Understanding Pulse Train Limits
Many compact PLCs control servo drives via pulse-train outputs, but each PLC has a maximum pulse frequency. For example, an Omron CP1H typically supports up to 100 kHz per axis, while a Mitsubishi FX5U can reach 200 kHz on specific outputs. If your servo requires 500,000 pulses per revolution for fine positioning, exceeding the PLC’s output limit will cause missed steps or positioning failure.

Calculating Your Required Frequency
Determine the servo’s encoder resolution and your target speed. A Panasonic MINAS A6 servo with a 23-bit absolute encoder requires a high pulse count for full resolution. If you need 3000 RPM with a 10,000 pulse/rev setting, the required frequency is 500 kHz—far beyond most built-in PLC outputs. In this case, you must either reduce resolution via electronic gear ratio or select a PLC with high-speed motion modules.

Practical Workarounds
Use the servo drive’s electronic gear function to reduce the command pulse count. For Siemens S7-1200 with V90 drives, PROFINET communication eliminates pulse-train limitations entirely. If you must use pulse control, verify the PLC’s output specification against the drive’s maximum input frequency before purchasing. Mismatching here is one of the most common causes of “motor runs but position is wrong” complaints.

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