FX5U Transistor Output Wiring to VFD STF Terminal Guide

ByHANI_AUTOMATION HANI

Question: I have a Mitsubishi FX5U-32MT/ES PLC with sinking (NPN) transistor outputs. I want to directly control the forward and reverse rotation of a D700 series VFD using the Y outputs. I know how to wire relay outputs, but I’ve never used transistor outputs for this. Can anyone guide me on the correct wiring method?

Understanding the FX5U Transistor Output Type

The FX5U-32MT/ES is a transistor output PLC with sinking (NPN) outputs. This means the output terminal (Y) switches to the common negative (0V) when activated. The common terminal for the outputs is typically labeled COM0, COM1, etc., and must be connected to the 0V of the external power supply. The load (in this case, the VFD input terminal) is connected between the positive supply (+24V) and the Y output. When the output turns on, current flows from +24V through the load, into the Y terminal, and down to 0V via the internal transistor.

⚠️ Important: Always check the polarity and voltage ratings. The FX5U transistor outputs are typically rated for 5-30V DC, 0.5A per point. The D700 VFD control inputs (STF, STR, etc.) are designed for 24V DC logic. Never apply AC voltage to these terminals.

D700 VFD Control Terminal Overview

The Mitsubishi D700 series VFD has dedicated control terminals for forward and reverse commands:

  • STF – Forward rotation start (contact closed = forward run)
  • STR – Reverse rotation start (contact closed = reverse run)
  • SD – Common terminal for control inputs (sink logic common)
  • PC – 24V DC power output for control logic (source)

The D700 can be configured for either sink or source logic. In the default sink logic mode (factory setting), the SD terminal is the common for the digital inputs. To activate an input, you connect it to the PC terminal (24V) through a switch or PLC output. This matches perfectly with the FX5U sinking output.

Step-by-Step Wiring Diagram

Here is the recommended wiring for forward/reverse control using two FX5U outputs (e.g., Y0 and Y1):

FX5U Terminal Wire Color (suggested) D700 Terminal Notes
Y0 (Forward) Red STF Forward run command
Y1 (Reverse) Blue STR Reverse run command
COM0 (common for Y0-Y3) Black SD (common) Connect to 0V of external 24V supply
External 24V DC + Brown PC Provides 24V for inputs
External 24V DC – Black SD (common) Shared with COM0

Power Supply Note: You can use the VFD’s internal 24V supply (terminal PC) to power the control circuit, but ensure the total current does not exceed the VFD’s rating (typically 100mA). If you have many outputs or other devices, use an external 24V DC power supply. Connect the external supply’s +24V to PC and 0V to SD, and also connect the PLC’s COM0 to the same 0V.

PLC Programming Considerations

In your ladder logic, you must ensure that Y0 and Y1 are never on simultaneously, as this would command both forward and reverse at the same time, potentially causing a fault or damage. Use interlocks:

--[ X0 ]-------------------( Y0 )  // Forward start
--[ X1 ]-------------------( Y1 )  // Reverse start

// Interlock logic
--[ Y0 ]----[/ Y1 ]--------( Y0 )  // Y0 only if Y1 is off
--[ Y1 ]----[/ Y0 ]--------( Y1 )  // Y1 only if Y0 is off

Also, set the VFD parameters for external terminal control:

  • Pr.79 = 2 (External operation mode, fixed)
  • Pr.178 = 60 (STF terminal function, default is forward)
  • Pr.179 = 61 (STR terminal function, default is reverse)

Common Mistakes and Troubleshooting

Symptom Possible Cause Solution
VFD does not start Missing 24V supply, incorrect common wiring, or VFD not in external mode Check voltage between PC and SD (should be ~24V). Verify Pr.79=2. Ensure COM0 is connected to SD.
Output LED on PLC lights but VFD doesn’t respond Wiring polarity reversed or VFD input logic set to source Confirm STF/STR receive 24V relative to SD when output is on. Check Pr.185-189 for input polarity.
Both forward and reverse active at same time Missing interlock in PLC program Add normally closed contacts of opposite direction in ladder.

Safety Recommendations

  • Always install an emergency stop circuit that cuts power to the VFD or disables the run command independently of the PLC.
  • Use shielded twisted-pair cable for control wiring to reduce noise interference.
  • Separate control wiring from power wiring by at least 10 cm.
  • Verify all connections with a multimeter before applying power.

✅ Summary: Wiring a sinking transistor output PLC to a VFD is straightforward once you understand the current flow. The key is to connect the PLC’s common (COM) to the VFD’s common (SD), and the PLC outputs (Y) to the VFD inputs (STF/STR). The 24V supply positive goes to the VFD’s PC terminal. Always double-check your wiring and use proper interlocks in the program.

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