A HLDC Mode is a control state for HLDC which decides the interactions between primary stations and secondary stations.
| Mode | Primary and Secondary Role | Transmission Mode |
|---|---|---|
| Normal Response Mode (NRM) | Primary controls all communication | Half-Duplex |
| Asynchronous Response Mode (ARM) | Secondary may transmit at any time | Duplex |
| Asynchronous Balanced Mode (ABM) | Both Primary or Secondary may transmit at any moment | Duplex |