What products are involved?
SRV – Inline Viscosity Meter | SRD – Inline Density & Viscosity Meter | SME Electronics
What is the purpose of this article?
This article explains how to wire and configure the 4-20 mA analog output from a Rheonics sensor's SME electronics into a Mitsubishi MELSEC PLC (FX3U and FX5 series), covering PLC-side wiring, the Rheonics control panel settings, and the PLC-side ladder programming needed to read and scale the signal into an engineering unit (e.g., viscosity or temperature).

TABLE OF CONTENTS

1. Getting Started

This guide uses the MELSEC-F/FX line as the reference platform because it is Mitsubishi's introductory, most cost-effective series and one of the first MELSEC lines still in production. The wiring and scaling principles shown here also apply to other MELSEC lines that use an analog input module in current mode. The example uses the FX3U-4AD expansion module, the most basic analog input expansion available for the FX3U series.

Figure 1: The FX3U-4AD analog input expansion module used.

2. PLC Wiring

2.1 MELSEC Analog input guidelines - terminal layout

Before wiring, identify the terminal layout for the analog input module you are using. Terminal numbering and channel assignment vary by model, so always confirm with the module's own manual before connecting field wiring.

Figure 2: Terminal layout of the FX3U-4AD analog input module.

2.2 Important wiring for sink input

⚠ Important — sink wiring: To operate the module as a sink input, the S/S terminal must be connected to the 24 V terminal on the main PLC unit. Confirming this jumper before powering the loop avoids miswired current-loop faults.
Figure 3: S/S-to-24V jumper required to configure the module for sink input mode operation

2.3 General connection schematic

When connecting the SME electronics' 4-20 mA output, the corresponding analog input channel on the MELSEC module must be configured for current input mode, not voltage mode.

Figure 4: General connection schematic between the SME electronics and the MELSEC analog input channel in current mode.

2.4 Grounding

Follow the module's grounding guidelines for the analog input terminals to keep the 4-20 mA signal free of induced noise (Figure 5). The complete signal wiring, including grounding, should match the diagram in Figure 6.

Figure 5: Grounding guidelines for the analog input terminals.
Figure 6: Complete signal wiring diagram, including grounding, from the SME electronics to the MELSEC PLC.

3. Rheonics control panel settings

→ See: Analog Signal Integration with Micro820 — Section 3, Rheonics control panel settings. The SME electronics' analog output configuration (4-20 mA range, output assignment) is identical regardless of the PLC brand connected, so the same steps apply here.

4. PLC setup

4.1 Mitsubishi software

Mitsubishi's PLC programming software is not free, but a demo version is available. Note that the GX Works3 demo supports the FX5 series but does not support the FX3 line. For FX3U programming, use GX Works2 instead.

Figure 7: Mitsubishi GX Works3 demo software

4.2 Instructions for FX5 series

Requirements: GX Works3, FX5 series CPU, and an analog input block (built-in or expansion).

  1. Create a new Project in GX Works3 for your FX5 CPU model.
  2. Add the analog module. In the Navigation Panel, select the CPU. Right-click on Module information → Add New Module, and add the module matching your model name.
Figure 8: Adding the analog module in the GX Works3 Navigation Panel.
  1. Open the module. Double-click the newly added module in the Navigation panel to open its configuration.
Figure 9: Opening the module configuration window in GX Works3.
  1. Configure the channel settings under Basic Settings.
ItemSetting nameValue
ChannelCH1CH1
A/D Conversion Enable/DisableDisable / Enable SettingEnable
A/D Conversion MethodSet A/D Conversion methodSampling Processing
Range switching functionInput rangeInput Current (4 to 20mA)

Table 1: Basic Settings

Figure 10: Basic Settings screen in GX Works3 showing the A/D conversion configuration.
Figure 11: Setting the input range to Input Current (4 to 20 mA).
  1. Configure Application Settings - Scaling. The FX5 module includes built-in scaling, so the read value can be converted to your engineering unit without additional program blocks. Figure 12 gives an example of scaling the input for viscosity.
Figure 12: Example of scaling the viscosity value in the RCP.
ItemValue
Scaling enable/DisableEnable
Scaling Upper Limit5000
Scaling Lower Limit0

Table 2: Application Settings

Figure 13: Application Settings screen with scaling enabled.
Figure 14: Scaling function reference
  1. Finding the register (special device) for the input channel. Use the intelligent function module reference or the module's manual to identify the assigned SM and SD registers for the channel.
Figure 15: Intelligent function module reference listing the SM and SD special registers.
Figure 16: Locating the SM register assigned to the analog input channel.
Figure 17: Locating the SD register that holds the scaled analog value.
  1. Move the register value into a data register. Use the SM6300 special memory bit as the enable condition. Then use a MOV instruction to move the SM6030 scaled value into a data register, e.g., D1.
Figure 18: Ladder logic using a MOV instruction to move the scaled value into data register D1.

Further information

PLC Integration

4-20 mA & RCP configuration

Products

References

  • [1] Mitsubishi Electric — FX3S/FX3G/FX3GC/FX3U/FX3UC Series User's Manual, Analog Control Edition: PDF
  • [2] Mitsubishi Electric — FX3U Series User's Manual, Hardware Edition: PDF
  • [3] Mitsubishi Electric — FX PLC Main Unit product page: Link
  • [4] Mitsubishi Electric — FX Analog Expansions product page: Link