It´s master-slave comunication over uart in one wire configuration. My arduino is the master, so I send a modbus request, in this case are the first 8 bytes of the received frame. Because in this configuration the Rx and Tx are together with a little circuitery attached, when you transmit data, you receive this data first and then the response data from the slave component.
If I send:
0x01 (ID)
0x03 (read)
0x88 and 0xB8 (coil)
0x00 and 0x01 (1 register request)
0x2F and 8F (CRC16 RTU checksum)
I get this response:
First, i receive the same that I sent: 0x01 0x03 0x88 0xB8 0x01 0x2F 0x8F
and then I receive the answer from the slave:
0x01 (ID)
0x03(read)
0x02(number of bytes of the answer)
0x04 and 0xE2 (answer)
0x3A and 0xCD (checksum, CRC16)