Hey, that's a classic challenge when working with legacy A-series PLCs! The 10ms scan time limitation is definitely a bottleneck for sub-millisecond position control. Here's what I'd recommend based on industry practices:

The most reliable approach is to use a dedicated high-speed pulse output module or external motion controller. The Mitsubishi A-series can work with modules like the AD75 positioning modules that handle pulse generation independently from the main PLC scan. These modules generate pulses directly to the servo drives using their own dedicated processors, bypassing the 10ms scan limitation entirely.

Another solid option is to implement interrupt-driven routines. Configure specific inputs as interrupt triggers that can execute custom functions immediately when activated, regardless of where the main scan is. This gives you near-real-time response for critical positioning events.

For modern servo drives, you'll want to use the PLC's high-speed pulse output functions (like DDRVI and DPLSY commands) and let the servo drive handle the actual position loop control. The PLC just sends position commands, and the drive's internal processor maintains the sub-millisecond accuracy.

Also consider optimizing your ladder logic - move non-critical operations to slower scan sections and keep positioning logic as lean as possible. This won't solve the fundamental limitation but can help minimize scan time variations.

The key is recognizing that the PLC should handle sequence logic and motion commands, while the modern servo drive executes the high-frequency control. This separation of responsibilities is what makes retrofits like this workable!