Configuring CJ1M-CPU22 RS-232 Port for Arduino Serial

Problem Definition

A common commissioning challenge on an Omron CJ1M-CPU22 (or CJ2M-CPU31) is sending data from the controller's serial port to an external device such as an Arduino Mega using the RS-232 protocol. Symptoms reported in the field include:

• The TXD(236) instruction executes but no bytes appear on the DB-9 line when probed with LEDs.
• Status flag A392.05 (Port 2 Send Ready) never goes ON.
• Flag A392.04 (Port 2 Transmission Error) latches ON instead, indicating the port cannot initialize.
• The CPU's COMM LED never blinks, confirming no edge transitions are present on TXD/RXD.

The root cause is almost always a combination of three issues: (1) the wrong option board installed in the CPU socket, (2) a logic-level mismatch between the Arduino (TTL 0–5 V) and the PLC port (RS-232 ±3 to ±15 V), and (3) port parameters in CX-Programmer set incorrectly or never set at all. This reference walks through every layer of the problem, from hardware selection to field verification with a serial monitor.

Hardware Identification & Required Components

The first step in any CJ-series serial commissioning is to confirm exactly which CPU you have and which port you are addressing. Many CJ1M and CP1H SKUs ship without a built-in DB-9 connector on the CPU module itself; the serial port is delivered as a snap-in option board. Misidentifying the SKU is the most frequent cause of the symptom "the port is configured but nothing transmits."

For the CJ1M-CPU22 referenced in the field, the bottom socket of the CPU accepts an option board. The required board for true RS-232 on the CPU module is:

• CP1W-CIF01 – RS-232C option board, DB-9 male, signals on pins 2 (RXD), 3 (TXD), 5 (GND). This is NOT the same as the CP1W-CIF11 (RS-422/485) or CP1W-CIF12 (RS-485 isolated) variants.

Alternatives for older installations: the older CPM1A-CIF01 (RS-232C) and CPM1A-CIF11 (RS-422) boards are functionally compatible when CJ1M is configured to recognize them, but the CP1W form factor is the modern recommendation.

Electrical Interface: RS-232 vs TTL

The Arduino Mega uses an ATmega2560 UART with TTL signal levels (0 V / 5 V), with logic HIGH ≥ 2.0 V and LOW ≤ 0.8 V, where the controller and listener share a common ground. RS-232C, in contrast, defines a single-ended asymmetric interface with a logic "1" (mark/Space) between –3 V and –25 V and a logic "0" (mark) between +3 V and +25 V, with the standard voltage range of ±5 V to ±15 V used by most transceivers, including the MAX232.

Connecting a 5 V Arduino TX pin directly to a PLC DB-9 RX pin will fail for two reasons:

1. The Arduino's 0 V "space" will sit inside the PLC's ±3 V dead band, producing a logic-low error.
2. The PLC's negative voltage swing can forward-bias the ATmega's input clamping diodes, risking permanent damage to the MCU if the PLC's –10 V appears on the Arduino's RX.

The correct bridge is a charge-pump transceiver such as the MAX232CPE (or 3.3 V-tolerant MAX3232). The chip accepts 5 V TTL on T1IN/T2IN and produces ≈ ±9 V on T1OUT/T2OUT. The MAX232 requires five external capacitors (typically 1.0 µF). Refer to the Maxim datasheet for the exact capacitor values and DC-DC polarity.

CX-Programmer PLC Setup – Serial Port Configuration

On a fresh project, the port parameters for the CJ1M serial option must be set both in CX-Programmer (offline) and downloaded to the CPU. The PLC Setup table holds the entries that the TXD instruction uses at runtime. The two relevant settings are:

Step-by-Step Procedure (CX-Programmer / Sysmac Studio)

1. Open the project and go online. Right-click the CPU in the project tree and select PLC Setup → Serial Port 2 (or "SERIAL PC" depending on the version).
2. Set the following fields for the Arduino link configured at 9600, 7, E, 2:

3. Transfer the change: PLC → Transfer → To PLC. Confirm with the dialog box, switch to Program mode if prompted, then back to Run/Monitor.
4. Verify that the new settings are now visible at CPU → Edit PLC Setup → Serial Port while online.

TXD Instruction Implementation

The TXD(236) instruction reads bytes from a consecutive block of words and pushes them out Port 1 or Port 2. The operands are:

Minimal Ladder Example

|
|      P_On                   A39205          TXD(236)
|-----||----------------------||--------------------------------(S  C  P)|
|                                                  (D0,#0010,#0010)
|

Here, P_On is the always-closed flag (CF100 or CIO W0.00) and A392.05 is the "Port 2 Send Ready" flag – TXD interlocked against A392.05 ensures the previous transmission has finished before a new one starts, which is mandatory because TXD will not queue; it will silently drop bytes if the port is busy.

Loading the Source Buffer

Words D0..D7 must be pre-loaded with the bytes to send, in little-endian order per word:

MOV(021)  #5441  D0     ; "AT"  ASCII 0x41 ('A'), 0x54 ('T')
MOV(021)  #2B0D  D1     ; "\r+"  ASCII 0x0D (CR), 0x2B ('+')

For testing, a simple two-byte stream such as "\r\n" at 9600 7E2 helps isolate whether the issue is the data or the framing.

Status Flag Diagnostics

The CJ1M exposes serial port status in the AR (Auxiliary Relay) area. The most useful flags during commissioning are:

Field interpretation:

• A392.04 ON and A392.05 OFF at the same time means the port never came up. The most common cause is mismatched settings (baud rate wrong, parity wrong, port in Host Link mode without a host talking to it).
• A392.04 OFF and A392.05 OFF means the previous TXD is still in progress or the buffer is full.
• A392.05 ON while A392.04 OFF is the healthy state; if you still see no activity, the fault is downstream in the wiring or the converter.

MAX232 Wiring & DB-9 Pinout

The MAX232 in a 16-pin DIP requires four capacitors plus an optional fifth for the voltage doubler. The TTL side connects to the Arduino; the RS-232 side to the PLC DB-9.

Pinout Table

Inline Wiring Diagram (SVG)

Three rules are repeated in every CJ-series serial guide:

1. Use only pins 2, 3, and 5 of the DB-9. Pins 4 (RTS) and 6 (DSR) are bridged inside the PLC; pins 7/8 are internal handshake and may be left open.
2. Tie all grounds together: PLC ground, MAX232 ground, and Arduino ground must be on the same 0 V reference.
3. Keep the cable short. RS-232 is rated for 15 m at the slowest common baud rates, but field installations beyond 5 m should be probed with an oscilloscope rather than LEDs.

Arduino Mega Serial1 Configuration

On the ATmega2560, Serial goes to the USB-UART bridge (pins 0/1), while Serial1 is the hardware USART1 on pins 18 (RX1) and 19 (TX1). Use Serial1 for the PLC link so you keep the USB monitor free for diagnostics.

// Arduino Mega sketch for 9600 7E2 receive from PLC
#include <Arduino.h>

void setup() {
  Serial.begin(115200);          // USB monitor
  Serial1.begin(9600, SERIAL_7E2); // 7 data, Even parity, 2 stop bits to CJ1M
  while (!Serial) {}
  Serial.println(F("PLC link ready @ 9600 7E2"));
}

void loop() {
  static uint8_t buf[64];
  static uint8_t idx = 0;

  while (Serial1.available() > 0) {
    uint8_t c = Serial1.read();
    if (idx < sizeof(buf)) buf[idx++] = c;
  }

  // Echo each message back to the USB monitor
  if (idx > 0) {
    Serial.print(F("RXD: "));
    for (uint8_t i = 0; i < idx; i++) {
      Serial.print(buf[i] < 0x10 ? "0" : "");
      Serial.print(buf[i], HEX);
      Serial.print(' ');
    }
    Serial.println();
    idx = 0;
  }
}

The key call is Serial1.begin(9600, SERIAL_7E2);. The Arduino's parameter set on the PLC must match this exactly: any deviation produces framing errors that surface on the PLC as A392.04 ON.

Communication Verification with RealTerm

Before chasing firmware bugs, verify the byte stream from the PLC with a passive monitor on the line. The recommended tooling:

• Hardware: FTDI USB-to-Serial adapter tapped to pins 2, 3, 5 of the DB-9. Tie the grounds; do not rely on the chassis.
• Software: RealTerm configured at 9600 7E2, Display as HEX, Half Duplex off, capture to file enabled.
• Exercise: Cycle the PLC, run TXD with a known buffer such as 0x54 0x41 0x0D 0x0A ("AT\r\n") and watch for the same bytes in RealTerm.

Step-by-Step Commissioning Procedure

1. Confirm hardware: Power off the CPU, insert CP1W-CIF01 into the option slot until the latch clicks. Power up. The CPU COMM LED should remain OFF.
2. Configure port: In CX-Programmer, open PLC Setup → Serial Port 2, set 9600 7E2, Host Link OFF, transfer to PLC, cycle power.
3. Stage the buffer: Write D0 = 0x5441 ("AT") and D1 = 0x0D0A ("\r\n") via MOV instructions on first scan, then latch them.
4. Wire the TXD rung: Use the ladder in the previous section. Put the CPU in Monitor mode.
5. Verify with RealTerm: Tap the line with a USB-serial sniffer, confirm the bytes 41 54 0D 0A appear once per TXD scan when a rising-edge trigger fires the rung.
6. Verify the round trip: Have the Arduino echo the message back through Serial1. The PLC RXD(235) instruction should pulse A392.07 with each byte received.
7. Log and document: Save the RealTerm capture as evidence for the project folder.

Troubleshooting Matrix

Frequently Asked Questions