Question: Can I use more than the 8 inputs 0 to 7 on the MC4N/MC6N as registration inputs, using REGIST(21 ... ) ?
Suggestion & resolution:
The hardware that does the registration is only there on inputs 0 to 7. You can do a "soft" regist that is polled at the servo_period time by using REGIST(20, input_number, 4). It's not a true high speed capture but might suffice.
Alternatively on an EtherCAT system, the servodrives may support Touch Probe and that can be used along with REGIST(20, .. ) as well. The touch probe objects must be included in the PDO.