GFK-0825F Appendix C The Genius Serial Bus C-9
C
Bus Access
All devices must receive th e current SBA and the stop char a cter even though the data is irr elevant
locally. After the stop control character is received, each device on the bus starts a timer. The time
delay is equal to a “skip time” times the difference between the device Serial Bus Address (SBA)
and the last SBA received. Th e device will transmit after the time delay if no other start bits are
detected first. Thus ea ch d evi ce t akes turn in order of SBA. Unused SBAs result in longer times
between messages. All devices must detect messages within this skip time delay. A bus “collision”
(two sources transmitting simultaneously) results if this sequence is missed. The skip time value is
equal to one bit period, except on the 153.6e rate, where it is two bit periods long. The longer
interval is useful to accommodate the longer propagation delays due to longer bus cables, or when
delays are introduced b y fib er optic or other repeater s, The worse case is when adjacen t SBAs are
physically located at opposite ends of a long bus. For example, assume SBA 4 and 6 are at one end
of a 2000 foot bus and SBA5 at the other, operating at 153.6s Kb. When SBA 4 end character is
detected, SBA6 immediately starts timing 2 skip times (13 uSec) to start of it's transmission. SBA5
receives the end chara ct er 3 uSec later, and starts timin g 1 skip time (6.5 uSec). Thus S BA 5 wil l
start transmitting 9.5 uSec after SBA 4 quit. This allows 3.5 uSec for the signal to get back to
SBA6 to cancel its transmission turn. The 3 uSec transmission delay leaves only 0.5 uSec to do this
and avoid a collision between SBA5 and 6.
Bus collisions result in missing data or detected CRC errors. Problems resulting from bus collisions
can be fixed by not using (skipping) a SBA, resequencing SBAs in order along the bus, going from
153.6s baud to the 153.6e, or a lower baud rate.