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4-12 Field Control™ Genius® Bus Interface Unit User’s Manual
–
October 1999 GFK-0825F
4
Genius Bus Scan Time
The Genius bus scan time is dependent on the number of devices and amount of data traffic on the
bus. The bus scan time may vary from 3-400mS, but 20-30mS is typical. Bus Controllers impose a
minimum Genius bus scan time of 3mS. Therefore, Genius bus scan time can never be less than
3mS. The Genius bus scan time contribution for Field Control station depends on its I/O data
usage. The table below shows the scan time contribution, at each baud rate, for stations with a total
of 16, 32, 64, 128, and 256 bytes, when the Bus Interface Unit receives outputs from
only one bus
controller at a time.
To find the exact scan time contribution for a station, follow the procedure below.
Contribution time in mS at each baud rate
Total Amount of Input and
Output Data for
Field Control Station
153.6 Kb
std
153.6 Kb
ext
76.8 Kb 38.4 Kb
16 bytes 2.09 2.16 3.83 7.16
32 bytes 3.24 3.31 6.12 11.74
64 bytes 5.52 5.60 10.69 20.89
128 bytes 10.10 10.17 19.85 39.20
256 bytes (fully-loaded) 19.25 19.32 38.15 75.80
Procedure for Estimating Bus Scan Time for a Field Control Station
1. Find the total number of input bytes and output bytes. (Each analog channels is 2 bytes. Eight
discrete points are one byte).
number of input bytes = ________
number of output bytes = ________
total bytes = ________
2. With this total, calculate a scan time contribution using the formula below that corresponds to
the Genius bus baud rate.
Formula for 153.6 Kbaud Standard:
0.943mS + (0.0715 x total bytes) = ________ mS
Formula for 153.6 Kbaud Extended:
1.015mS + (0.0715 x total bytes) = ________ mS
Formula for 76.8 Kbaud:
1.538mS + (0.143 x total bytes) = ________ mS
Formula for 38.4 Kbaud:
2.583mS + (0.286 x total bytes) = ________ mS
Other devices on the bus, including the Bus Controller, Hand-held Monitor, and I/O blocks also
add to the bus scan time. The
Genius I/O System Manual
(Volume 1) shows how to calculate
overall bus scan time for all of the devices on a bus. Remember that the Genius bus scan time can
never be less than the 3mS minimum enforced by the bus controller.