The FX2N-8AD is Mitsubishi Electric’s 8-channel high-accuracy analogue input special function module for the MELSEC FX2N compact PLC series, providing eight independent channels of 12-bit analogue-to-digital conversion with software-configurable input ranges and improved ±0.5% accuracy — delivering the highest channel count and best measurement accuracy available in the FX2N analogue input module range for large-scale multi-variable process monitoring and high-precision measurement applications. As the premium analogue input module for the FX2N platform, the FX2N-8AD is specified for installations requiring simultaneous monitoring of eight process variables with better accuracy than the ±1% specification of the FX2N-4AD, such as multi-zone temperature monitoring in thermal processing equipment, multi-axis load cell systems, and comprehensive environmental monitoring installations. The FX2N-8AD is available through Atlantech Drives — quote within 24-48 hours, worldwide express shipping, 12-month warranty.
What Is the FX2N-8AD?
The FX2N-8AD is a MELSEC FX2N series special function module providing eight channels of analogue input with 12-bit resolution and software-configurable input ranges, improved to ±0.5% accuracy at 25°C compared to the ±1% specification of the FX2N-2AD and FX2N-4AD. The module installs in the FX2N extension bus and communicates via FROM/TO buffer memory instructions — channel range settings are written via TO instructions at initialisation, and converted values are read via FROM instructions each scan cycle. Each of the eight channels can be independently configured for voltage (0–5 VDC, 0–10 VDC, -10 to +10 VDC) or current (4–20 mA, 0–20 mA) input via buffer memory range configuration registers, enabling the eight channels to measure signals of different types simultaneously from the same module. The FX2N-8AD’s doubled channel count compared to the FX2N-4AD enables systems requiring comprehensive process monitoring — eight temperature zones, eight pressure points, eight flow streams — to use a single special function module slot rather than two FX2N-4AD modules, saving an extension bus position for other special function modules.
Key Technical Specifications
- Model: FX2N-8AD
- Analogue Input Channels: 8
- Resolution: 12-bit (4,095 divisions unipolar)
- Input Range (Voltage): 0–5 VDC, 0–10 VDC, -10 to +10 VDC (software configurable per channel)
- Input Range (Current): 4–20 mA, 0–20 mA (software configurable per channel)
- Input Accuracy: ±0.5% of full scale at 25°C (±1.0% over full temperature range)
- Conversion Speed: Approx. 2.5 ms per channel (20 ms for all 8 channels)
- Averaging Function: Configurable 1–4,096 sample averaging per channel
- Extension Bus Occupation: 8 I/O points
- External Power Supply: 24 VDC ±10%, 80 mA (required)
- Operating Temperature: 0°C to 55°C
- Compatible FX2N base units: All FX2N models
- Weight: Approx. 0.20 kg
Typical Industrial Applications
The FX2N-8AD is deployed in applications where eight simultaneous high-accuracy analogue measurements are required from a single FX2N special function module, saving the extension bus slot and cost of a second FX2N-4AD. In multi-zone industrial oven and kiln temperature monitoring, the FX2N-8AD measures eight thermocouple transmitter outputs (4–20 mA from external transmitters) across eight temperature zones simultaneously, providing the FX2N PID program with all zone process variables in a single module read cycle. Pharmaceutical freeze dryer (lyophiliser) systems use the FX2N-8AD to monitor shelf temperature, condenser temperature, chamber pressure (via vacuum transducer), shelf inlet and outlet coolant temperatures, and product probe temperatures — six to eight measurements all from a single FX2N-8AD module. In automotive paint shop ovens, the FX2N-8AD monitors inlet air temperature, outlet air temperature, conveyor zone temperatures, and recirculation fan pressures from a single module, providing the FX2N controller with comprehensive thermal process data for energy management and quality control logging. Multi-axis load monitoring systems for press machines, injection moulding platens, and material testing rigs use the FX2N-8AD’s ±0.5% accuracy to measure strain gauge amplifier outputs from eight load cells simultaneously, enabling the FX2N to compute total load distribution and individual axis force balance for process quality verification. Boiler and heat exchanger monitoring in process plants uses the FX2N-8AD for feed water temperature, steam temperature, flue gas temperature, stack draft pressure, fuel pressure, and combustion air flow measurements from six to eight points in a single module.
Replacement & Upgrade Notes
The FX2N-8AD is a discontinued Mitsubishi product available for maintenance replacement through specialist distributors. For FX2N systems requiring eight analogue inputs where the FX2N-8AD has failed and cannot be sourced, two FX2N-4AD modules can provide equivalent channel coverage (8 channels total) at the cost of an additional extension bus slot and slightly different buffer memory addressing in the FX2N program. When using two FX2N-4AD modules as a replacement for one FX2N-8AD, the ladder program’s FROM/TO instruction module numbers must be updated to reflect the two separate module addresses, and the scan time overhead of two separate module read cycles will be higher than a single FX2N-8AD read cycle. For new system designs or complete platform upgrades, the FX3U platform with FX3U-4AD-ADP left-side adapter modules provides superior analogue input capability — four FX3U-4AD-ADP adapters in the four left-side adapter positions provide 16 analogue input channels at ±0.5% accuracy without consuming any extension bus slots, substantially exceeding the FX2N-8AD’s 8-channel capacity. When migrating FX2N systems using FX2N-8AD modules to FX3U, the FROM/TO buffer memory addressing must be completely rewritten as the FX3U adapter interface uses a different buffer memory structure than the FX2N special function module bus — GX Works2’s FX2N-to-FX3U migration guide provides specific guidance on analogue module address remapping.
Frequently Asked Questions
Q: What is the conversion time for all 8 channels of the FX2N-8AD, and is this adequate for FX2N PID control feedback?
A: All 8 channels complete one conversion cycle in approximately 20 ms (8 channels × 2.5 ms per channel). For FX2N PID control applications with a 500 ms sampling interval — the standard PID instruction sampling period in most FX2N temperature control programs — the 20 ms conversion cycle is entirely adequate, providing fresh process variable data approximately 25 times per PID sampling interval. For faster PID loops requiring sub-50 ms sampling, the 20 ms conversion time consumes a significant portion of the available feedback delay budget and should be considered when designing tight control loops.
Q: Can the FX2N-8AD’s 8 channels be split across two separate FX2N PID control loops using different sampling intervals?
A: Yes. Any subset of the FX2N-8AD’s 8 channels can be assigned to different FX2N PID instructions operating at different sampling intervals. The FX2N PID instruction reads its process variable from a designated D register — assign the FROM instruction output for the relevant channel to the specific D register used by each PID instruction, and configure each PID instruction with its own sampling interval independently. Multiple simultaneous PID loops reading from the same FX2N-8AD module operate correctly as long as each loop reads from its own dedicated channel.
Q: Does the FX2N-8AD provide input disconnection detection for open 4–20 mA loops?
A: Unlike the FX2N-4AD-TC thermocouple sub-variant which detects thermocouple wire break, the standard FX2N-8AD does not provide dedicated hardware disconnection detection for 4–20 mA inputs. An open-circuit 4–20 mA loop results in the module reading 0 mA (0 counts), which the FX2N program can detect by monitoring for converted values below a threshold corresponding to approximately 3 mA (approximately 615 counts for a 4–20 mA range). Implement an under-range alarm in the FX2N ladder program: if any channel’s FROM value is below the 3 mA threshold, set an alarm flag and inhibit the affected PID control loop output to prevent a false control response to the sensor failure reading.
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