Firewire 800 Imac
Beta mode is based on 8B/10B (from Gigabit Ethernet, also used for many other protocols). 8B/10B encoding involves expanding an 8 bit data word into 10 bits, with the extra bits after the 5th and 8th data bits. The partitioned data is sent through a Running Disparity calculator function. The Running Disparity calculator attempts to keep the number of 1s transmitted equal to 0s, thereby assuring a DC-balanced signal. Then, the different partitions are sent through a 5B/6B encoder for the 5 bit partition and a 3B/4B encoder for the 3 bit partition. This gives the packet the ability to have at least two 1s, ensuring synchronization of the PLL at the receiving end to the correct bit boundaries for reliable transfer. An additional function of the coding scheme is to support the arbitration for bus access and general bus control. This is possible due to the "surplus" symbols afforded by the 8B/10B expansion. (While 8-bit symbols can encode a maximum of 256 values, 10-bit symbols permit the encoding of up to 1024. ) Symbols invalid for the current state of the receiving PHY indicate data errors.