Once per MOST Network frame, the MediaLB Controller generates a unique FRAMESYNC pattern on the MLBS line. For all Devices on the bus, the end of the FRAMESYNC pattern defines the byte boundary and the channel boundary for the MLBS and MLBD lines.
Each four-byte wide block (quadlet) in a 3-pin MediaLB frame is defined as a physical channel. Physical channels can be grouped into multiple quadlets (which do not have to be consecutive) to form a logical channel. The MediaLB Controller handles channel arbitration, allocates channel bandwidth for MediaLB Devices, and manages the unique ChannelAddresses for referencing logical channels.
The MediaLB Controller initiates communication with MediaLB Devices by sending an assigned ChannelAddress on MLBS in each logical channel. This ChannelAddress indicates which MediaLB Device will transmit data and which MediaLB Devices will receive data in the following logical channel.
One physical channel after the ChannelAddress is sent on MLBS, the transmitting MediaLB Device associated with that ChannelAddress outputs a command byte (Command) on MLBS and respective data (Data) on MLBD, concurrently. The Command byte contains information about the data simultaneously being transmitted. The MediaLB Device receiving the data outputs a status byte (RxStatus) on MLBS after the transmitting Device sends the Command byte. This status response can indicate that the Device is ready to receive the data, or that the receiving Device is busy (e.g. cannot receive the data at present). Since synchronous stream data is sent in a broadcast fashion, Devices receiving synchronous data can never return a busy status response. In this situation, the RxStatus byte must not be actively driven onto the MLBS line by Devices receiving synchronous data.
The ChannelAddresses output by the Controller for each logical channel are used in normal data transport and can be statically or dynamically assigned. To support dynamic configuration of MediaLB Devices, a unique DeviceAddress must be assigned to all MediaLB Devices before startup. DeviceAddresses allow the External Host Controller (EHC) and MediaLB Controller to dynamically determine which Devices exist on the bus. At the request of a MediaLB Device (e.g. EHC), the Controller scans for DeviceAddresses in the System Channel. Once a Device is detected, a ChannelAddress for each logical channel can be assigned.
The DeviceAddress, ChannelAddress, Command, and RxStatus structures are described in the Link Layer section.