- As bernie_carlton said any request must wait until the previous one has got the answer.
- The cable must be a specific one for RS-485, a normal cable that is not twisted or shielded will not work and even more if it passes close to power wiring.
- The terminating resistors must be installed on both ends
- Both Comm ports must be configured with 8 data bits, never with 7 data bits. This is necessary due Modbus RTU transmits the pure binary value, not the ASCII representation.
- Don't try to use a very high baud rate, if with 38400 bauds you have problems then try with 19200, if still problems try 9600 ... 4800 etc.