MASSO supports 0–10V signals, as well as clockwise and counter-clockwise signals, to directly control VFDs.

Follow the 7 steps shown in the video to connect your VFD.

For troubleshooting a VFD or spindle problem, refer to the Troubleshooting guide.

For additional VFD wiring diagrams, refer to the MASSO Forum VFD wiring section.

Connecting a VFD Forward/Reverse Input Using an Opto Output

Many VFDs use digital input terminals, such as:

• STF = Forward run
• STR = Reverse run
• SD / COM = input common
• PC / +24V = internal 24V supply

Different VFDs can use different input logic, so always check the wiring before connecting.

Step 1 — Check the VFD input type

Look in the VFD manual for one of these terms:

• Sink logic
• NPN input
• Source logic
• PNP input

If the manual shows Sink / NPN logic, the input turns ON when the input terminal is connected to the common.

Example:

• STF - SD = Forward ON
• STR - SD = Reverse ON

If the manual shows Source / PNP logic, the input turns ON when +24V is applied to the input terminal.

Example:

• PC / +24V - STF = Forward ON
• PC / +24V - STR = Reverse ON

Step 2 — Test with a multimeter

Power the VFD, but keep the machine safe and motor disabled if possible.

Measure DC voltage between:

• STF and SD / COM

If you measure around +24V, the input is most likely a sink / NPN input.

Use this wiring:

• Opto collector - STF or STR
• Opto emitter   - SD / COM

If you measure around 0V, then measure between:

• PC / +24V and STF

If you measure around +24V, the input is most likely a source / PNP input.

Use this wiring:

• Opto collector - PC / +24V
• Opto emitter   - STF or STR

Step 3 — Manual test before wiring the opto

Before connecting the opto, test the VFD input with a short wire.

For sink / NPN input:

• Short STF to SD / COM = Forward ON
• Short STR to SD / COM = Reverse ON

For source / PNP input:

• Short PC / +24V to STF = Forward ON
• Short PC / +24V to STR = Reverse ON

Only short terminals that the VFD manual shows as valid control input terminals.

Do not short PC / +24V to SD / COM unless the VFD manual specifically says this is allowed.

Step 4 — PC817 / opto wiring

A PC817 output is a transistor, not a relay contact. Polarity matters.

General rule:

• Collector = more positive side
• Emitter   = more negative side

For sink / NPN VFD inputs

• VFD STF / STR - PC817 collector
• VFD SD / COM - PC817 emitter

When the opto turns ON, it connects the input to the common.

For source / PNP VFD inputs

• VFD PC / +24V - PC817 collector
• VFD STF / STR - PC817 emitter

When the opto turns ON, it applies +24V to the input.

Step 5 — Forward and reverse safety

Use separate outputs for Forward and Reverse.

• Output 1 - Forward input
• Output 2 - Reverse input

Never turn ON Forward and Reverse at the same time.

Recommended control sequence when changing direction:

• Forward OFF
• Wait 200–500 ms
• Reverse ON

or

• Reverse OFF
• Wait 200–500 ms
• Forward ON

Important note

A PC817 is not a true dry contact. If the collector and emitter are swapped, some VFDs may still run because a small leakage current can trigger the VFD input.

For best universal compatibility, use a relay contact or PhotoMOS output when the VFD input type is unknown.

• Relay contact = behaves like a real switch
• PC817 output  = polarity-sensitive transistor