification Procedures

After establishing the link, run the following checks before assuming the connection is healthy.

Round-Trip Communication Test

1. From CX-Programmer, open PLC → Memory → CIO or DM view.
2. Write a known pattern to a free DM word (e.g., DM0100 = A5A5).
3. Issue PLC → Transfer → From PLC and verify DM0100 returns the same pattern.
4. Clear the test word to 0000 to leave the system in a clean state.

Online Monitoring Test

1. Open the main ladder program and click Monitor (Ctrl+M).
2. Confirm contact and coil status update in real time as inputs change.
3. Force an output (in PROGRAM mode only) and observe the corresponding indicator LED on the output module.

PLC Setup Verification

1. Open PLC → Edit PLC Setup. Read DM6645 and DM6646.
2. Confirm mode is Host Link, baud is 9600, frame is 7-E-2, and unit number matches the CX-Programmer connection setting.

Troubleshooting Matrix

Alternative Connection Paths

If the RS-232C port is occupied or the cable path is impractical, the CQM1H-CPU21 supports two further connection options documented in the CQM1H catalog (P050-E1-07).

• Peripheral port + CS1W-CN626 or CBL-202: Brings the mini-DIN peripheral port to a DB9 RS-232C interface. Same Host Link parameters and CX-Programmer settings apply.
• Communications board / expansion: A CQM1H-SCB41 serial communications board can add a second RS-232C + RS-422/485 channel, useful when the built-in port is already serving an HMI in NT Link mode.

FAQ