The FX3U-4AD is Mitsubishi Electric’s 4-channel analog input special function module for the MELSEC FX3U compact PLC series — providing 4 independent analog input channels with 12-bit resolution (4,096 counts) for voltage (0–10 VDC, ±10 VDC) and current (4–20 mA, 0–20 mA) measurement, enabling the FX3U base unit to acquire real-world analog process signals from temperature transmitters, pressure sensors, flow meters, position transducers, and other 4–20 mA or 0–10 VDC signal sources for PLC-based process monitoring and control. Atlantech Drives holds stock of the FX3U-4AD. Contact us for fast worldwide delivery and competitive pricing.
What Is the FX3U-4AD?
The FX3U-4AD is a MELSEC FX3U series special function module providing 4 analog input channels — the standard analog input module for FX3U PLC systems requiring multi-channel analog signal acquisition from process sensors and transducers. The module connects to the FX3U base unit’s right-side extension bus and occupies one special function module slot in the FX3U extension chain, providing 4 independently configurable analog input channels whose measured values are read by the FX3U CPU program using FROM instructions (or auto-refresh via GX Works2 parameter settings). Each of the 4 channels can be individually configured for voltage input (0–10 VDC, ±10 VDC) or current input (4–20 mA, 0–20 mA) by the wiring and parameter settings — the channel input type is set via the FX3U-4AD buffer memory parameter (BFM#0 input mode selection) written by a TO instruction from the FX3U program. The FX3U-4AD’s 12-bit conversion resolution (4,096 counts over the full input range) provides 2.44 mV resolution on the 0–10 VDC range and 3.9 µA resolution on the 4–20 mA range — adequate for the majority of industrial process measurement requirements including temperature (with 4–20 mA transmitters), pressure, flow, level, and position measurement. The module samples all 4 channels sequentially with a conversion cycle time of approximately 15 ms per channel (60 ms total for all 4 channels) — sufficient update rate for slow process variables but not adequate for rapidly changing signals requiring faster analog acquisition.
Key Technical Specifications
- Model: FX3U-4AD
- Analog Input Channels: 4
- Input Signal Types: 0–10 VDC, ±10 VDC, 4–20 mA, 0–20 mA (per channel, selectable)
- Resolution: 12-bit (4,096 counts)
- Conversion Accuracy: ±0.5% of full scale (at 25°C)
- Conversion Time: Approx. 15 ms per channel (60 ms total for 4 channels)
- Input Impedance: Approx. 200 kΩ (voltage), approx. 250 Ω (current)
- Isolation: Between channels and FX3U internal bus (optical isolation)
- Extension Bus Occupation: 8 I/O points (counts as special function module)
- Power Supply: From FX3U base unit internal bus (no external supply required)
- Compatible Base Units: FX3U, FX3UC (via adapter)
- Operating Temperature: 0°C to 55°C
- Dimensions (W × H × D): 40 mm × 90 mm × 87 mm
- Weight: Approx. 0.17 kg
Architecture & Design Overview
The FX3U-4AD implements a successive approximation analog-to-digital converter (SAR ADC) architecture with 12-bit resolution — the SAR ADC compares the input voltage to an internal reference using a binary search algorithm, completing one conversion per channel in approximately 15 ms. The 4 channels are converted sequentially (not simultaneously), so all 4 channel values are updated every 60 ms in normal operation. The module’s analog input terminals are optically isolated from the FX3U internal bus — field sensor ground loops and common-mode voltage differences between sensor and PLC cannot affect the FX3U program or other modules, providing robust isolation for sensors installed at remote locations with different earth potential. The FX3U-4AD communicates with the FX3U CPU via the special function module buffer memory (BFM — Buffer Function Module memory) — a block of 16-bit registers in the module that the FX3U CPU reads (FROM instruction) and writes (TO instruction) to exchange configuration data and measured values. BFM#0 configures each channel’s input type (voltage or current), BFM#1–BFM#4 hold the current converted digital values for channels 1–4, and BFM#5–BFM#24 provide averaging, scaling, and error status registers. For GX Works2 users, the FX3U-4AD auto-refresh parameter (in the special function module parameter settings) automatically transfers BFM values to/from FX3U device addresses each scan cycle without explicit FROM/TO instructions in the ladder program — simplifying analog input programming for engineers unfamiliar with buffer memory instruction syntax. The module’s built-in averaging function (configurable from 1 to 4,096 samples per displayed value) reduces measurement noise from electrically noisy environments — 4–20 mA signals from transmitters in VFD-rich panels can carry switching noise superimposed on the DC signal, and the FX3U-4AD’s averaging effectively filters this noise without requiring external filtering hardware.
Frequently Asked Questions
Q: How does the FX3U-4AD read analog values in the FX3U ladder program using FROM instructions?
A: The FROM instruction reads data from the FX3U-4AD buffer memory into FX3U data registers. The instruction syntax is FROM K1 K1 D100 K1 — where K1 is the module position in the extension chain (first special function module = 1), K1 is the BFM number to read (BFM#1 = channel 1 digital value), D100 is the destination data register in the FX3U, and K1 is the number of registers to read. To read all 4 channel values simultaneously: FROM K1 K1 D100 K4 — this reads BFM#1 through BFM#4 (channels 1–4) into D100 through D103 in one instruction. The digital value in D100 represents the analog input linearly scaled to the configured input range (0–4,095 counts for 0–10 VDC or 4–20 mA).
Q: Can the FX3U-4AD measure signals from 2-wire 4–20 mA transmitters (loop-powered)?
A: Yes, with an external 24 VDC loop power supply. 2-wire 4–20 mA transmitters require a loop power supply in series with the transmitter and the FX3U-4AD’s current input terminal. Connect the 24 VDC supply positive to the transmitter’s positive terminal, the transmitter’s negative terminal to the FX3U-4AD’s channel input (I+) terminal, and the FX3U-4AD’s current input common (COM) terminal to the 24 VDC supply negative. The FX3U-4AD’s 250 Ω internal current input resistor provides the voltage drop for current measurement while the loop supply powers the 2-wire transmitter.
Q: What is the maximum number of FX3U-4AD modules that can be connected to a single FX3U base unit?
A: The FX3U base unit supports up to 8 special function modules in the right-side extension chain — a maximum of 8 FX3U-4AD modules providing 32 analog input channels total. In practice, the FX3U-4AD’s extension bus current consumption and the FX3U base unit’s internal bus current capacity may limit the practical number below 8 before the 8-module limit is reached — calculate the total bus current consumption of all extension modules and verify it is within the FX3U base unit’s supply capacity before specifying the maximum module count.
Do you need more references from this brand? Explore our full catalog of Mitsubishi here: https://atlantechdrives.com/manufacturers/mitsubishi/
