Sn_TX_RR (Socket n TX Read Pointer Register)
This register shows the address that transmission is finished at the TX Memory. With the SEND command of Socket n Command Register, it transmits data from current Sn_TX_RR to Sn_TX_WR and automatically changes after transmission is finished. Therefore, after transmission is finished, Sn_TX_RR and Sn_TX_WR will have same value. When reading this register, user should read upper byte (0x0422, 0x0522, 0x0622, 0x0722) first and lower byte
(0x0423, 0x0523, 0x0623, 0x0723) later to get the correct value.