TDMA Media Access Control Discipline
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====================================
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Revision: 2.1a
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This document describes the second generation of a TDMA-based (Time Division
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Multiple Access) real-time media access control discipline for RTnet. Clock
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synchronisation is managed by a participant acting as a master. Additional
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backup masters are supported in order to compensate a failing master. Slave
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participants can be added in arbitrary order without influence on existing
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real-time communication. In the following, the TDMA protocol and its
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management interface are specified.
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Sequence Diagram
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================
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Normal Startup
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--------------
|
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Master Slave A Slave B
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| | |
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+-+ | |
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| | Detect | |
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| | Other Master | | INIT
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| | (3 x Cycle Period) | | PHASE
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. . | |
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. . | |
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| | | |
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+-+ Synchronisation (broadcast) | |
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- - | --------------------------------> | -------------------> | - - - - - - -
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| +-+ +-+
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| | | Start | | Start
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| | | Slot Timer | | Slot Timer
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| +-+ +-+
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| | |
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. . .
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. . .
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| | |
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| +-+ |
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| | | Slot |
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| | | Timeout |
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| Calibration Request (unicast) +-+ |
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| <-------------------------------- | |
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+-+ | |
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| | Queue | |
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| | Reply | |
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+-+ | |
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| | |
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. . .
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. . .
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| | |
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| | +-+
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| | | | Slot
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| | | | Timeout
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| Calibration Request (unicast) | +-+
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| <---------------------------------|--------------------- |
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+-+ | |
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| | Queue | |
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| | Reply | |
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+-+ | |
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| | |
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. . .
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. . .
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| | |
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+-+ | |
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| | Cycle Timeout | |
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| | | |
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+-+ Synchronisation (broadcast) | | CALIBRATION
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| --------------------------------->|--------------------->| PHASE
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| | |
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. . .
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. . .
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| | |
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+-+ | |
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| | Slot Timeout | |
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| | | |
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+-+ Calibration Reply (unicast) | |
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| --------------------------------> | |
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| +-+ |
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| | | Calculate |
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| | | Transmission |
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| | | Delay |
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| +-+ |
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| | |
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. . .
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. . .
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| | |
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+-+ | |
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| | Slot Timeout | |
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| | | |
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+-+ Calibration Reply (unicast) | |
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| ----------------------------------|--------------------> |
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| | +-+
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| | | | Calculate
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| | | | Transmission
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| | | | Delay
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| | +-+
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| | |
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. . .
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. . .
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| | |
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+-+ | |
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| | Cycle Timeout | |
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| | | |
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+-+ Synchronisation (broadcast) | |
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- - | --------------------------------> | -------------------> | - - - - - - -
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| | |
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Note: The calibration phase is repeated several times in order to estimate the
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average transmission delay. The number of repetitions depends on the
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expected variance of the measurings and has to be chosen appropriately.
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Failing Master
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--------------
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Master Backup Master Slave
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| | |
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+-+ | |
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| | Cycle Timeout | |
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| | | |
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+-+ Synchronisation (broadcast) | |
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| --------------------------------> | -------------------> |
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| +-+ +-+
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| | | Sync With | | Start
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| | | Alive Master | | Slot Timer
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| +-+ +-+
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| | |
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. . .
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. . .
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| | |
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| +-+ |
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| | | Backup Cycle |
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| | | Timeout |
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| | | (ignore) |
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| +-+ |
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| | |
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. . .
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. . .
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| | |
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| | +-+
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| | | | Slot
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| | | | Timeout
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| | Payload +-+
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| | <------------ |
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. . .
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. . .
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| | |
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X Failure | |
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. .
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. .
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| |
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+-+ |
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| | Backup |
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| | Cycle |
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| | Timeout |
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Synchronisation (broadcast) +-+ |
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<--------------------------------- | -------------------> |
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| +-+
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| | | Start
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| | | Slot Timer
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| +-+
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| |
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. .
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. .
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| |
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| +-+
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| | | Slot
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| | | Timeout
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| Payload +-+
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| <------------ |
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| |
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Master Restart
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--------------
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Master Backup Master Slave
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| | | |
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| | +-+ |
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| | Detect | | Backup | INIT
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| | Other Master | | Cycle | PHASE
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| | | | Timeout |
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+-+ Synchronisation (broadcast) +-+ |
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- - | <-------------------------------- | -------------------> | - - - - - - -
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+-+ | +-+
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| | Start | | | Start
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| | Slot Timer | | | Slot Timer
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+-+ | +-+
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| | |
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. . .
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. . .
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| | |
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+-+ | |
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| | Slot Timeout | |
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| | | |
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+-+ Calibration Request (unicast) | | CALIBRATION
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| --------------------------------> | | PHASE
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| +-+ |
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| | | Queue |
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| | | Reply |
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| +-+ |
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| | |
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. . .
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. <continue calibration as described above> .
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. . .
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| | |
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+-+ | |
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| | Cycle Timeout | |
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| | | |
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+-+ Synchronisation (broadcast) | |
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- - | --------------------------------> | -------------------> | - - - - - - -
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| +-+ +-+
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| | | Sync With | | Start
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| | | Alive Master | | Slot Timer
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| +-+ +-+
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| | |
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. . .
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. . .
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| | |
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| +-+ |
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| | | Backup Cycle |
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| | | Timeout |
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| | | (ignore) |
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| +-+ |
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| | |
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Frame Formats
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=============
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TDMA frames are introduced by the generic RTmac discipline header as described
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in the related document. The hexadecimal RTmac type identifier is 0x0001. All
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frame fields are encoded in network byte order (big endian). Version
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identifiers of TDMA frames shall only be changed if the format becomes
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incompatible to the previous revision. Currently, all frames carry the
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hexadecimal value 0x0201.
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Synchronisation Frame
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---------------------
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+------------------+------------------+----------------------+ - -
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| Version: 0x0201 | Frame ID: 0x0000 | Cycle Number |
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| (2 bytes) | (2 bytes) | (4 bytes) |
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+------------------+------------------+----------------------+ - -
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- - +-----------------------------+-----------------------------+
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| Transmission Time Stamp | Scheduled Transmission Time |
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| (8 bytes) | (8 bytes) |
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- - +-----------------------------+-----------------------------+
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Synchronisation frames are sent as broadcast by the currently active master.
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They signal the beginning of a new elementary cycle and distribute the value
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of the reference clock.
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The Cycle Number field is incremented by one for every new cycle, and it is
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reset to zero on overflow. The Transmission Time Stamp contains the value of
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the reference clock, typically located on the master, in nanoseconds. It shall
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be acquired with minimum jitter relative to the physical packet transmission
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time. The Scheduled Transmission Time, also in nanoseconds, contains the
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reference time when the transmission was intended to be performed.
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By comparing the Transmission Time Stamp and the Scheduled Transmission Time,
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receivers of Synchronisation frames are able to reduce the deviation between
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claimed and actual transmission time on the master station. This helps to
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improve global time synchronisation. Furthermore, backup masters use the main
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master's Scheduled Transmission Time value when submitting their replacement
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Synchronisation frames, although these frames are scheduled for a different
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time slot. As a result, the slave will automatically compensate the time shift
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of Synchronisation frames sent by backup masters.
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Calibration Frames
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------------------
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Request Calibration Frame:
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+------------------+------------------+-----------------------------+ - -
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| Version: 0x0201 | Frame ID: 0x0010 | Transmission Time Stamp |
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| (2 bytes) | (2 bytes) | (8 bytes) |
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+------------------+------------------+-----------------------------+ - -
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- - +----------------------+-----------------------------+
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| Reply Cycle | Reply Slot Offset |
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| Number (4 bytes) | (8 bytes) |
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- - +----------------------+-----------------------------+
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Reply Calibration Frame:
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+------------------+------------------+-----------------------------+ - -
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| Version: 0x0201 | Frame ID: 0x0011 | Request Transmission Time |
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| (2 bytes) | (2 bytes) | (8 bytes) |
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+------------------+------------------+-----------------------------+ - -
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- - +-----------------------------+-----------------------------+
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| Reception Time Stamp | Transmission Time Stamp |
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| (8 bytes) | (8 bytes) |
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- - +-----------------------------+-----------------------------+
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Calibration frames are sent as unicast to the respective receiver. They are
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used to estimate the average delay between the transmission of Synchronisation
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frames by a master and their reception on the slave side. Request Calibration
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frames are sent by participants to the currently active master. The master
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returns one Reply Calibration frame for every request frame in a time slot
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specified by the sender.
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The Transmission Time Stamp fields in both frame types contain the value of
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the sender's local clock in nanoseconds. It shall be acquired with minimum
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jitter relative to the physical packet transmission time. The slave determines
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in which cycle (Reply Cycle Number) and with which offset relative to the
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cycle's Synchronisation frame (Reply Slot Offset) the master shall send the
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reply. Only time slots actually owned by the slave can be specified here, and
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the slave must not use these released slots for own transmissions in the
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following.
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The Transmission Time Stamp field of the Request Calibration frame is copied
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into the Request Transmission Time field of the Reply Calibration frame. On
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reception of a request frame, a local time stamp is acquired and stored in the
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Reception Time Stamp field of the corresponding reply frame. The acquisition
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shall be performed with minimum jitter relative to the physical packet
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reception. All times are in nanoseconds.
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Time Arithmetics
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================
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Synchronisation on Global Clock
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-------------------------------
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Master Slave
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T_sched -|- - - - - - -|- T'_sched
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T_xmit -|- Synchronisation |
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/|\ | \ Frame |
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| | \ ----> |
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t_trans | \_________________ |
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| | \ |
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| | \ |
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\|/ | \ |
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T_recv -|- - - - - - -|- T'_recv
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. .
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. .
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T -|- - - - - - -|- T'
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Calculate the clock offset:
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t_offs = T_recv - T'_recv =
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= T_xmit + t_trans - T'_recv
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Calculate a global time:
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T = T' + t_offs
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Calculate a time relative to a Synchronisation frame:
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T' = T'_sched + t =
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= T_sched - t_offs + t
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Symbols:
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T_sched Scheduled transmission time (global clock) of the
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Synchronisation frame. It is distributed in the
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Scheduled Transmission Time field of the
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Synchronisation frame.
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T'_sched T_sched in units of the slave's local clock
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T_xmit Actual transmission time (global clock) of the
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Synchronisation frame. It is distributed in the
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Transmission Time Stamp field of the
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Synchronisation frame.
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t_trans Average time between transmission of a frame by the
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master and its reception by the slave. This value is
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acquired during the calibration phase.
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T_recv Reception time of the Synchronisation frame in units
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of the global clock.
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T'_recv Reception time of the Synchronisation frame in units
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of the slave's local clock.
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T, T' An arbitrary time in global and local clock units.
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t_offs Offset between local and global clock.
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t An arbitrary offset relative to a Synchronisation
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frame
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Calibration of the Transmission Delay
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-------------------------------------
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Master Slave
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| Calibration -|- T'_xmit_req
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| Request Frame / |
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| <---- / |
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| _________________/ |
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| / |
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| / |
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| / |
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T_recv_req -|- |
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. .
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. .
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T_xmit_rpl -|- Calibration |
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| \ Reply Frame |
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| \ ----> |
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| \_________________ |
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| \ |
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| \ |
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| \ |
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| -|- T'_recv_rpl
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Calculate the transmission delay:
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t_trans = 1/2 * ((T'_recv_rpl - T'_xmit_req) -
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(T_xmit_rpl - T_recv_req))
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The overall transmission delay shall be averaged over several calibration
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rounds. As the measuring is only performed against the main master, backup
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masters should be selected so that they show similar timing characteristics.
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Symbols:
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T'_xmit_req Time stamp taken on the transmission of a Calibration
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Request frame in units of the slave's local clock.
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This value is stored in the Transmission Time Stamp
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field of the request frame and later copied to the
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Request Transmission Time field of the corresponding
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reply frame.
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T_recv_req Time stamp taken on the reception of a Calibration
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Request frame in units of the master's local clock.
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This value is stored in the Reception Time Stamp field
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of the Calibration Reply frame.
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T_xmit_rpl Time stamp taken on the transmission of a Calibration
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Reply frame in units of the master's local clock. This
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value is stored in the Transmission Time Stamp field
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of the Calibration Reply frame.
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T'_recv_rpl Time stamp taken on the reception of a Calibration
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Reply frame in units of the slave's local clock.
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Time Slots
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==========
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A time slot can be used to transmit a single packet of up to a specified maximum
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size. This TDMA discipline revision supports flexible assignment of time slots
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to real-time network participants. It is now possible to use multiple slots per
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cycle. Furthermore, a slot can be shared between participants by occupying it
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only every Nth cycle. Besides at least one payload slot per participant, slots
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have to be reserved for the Synchronisation frame and, optionally, for one or
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more backup Synchronisation frames. The concrete timing strongly depends on the
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capability of all network participants. Therefore, timing requirements like
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worst case jitters or minimum slot gaps are not specified here.
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In contrast to earlier TDMA discipline revisions, the slave configuration is
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no longer distributed by the TDMA master. This means that the slaves have to
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be aware of their slot setup before sending any data to a TDMA-managed
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network. Therefore, the required settings either have to be stored on the
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slaves or, if a centralised management is desired, the RTnet configuration
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service RTcfg has to be used (see related specification for further details).
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Slot Identification and Selection
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---------------------------------
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NOTE: The following specifications are OPTIONAL. They describe the internal
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realisation of this TDMA discipline as applied to the first
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implementation in RTnet.
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Time slots carry an internal ID number, unique per participant. These numbers
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are used when determining the slot in which an outgoing packet shall be
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transmitted. The TDMA discipline contains no automatic scheduling mechanism.
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Instead, the sender, i.e. an user or a service, either explicitly provides a
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desired slot ID or a default slot is used.
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Slot ID | Description
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---------+-----------------------------------------------------------------
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0 | default slot for RT; also default NRT slot if slot 1 is missing
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1 | non-RT slot; if missing, slot 0 is used
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2 | user slots, used for explicitly scheduled packets
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: |
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Configuration Example
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---------------------
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An exemplary configuration consisting of two masters, one serving as backup,
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and three slaves is shown below. The slot period is expressed in the form
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<phasing>/<period>. For instance, 1/3 means that this slot will be used in
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every first of three cycles, while 3/3 means in every third or three.
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+------+ +----------+ +---------+ +---------+ +----------+
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| | | Master 2 | | Slave A | | Slave B | | Master 1 |
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| Sync | | Backup | | Slot 0 | | Slot 0 | | Slot 0 |
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| | | Sync | | RT/NRT | | RT | | RT/NRT |
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| 1/1 | | 1/1 | | 1/1 | | 1/1 | | 1/1 |
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--+------+--+----------+--+---------+--+---------+--+----------+--...
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+----------+
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| Slave C |
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| Slot 3 |
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| RT |
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| 3/3 |
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+---------+ +----------+
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| Slave C | | Master 2 |
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| Slot 0 | | Slot 0 |
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| RT/NRT | | RT/NRT |
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| 2/2 | | 2/3 |
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+---------+ +---------+ +----------+ +------+
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| Slave B | | Slave C | | Slave A | | |
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| Slot 1 | | Slot 2 | | Slot 2 | | Sync |
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| NRT | | NRT | | RT | | |
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| 1/2 | | 1/4 | | 1/3 | | 1/1 |
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...--+---------+--------+---------+--+----------+-------------+------+-->
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Management Interface
|
====================
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NOTE: The following specifications are OPTIONAL. They describe the internal
|
realisation of this TDMA discipline as applied to the first
|
implementation in RTnet.
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The TDMA discipline is managed by the command line tool tdmacfg. In the
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following, the usage of this tool is described.
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Commands
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--------
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tdmacfg <dev> master <cycle_period> [-b <backup_offset>]
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[-c calibration_rounds] [-i max_slot_id] [-m max_calibration_requests]
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Starts a TDMA master on the specified device <dev>. The cycle period length is
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given in microseconds using the <cycle_period> parameter. If <backup_offset>
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is provided, the master becomes a backup system. In case the main master
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fails, the backup master with the smallest <backup_offset> will start sending
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Synchronisation frames with the specified offset in microseconds relative to
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the scheduled cycle start. <calibration_rounds> specifies the number of clock
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calibration requests the master will send to any other potentially already
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active master during startup. By default, 100 rounds are performed. The
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calibration will be performed when the first slot is added. By default, a
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master can handle up to 64 calibration requests at the same time. This value
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can be adapted by specifying the <max_calibration_requests> parameter. The
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largest used slot ID is tunable by providing <max_slot_id> or will be limited
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to 7 if this parameter is omitted.
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tdmacfg <dev> slave [-c calibration_rounds] [-i max_slot_id]
|
|
Starts a TDMA slave on the specified device <dev>. <calibration_rounds>
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specifies the number of clock calibration requests the slave sends to the
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active master during startup. By default, 100 rounds are performed. The
|
calibration will be performed when the first slot is added. The largest used
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slot ID is tunable by providing <max_slot_id> or will be limited to 7 if this
|
parameter is omitted.
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tdmacfg <dev> slot <id> [<offset> [-p <phasing>/<period>] [-s <size>]
|
[-j joint_slot] [-l calibration_log_file] [-t calibration_timeout]]
|
|
Adds, reconfigures, or removes a time slot for outgoing data on a started TDMA
|
master or slave. <id> is used to distinguish between multiple slots. See above
|
slot ID table for predefined values. If <offset> is given, the time slot is
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added or modified to send data with the specified offset in microseconds
|
relative to the scheduled cycle start, if omitted, the slot is removed from
|
the station's configuration.
|
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By default, a slot will be used in every cycle. When providing <phasing> and
|
<period>, the slot will only be occupied in every <phasing>-th of <period>
|
cycles. By assigning e.g. 1/2 to one and 2/2 to another slot, the usage of the
|
physical time slot will alternate between both slot owners. The <size>
|
parameter limits the maximum payload size in bytes which can be transmitted
|
within this slot. If no <size> parameter is provided, the maximum size the
|
hardware supports is applied. To share the same output queue among several
|
slots, secondary slots can be attached to a primary <joint_slot>. The slot
|
sizes must match for this purpose.
|
|
The addition of the station's first slot will trigger the clock calibration
|
process. To store the results of each calibration handshake, a
|
<calibration_log_file> can be provided. By default, this command will not
|
terminate until the calibration is completed. The <calibration_timeout>
|
parameter can be used to specify an upper time limit.
|
|
tdmacfg <dev> detach
|
|
Detaches a master or slave from the given devices <dev>. Past this command,
|
the write access to the device is uncoordinated again and may interfere with
|
remaining real-time network participants.
|
|
|
2004, 2005, Jan Kiszka <jan.kiszka-at-web.de>
|
