All transactions, read or write, are initiated by the MDIO master. All MDIO slave devices, when addressed, must respond. MDIO transactions take the form of an MDIO frame, containing fields for transaction type, address and data. This MDIO frame is transferred across the MDIO wire synchronously to MDC. The abbreviations used in this section are explained in the following table.
| Abbreviation | Term |
|---|---|
| PRE | Preamble |
| ST | Start of frame |
| OP | Operation code |
| PHYAD | Physical address |
| REGAD | Register address |
| TA | Turnaround |