Customized MITSUBISHI PLC Programmable Logic Controller FX3U-4AD-ADP

Mitsubishi FX3U-4AD-ADP | MELSEC-F FX3U Series 4-Channel Analog Input Adapter — 0–10VDC / 4–20mA, 12-Bit Voltage / 11-Bit Current, Programless, 24VDC, 27.4×90×75mm

Overview

The Mitsubishi FX3U-4AD-ADP is a four-channel analog input adapter for the MELSEC FX3U family of compact PLCs. The "ADP" designation distinguishes this module from the standard "special function unit" type — where a special function unit connects to the right side of the FX3U main unit on the DIN rail and counts toward the PLC's expansion unit budget, an ADP adapter attaches physically to the left side of the FX3U main unit and operates through a direct connection that consumes no I/O points and no special function unit slots.

This attachment method is what makes the FX3U-4AD-ADP particularly attractive for compact machine control applications where the FX3U is already close to its expansion limit, or where adding analog capability without any I/O point consumption is a system design priority.

A machine using an FX3U with a full complement of digital expansion modules can still add the FX3U-4AD-ADP to gain four analog inputs without any redesign of the I/O allocation or the special function unit chain.

The programless operation takes this convenience further. In a standard special function unit, the PLC programmer writes dedicated FROM/TO instructions (or buffer memory commands) to read analog values from the module and write them into the main program's data registers.

The FX3U-4AD-ADP bypasses this requirement entirely: once installed and configured, the adapter automatically transfers its four analog readings into pre-defined special data registers (D8280 through D8299 for the first adapter, with subsequent adapters in defined register ranges).

The PLC program simply reads these registers with standard MOV or arithmetic instructions — no dedicated analog communication instructions needed.

Key Specifications

Per-Channel Mode Selection — Voltage or Current

The FX3U-4AD-ADP handles both voltage and current signals, but the selection is per-channel — not per module. Each of the four channels can independently be configured for voltage (0–10V DC) or current (4–20mA), allowing a single adapter to simultaneously accept signals from both voltage-output and current-output transmitters in the same machine.

The configuration is done through the PLC program, writing to the appropriate special data registers that control the FX3U-4AD-ADP's operating mode. Once set, the mode remains in effect across power cycles as long as the setting is written in the PLC's initial processing.

For machines with mixed sensor types — some sensors outputting 0–10V (voltage-type proximity amplifiers, some temperature transmitters) and others outputting 4–20mA (most industrial loop-powered transmitters) — this per-channel flexibility means one FX3U-4AD-ADP covers the complete analog input requirement without needing separate modules for each signal type.

12-Bit Voltage Resolution — Measurement Precision

The 12-bit analog-to-digital conversion for voltage inputs divides the 0–10V range into 4096 discrete steps, giving a voltage resolution of approximately 2.5mV per step.

For most process measurement applications — temperature from a PT100 transmitter, pressure from a 0–10V pressure transducer, position from a linear potentiometer — this resolution is more than adequate.

A 0–10V pressure transmitter measuring 0–10 bar with 12-bit conversion resolves the pressure to approximately 2.4mbar, which is finer than the accuracy of most pressure transmitters themselves.

The current input resolution at 11-bit is slightly coarser — dividing the 4–20mA span into 2048 steps, corresponding to approximately 7.8µA per step.

Over a typical 4–20mA span representing a process range of 0–100%, this is equivalent to a resolution of about 0.05% of full scale, again more precise than the ±0.5–1.0% accuracy specification of the adapter itself.

In practice, measurement uncertainty is determined by the accuracy specification rather than the resolution for normal industrial applications.

Installation and Wiring

The FX3U-4AD-ADP connects to the left side of the FX3U main unit through a dedicated adapter connector — the same mechanical interface used by communication adapters (RS-232C, RS-485, USB) and other special adapters in the FX3U left-side expansion range. Up to four adapter units can be connected in this chain, with the adapters numbered from left to right (No. 1 closest to the main unit, No. 4 furthest left).

Field wiring connects through the screw-clamp terminals on the FX3U-4AD-ADP's terminal block.

Each channel has designated voltage input terminals (VIN+ and COM) and current input terminals (IIN and COM). 

The 24V DC external power supply required by the adapter connects through designated power terminals.

This 24V supply is separate from the FX3U's own power supply, though in practice the same 24V DC panel supply typically feeds both.

FAQ

Q1: Does the FX3U-4AD-ADP work with the older FX2N or FX1N PLC series?

No. The FX3U-4AD-ADP is mechanically and electrically designed for the FX3U adapter connection system, which is specific to the FX3U, FX3UC, FX3G, FX3GE, and FX3S platforms. The FX2N and FX1N series use a different expansion system (right-side extension units/blocks) and do not have the left-side adapter connector needed for ADP-type modules.

For analog input on FX2N systems, the FX2N-4AD (special function unit) or FX2N-4AD-TC (thermocouple) modules are the appropriate choices.

Q2: How many FX3U-4AD-ADP units can be connected to one FX3U main unit?

Up to four ADP adapter units in total can be connected to the left side of an FX3U main unit (combining all adapter types — analog input, analog output, communication, etc.).

Within this limit, multiple FX3U-4AD-ADP units can be used — for example, two FX3U-4AD-ADP adapters provide eight analog input channels while consuming zero I/O points. 

If eight or more analog channels are needed on a single FX3U main unit, a combination of ADP adapters and special function units (FX3U-4AD or FX3U-8AD) may be required.

Q3: Can the FX3U-4AD-ADP handle negative voltage inputs (below 0V) or signals outside the 0–10V / 4–20mA ranges?

The FX3U-4AD-ADP is designed for unipolar signals in the 0–10V and 4–20mA ranges. Negative voltages or signals below 4mA (for current mode) are outside the module's specified input range.

For applications requiring bipolar voltage measurement (−10V to +10V) or the full −20mA to +20mA current range, the FX3U-4AD (standard special function unit) supports these wider ranges with 15-bit and 14-bit resolution.

When input signals might fall outside the 4–20mA range (for example, in a sensor fault condition where a transmitter might output 0–3.6mA for fault signalling), input limiting and fault detection logic should be implemented in the PLC program.

Q4: The specifications list ±0.5% accuracy for voltage and ±1.0% for current. What are the conditions for these figures?

The accuracy figures apply at 25°C ambient temperature with the PLC in its normal operating orientation and with a stable power supply. At temperatures other than 25°C, the accuracy degrades with temperature coefficient — typically a few hundredths of a percent per degree Celsius.

The current input's lower accuracy (±1.0%) compared to voltage (±0.5%) reflects the additional uncertainty introduced by the shunt resistor used for current measurement.

For critical measurements where ±1.0% is insufficient, a current-to-voltage converter upstream of the adapter or a higher-accuracy special function unit (FX3U-4AD) with its greater bit depth should be considered.

Q5: Is special programming required to read the analog values from the FX3U-4AD-ADP in the PLC ladder program?

No dedicated analog programming instructions are needed. The FX3U-4AD-ADP's programless operation means the digital conversion values are automatically written to special data registers (D8280–D8283 for channel 1–4 of the first adapter, with subsequent adapters using defined register offsets).

The PLC ladder program reads these registers using standard data transfer instructions (MOV, arithmetic operations, comparison contacts) as if they were any other data register in the FX3U's memory.

The mode configuration (voltage vs current per channel) is set by writing to the corresponding configuration registers at program startup.