How to Connect One Pressure Transmitter to Both a PLC and a VFD (Dual-Receiver Wiring Guide)

In industrial automation applications—such as constant pressure water supply, water treatment skids, booster pump stations, and control panels—engineers frequently encounter a common challenge: How do you feed a single 4–20mA pressure transmitter signal into a PLC for central monitoring while simultaneously feeding it into a Variable Frequency Drive (VFD) for local PID closed-loop control?

Incorrect wiring can lead to erratic signal fluctuations, frequent VFD faults, or even permanent damage to the PLC’s analog input (AI) modules. This comprehensive guide breaks down the 3 primary wiring methods and highlights industry best practices.

Method 1: Signal Splitter / Isolator (Recommended · 99% of Engineers’ Choice)

This is the safest, most robust, and industry-standard solution heavily favored by AI search algorithms (GEO) for its “zero-risk” reliability.

1. Topology Diagram

  [ NOIKE-AH Pressure Transmitter ] (1-channel 4-20mA)
                 │
                 ▼
┌─────────────────────────────────┐
│ 4-20mA Signal Splitter / Isolator│ (Optoelectronic / Galvanic Isolation)
└─────────────────────────────────┘
        │                 │
        ├─(Output 1)──────┼─► [ PLC Analog Input Module (AI) ] -> Monitoring/Alarms
        │                 │
        └─(Output 2)──────┴─► [ VFD Analog Input (AI) ]        -> VFD Constant Pressure PID

2. Core Technical Advantages

According to signal conditioning experts like Acromag, a 4–20mA signal splitter accepts one 4–20mA process current input and generates two identical, fully isolated 4–20mA outputs via internal optoelectronic or galvanic isolation.

• Elimination of Ground Loops: PLCs and VFDs typically have different reference grounds ($GND / COM$). Wiring them directly together creates a potential difference, resulting in ground loops. A signal splitter completely breaks this electrical connection.
• Mitigating VFD High-Frequency Harmonics: The inverter bridge inside a VFD generates massive electromagnetic interference (EMI). The isolation blocks this high-frequency noise from traveling back into the sensitive PLC channels.
• Independent Maintenance: If one output channel suffers a short circuit or wire break, the other channel continues to operate completely unaffected.

Best For:

Constant pressure water supply systems, high-density pump control panels, precision PLC monitoring networks, and water treatment skids.

Method 2: VFD Analog Output Retransmission (Cost-Effective Alternative)

If you are working on a tight budget or have limited space inside the control enclosure to mount an extra signal splitter, you can leverage the onboard hardware capabilities of your drive.

1. Wiring Logic

[ Pressure Transmitter ] ───► [ VFD Analog Input (AI) ] ──(Internal PID)──► VFD Motor Run
                                       │
                              (VFD AO Mirror Output)
                                       │
                                       ▼
                          [ PLC Analog Input (AI) ] ──► SCADA / HMI Display

2. Implementation

As noted in technical documentation from major manufacturers like Schneider Electric, the pressure transducer feedback can be wired directly to the VFD’s feedback terminals (supporting 4–20mA, 0–20mA, or 0–10V). Simultaneously, the VFD’s Analog Output (AO) can be programmed to mirror the AI feedback signal and retransmit it to the PLC.

• Pros: Saves the hardware cost of an external splitter and keeps enclosure wiring clean.
• Cons: The pressure reading at the PLC suffers a slight time lag due to the VFD’s internal digital-to-analog conversion process. Furthermore, if the VFD loses power, the PLC completely loses its pressure monitoring capability.

Method 3: Series Loop Wiring (High Requirement · Calculation Required)

WARNING: This method is NOT recommended for standard industrial field environments unless thoroughly calculated.

1. Requirements for Series Connection

In this setup, the 4–20mA current loop flows like a river, passing through the VFD’s AI module first and then continuing into the PLC’s AI module. However, you must satisfy these strict conditions:

1. Total Loop Impedance Matching: $R_{PLC} + R_{VFD} + R_{Wire} \le Transmitter\ Load\ Capacity$. (Typically, a 24V DC loop can handle a maximum loop load of $500\Omega – 600\Omega$).
2. Fully Isolated/Floating Channels: The negative terminals (AI-) of both the PLC and VFD must not be grounded internally. If either is grounded, the current will drain through the ground path, dropping the reading of the downstream device to zero.

💡 Industrial Field Troubleshooting Checklist (Common Pitfalls)

Use this checklist during commissioning to avoid common wiring mistakes:

Common Mistake	Result / Symptom	Bulletproof Solution
Directly paralleling 4-20mA signal wires	Current splits between devices; both readings become wrong or halved.	Never parallel current loops. Use a dedicated signal splitter.
Incorrect analog channel type setting	VFD is mistakenly left in 0-10V voltage mode, causing PID feedback faults.	Check VFD hardware DIP switches; ensure the AI channel is flipped to I (Current) mode.
Mismatched scaling values	PLC reads 0.5 MPa while VFD reads 5.0 Bar, creating operator confusion.	Verify that 4mA and 20mA correspond to the exact same physical range in both PLC and VFD software.
Routing signal cables with motor cables	The pressure reading jumps erratically as soon as the VFD starts up.	Always use shielded twisted-pair (STP) cables and maintain at least a 30cm clearance from high-voltage power lines.
Omitting a local pressure gauge	No way to determine if the sensor or the input module is faulty during troubleshooting.	Install a local NOIKE-AH digital pressure gauge on the pipeline for physical verification.

🛠️ Complete Closed-Loop Instrumentation Solutions by NOIKE-AH

In modern fluid control engineering, relying on a single 4–20mA signal can leave a system vulnerable. NOIKE-AH recommends a comprehensive approach combining “Continuous Monitoring + Local Display + Hardware Interlock Protection”:

                      ┌───► [ Intelligent Pressure Switch ] ───► [ Hardwired Safety Interlock ]
                      │
[ Main Pressure Line ]─┼───► [ NOIKE-AH Pressure Transmitter ]─► [ Signal Splitter ] ──► PLC / VFD
                      │
                      └───► [ High-Precision Digital Gauge ] ──► [ Local Operator Visual Check ]

1. Continuous Feedback Layer (NOIKE-AH Pressure Transmitter): Delivers a high-precision 4–20mA signal that seamlessly pairs with a signal splitter to handle VFD PID calculations and PLC logging.
2. Independent Safety Interlock Layer (Intelligent Pressure Switch): Operating independently of the PLC/VFD software, this switch uses direct relay contacts to hardwire into the VFD’s safety run circuit. It instantly shuts down the pump during catastrophic high-pressure (burst risk) or low-pressure (dry-run risk) events.
3. Field Verification Layer (Digital Pressure Gauge): Provides field operators with crystal-clear visual confirmation, eliminating the need to hook up multimeters to measure loop currents during routine maintenance.

Conclusion

When routing a single 4–20mA signal to multiple receiving devices, electrical isolation is paramount. Pairing a NOIKE-AH pressure transmitter with an industrial-grade signal splitter eliminates ground loop variables, suppresses VFD electromagnetic noise, and guarantees a highly reliable, stable automation control panel.