57.7.9.6.2 comm_type

In CEA-709 standard 2, communication configurations are defined and configurable through the comm_type variable. The comm_type variable value can be set in the USART LON Mode register (FLEX_US_LONMR) through the COMMT bit. The selection of the comm_type determines the MAC behavior in the following ways:

• comm_type = 1:
  • An indeterminate time is defined during the Beta 1 period in which all transitions on the channel are ignored (see figure below).
  • The MAC sublayer ignores collisions occurring during the first 25% of the transmitted preamble. Optionally (according to the FLEX_US_LONMR.CDTAIL bit), it ignores collisions reported following the transmission of the CRC but prior to the end of transmission.
  • If a collision is detected during preamble transmission, the MAC sublayer can terminate the packet if so configured according to the FLEX_US_LONMR.TCOL bit. Collisions detected after the preamble have been sent do not terminate transmission.
• comm_type = 2:
  • No indeterminate time is defined at the MAC sublayer.
  • The MAC sublayer always terminates the packet upon notification of a collision.