C-8 Field Control™ Genius® Bus Interface Unit User’s Manual – October 1999 GFK-0825F
C
The half cycle pulse width, when measured between the positive and negative receiver thresholds,
denoted as Tp/2 in the figure, will vary along the waveform due to dispersion, and resembles a
frequency shift.. The digital input filter essentially is a band pass filter which looks at the half cycle
timing Tp/2, and the durat ion above the thresholds , T w. Th e limits are:
• Tp/2 = 0.6 Tp(normal) maximum
• Tw = 0.188 Tp(normal) mini mum
These measurements can be taken when evaluating the ma ximum length of an unspecifi ed ca ble.
Dispersion is much less of a problem with fiber optic links since the media is much wider
bandwidth, and therefore h as less distor tion.
Propagation Delay
The propagation delay is caused by travel time of the signal down the cable. Typical signal velocity
in data grad e c a bles is around 6 5- 78% of the spe e d of light. This r equires about 3 m i croseconds t o
travel a 2000 foot long bus. This is about half a bit time at 153,6 Kb. This skew could affect the
bus access sequence sin ce only one bit of quiet bus (skip ) t ime is usually allocated between
transmission of adjacen t addresses. (Refer to Bus Acces s Time section below.) The signal must
reach all devices on the bus within the period of one bit. Propagation delay causes the ultimate
limitation in bus length, even with ideal media. Propagation speed through fiber optic is not
significantly different than wire, and delays through the interfaces must be accounted for.
Serial Data Format
The Genius pr ot ocol is desi gn ed to produce m a ximum through put of data by usi n g 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 ar e sent with
no time space between them. The control bit is used to sign al the type of character being sen t . A 1
indicates a control ch ar a cter, and 0 a data character.
A minimum transmission is comprised of a start character, one or more data characters, and a stop
character. The Start char a cter data contains the ad dr ess and whether the transmiss i on is directed to
a specific address or a br oa d ca st to all. The End character con t ains the CRC-6 checksum. Mor e
complex transmissions may have additional start and end of block characters to break up the
message into “blocks” of data (not to be confused with Genius I/O “Blocks”). For example, a Bus
Controller can send device specific messages (blocks of data) to all devices on the bus during one
transmission cycle.