FANUC Robotics America GFK-1535A - Manual (Page 91)

A-6 VersaMax™ System Genius® Network Interface Unit User's Manual – November 2000 GFK-1535A
A
Serial Data Format
The Genius protocol produces maximum throughput of data by using a minimum
overhead of control and synchronizing characters.
Each character is 11 bits long, comprising a start bit (always 0), next a control bit,
followed by 8 bits of data, sent LSB first. The last bit is a stop bit, always 1.
Successive characters are sent with no time space between them. The control bit
indicates the type of character being sent. A 1 indicates a control character, and 0 a
data character.
A minimum transmission has a Start character, one or more data characters, and a
Stop character. The Start character data contains the address and whether the
transmission is directed to a specific address or broadcast to all. The End character
contains the CRC-6 checksum. Complex transmissions may have additional start
and end of block characters to break up the message into blocks of data. For
example, a Bus Controller can send device specific messages (blocks of data) to all
devices on the bus during one transmission cycle.
Bus Access
All devices receive the current SBA and the stop character even though the data is
not used. After receiving the stop control character, each device starts a timer. The
time delay is equal to a skip time, times the difference between the device SBA and
the last SBA received. The device will transmit after the time delay if no other start
bits are detected first. Thus each device takes 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 is equal to one bit period, except at
the 153.6e rate, where it is two bit periods long. The longer interval accommodates
greater propagation delays cause by longer bus cables or fiber optic or other
repeaters. The worse case is when adjacent 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 (52
µ
Sec) to start of its
transmission. SBA5 receives the end character 3
µ
Sec later, and starts timing 1 skip
time (26
µ
Sec). Thus SBA 5 will start transmitting 29
µ
Sec after SBA 4 quit. This
allows 23
µ
Sec for the signal to get back to SBA6 to cancel it's transmission turn.
The 3
µ
Sec transmission delay leaves only 20
µ
Sec 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 skipping an SBA, resequencing SBAs in order
along the bus, going from 153.6s baud to the 153.6e, or a lower baud rate.