Mitsubishi FX3U Octal I/O Addressing: GX Developer 32 I/O Setup

Overview

The Mitsubishi FX3U-32MR is a 32-point main-unit PLC (16 inputs / 16 outputs, relay output type) that often confuses first-time users of GX Developer / GX Works because only 8 inputs and 8 outputs appear in the project window on initial install. The cause is almost always a misunderstanding of the FX-series octal I/O numbering scheme, not a hardware limitation or a missing option. Once the PLC series is selected correctly in the programming software, all 32 I/O points become visible and addressable as X000–X017 and Y000–Y017.

This guide walks through the FX3U-32MR I/O map, the octal rule, the GX Developer 8.109P project configuration, the SC09 / USB-to-serial communication path, and the error messages (such as "Cannot communicate with the PLC") that surface when COM port or cable settings are wrong.

Prerequisites

• PLC: Mitsubishi FX3U-32MR (or FX3U-32MT, FX3U-32MS) main unit
• Programming software: GX Developer 8.109P (or GX Works 2 / GX Works 3, which use the same address model)
• Cable: Mitsubishi SC-09 RS-232 programming cable (DB9 male to mini-DIN 8-pin male), or a generic USB-to-RS232 adapter plus SC-09 equivalent
• USB-to-serial adapter: MOXA UPort 1110/1130 or any FTDI-based adapter with a stable Windows driver
• Windows driver: Confirm the COM port number assigned in Device Manager → Ports (COM & LPT) before launching GX Developer
• Power supply: 100–240 VAC on FX3U-32MR, 24 VDC sensor supply available on the input terminals

Understanding the FX3U-32MR I/O Architecture

The FX3U-32MR ships with 16 discrete inputs and 16 discrete outputs integrated on the main unit. The "32" in the part number refers to the total I/O count (16 in + 16 out), not the per-type count. The character "M" denotes main unit and "R" denotes relay output; "T" would be transistor (sink) and "S" transistor (source).

The Octal Addressing Rule Explained

Every Mitsubishi FX-series PLC numbers its I/O in base-8 (octal), not base-10 (decimal). The digits 8 and 9 simply do not exist in the I/O map. This is the most common reason new users "see only 8 inputs" – they are looking at the first valid octal block (X000–X007) and assuming the system stopped.

For comparison with a Siemens S7-200/300 mindset:

The same rule applies to outputs: Y000–Y007, Y010–Y017, Y020–Y027, Y030–Y037. Addresses containing 8 or 9 (X008, X009, X018, X019, Y008, Y009, …) are illegal and will either be auto-corrected to the next valid octal address or rejected with a syntax error in the ladder editor.

Why the Project Initially Shows 8 I/O

GX Developer 8.109P ships with a default PLC type of FXCPU (generic) or a previous project type, and the default I/O comment range is limited to the first 8 points per type. The software is not "broken" – it is configured to a smaller, different FX model. Selecting the correct PLC series opens the full range and re-enables the I/O comment table to the full 32-point width.

Step-by-Step: Configuring GX Developer for the FX3U-32MR

1. Start a new project. Launch GX Developer 8.109P. From the menu, choose Project → New.
2. Select the PLC series. In the New Project dialog, set:
   • PLC Series: FXCPU
   • PLC Type: FX3U/FX3UC
   • Project type: Simple project (or Structured if writing ST)
   • Label: leave default
3. Confirm the ladder list opens with the full 32-point I/O comment table (X000–X017, Y000–Y017). The lower 8 points will no longer be the only ones visible.
4. Set the COM port for programming. Go to Online → Transfer Setup → Serial and change COM port from COM1 to the virtual COM number reported by the USB-to-serial driver (commonly COM3, COM4, or COM5).
5. Set the transfer baud rate. Default is 9,600 bps; FX3U supports up to 115,200 bps with a USB SC-09-FX compatible adapter. Leave at 9,600 unless the driver and cable are known to support higher rates.
6. Click Connection Test. A successful response reads "Successfully connected with the PLC" along with the CPU model and serial number.
7. Write the program using X000–X017 for inputs and Y000–Y017 for outputs. Use Online → Write to PLC to download.

Communication Setup: SC-09 Cable + USB-to-Serial Adapter

The SC-09 is a Mitsubishi programming cable that converts the PLC's RS-422 mini-DIN 8-pin port to a DB9 RS-232 PC port. Modern laptops do not have DB9 connectors, so a USB-to-serial adapter is required. The MOXA UPort 1110 / 1130 series and FTDI-based adapters are the most reliable in industrial environments because of their robust drivers and ±15 kV ESD protection.

SC-09 Pinout Reference

The SC-09 contains an internal active converter – it is not a passive null-modem cable. A generic null-modem DB9-DB9 cable will not work between the USB-to-serial adapter and the PLC; you must use a true SC-09 or a third-party equivalent (e.g., USB-SC09-FX).

Common Errors and Solutions

Error: "Cannot communicate with the PLC"

Displayed when the programming tool cannot complete the handshake with the FX3U. Most common causes in field installations:

1. Wrong COM port – GX Developer is still set to COM1 while the USB-to-serial enumerates as COM3/4/5. Verify in Device Manager.
2. Wrong cable type – A plain DB9 null-modem was used instead of the SC-09. Replace the cable.
3. USB driver not installed – the yellow warning triangle in Device Manager indicates a driver load failure. Reinstall the MOXA / FTDI driver.
4. Port held by another process – close any terminal emulator (PuTTY, Tera Term) that opened the COM port before the connection test.
5. Baud-rate mismatch – if the PLC was previously programmed at 19,200 bps for a GOT HMI, the default 9,600 will fail. Match the COM port baud rate to the PLC system register D8120 value.

Error: "Invalid input/output number" / Silent Address Shift

Typing X008, X009, X018, X019, X028, X029, Y008, Y009, etc. in the ladder editor triggers a syntax warning or, in some GX Developer builds, silently rewrites the address to the next valid octal value. Always inspect the address displayed in the tooltip after entry.

Error: Program Uploads as Blank After "Read from PLC"

If the PLC has the PROTECT keyword in its password table (D register 8101 / D8104 in older firmware), Read from PLC may return a blank project. Clear the password via Online → Password → Clear with the original 8-character keyword, or use the backup battery to maintain memory.

Verification: Confirming All 32 I/O Are Live

1. Place the PLC in RUN mode (switch on the right side of the CPU).
2. In GX Developer, open the Online → Monitor → Monitor Mode (or press F3).
3. Force each input X000 through X017 to ON from the field side (24 V to the S/S terminal and the corresponding X terminal). Each one should turn green in the monitor window.
4. Write a one-line test program:

   LD X000
OUT Y000

   Verify Y000 energizes the moment X000 is closed. Repeat the test for every input/output pair to confirm the entire 32-point I/O is wired and the software can address it.

Troubleshooting Matrix

Specifications Quick Reference

Notes for Migration to GX Works 2 / 3

Newer Mitsubishi toolchains (GX Works 2 and GX Works 3) replace GX Developer 8.109P but preserve the octal addressing model exactly. When importing a GX Developer 8.109P project, the I/O ranges are translated 1:1, so an existing X017 reference continues to function. The main differences are:

• GX Works 2 uses Structured Project with POU tags; GX Works 3 introduces the iQ-R-style label concept.
• FX3U CPU firmware Ver. 2.20 or later is required for full GX Works 3 compatibility.
• The SC-09 cable is still supported; an additional USB adapter for FX series (USB-SC09-FX) is now available for direct USB connection without a separate USB-to-serial converter.