Modbus RTU is a simple, register-based poll-and-answer protocol over RS-485. It is ideal for reading and writing parameters and for simple commands; it was not designed for synchronised motion.
Commissioning 01
RS-485 is differential over a twisted pair and needs a line topology: connect the devices one after another like a chain, put 120 Ω termination at both ends and avoid star connections. A/B naming is reversed between some manufacturers; if nothing answers, the first thing to try is swapping the two wires.
Troubleshooting 02
Manuals often number holding registers from 40001 ("register number"), while the protocol addresses them from 0 ("address"): register 40001 is protocol address 0. Check which of the two your master software expects; if they differ, everything shifts by one. For 32-bit values the word order must also match.
Troubleshooting 03
No answer means the slave never understood the request: the slave address, baud rate, parity and stop bits must match the master exactly. If those are right, swap A and B, check the termination and make sure the master's response timeout is longer than the slave's reply time.
Basics 04
It is not recommended. Modbus RTU is poll-and-answer: the master queries devices one after another, so as the axis count grows each axis waits longer for its command and the axes never receive it at the same moment. It suits giving independent axes a target and a speed, reading parameters and monitoring; use EtherCAT for interpolation and synchronised motion.
EtherCATDrive code, motor, controller and the alarm code you see: with those we reach the right answer faster.
