what happens if you send a value that is 10 times the value you want?
The problem is likely byte swapping: a difference in byte order between the sending and receiving modbus nodes. The bits in a 16-bit, 2-byte integer with a value of 10 on the Modbus source node are
0000 0000 0000 1010
The blue bits compose the Most Significant Byte (MSB); the red bits compose the Least Significant Byte (LSB). The integer is sent as two bytes, in some order, either MSB then LSB or LSB then MSB, by the Modbus source node and received as two bytes by the Modbos destination node. If the destination node interprets the order of the bytes received differently than the source node, then it arranges the bits in a 16-bit, 2-byte integer like this:
0000 1010 0000 0000
where the red bits are now the MSB and the blue bits are now the LSB. The value of the integer evaluated from those bits is 2560 i.e. 256 (= [1 0000 000]) times the original LSB or 10.
N.B. this is a special case: the factor of 256 only applies when the original MSB is 0. More generally, the destination value will be MSB + 256 * LSB, which reduces to 256 * LSB when MSB is 0, although there is also the sign bit to consider since the integer is probably a signed 16-bit integer.
If you are never going to send a value greater than 127 from the Micro830 to the motor, then you could simply multiply it by 256 before sending it and move on. The more general case, to handle any possible value, including negative values, is a bit more involved but not too difficult; I think the BSL and BSR instructions might be the way to go.
