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GFK-0825F Appendix C The Genius Serial Bus C-11
C
Bus Ambient Electrical Information
Most capacitively- and magnetically-coupled noise shows up as common mode voltage on the bus.
The bus provides a 60 dB common mode rejection ratio. A noise spike above 1000 volts would be
required to corrupt the data. The bus receivers filter out corrupted data and perform a 6-bit cyclic
redundancy check to reject bad data. Corrupted signals due to noise show up as missed data rather
than incorrect data. The bus continues operating to the maximum extent possible when bus errors
are detected; random bus errors do not shut down communications. Bad data is rejected by the
receiving device and excessive errors are reported to the controller. Bus errors are indicated by
flickering of I/O block and bus controller LEDs. If excessive bus errors occur, the problem should
be found and corrected.
Lightning Transient Suppression
Running the bus cable outdoors or between buildings may subject it to lightning transients beyond
the 1,500 volt transient rating of the system. Installing cable underground reduces the probability of
a direct lightning strike. However, buried cables can pick up hundreds of amperes of current when
lightning contacts the ground nearby.
Therefore, it is important to protect the installation by including surge protectors on underground
data lines. The cable shields should be grounded directly. Surge suppressors and spark gaps should
be used to limit the voltage that might appear on the signal lines. It is recommended to install two
(only) silicon surge suppressors or spark gaps to control transients of 1 to 25 Kilovolts from 100 to
1000 amps or more. These devices should be installed close to the entrance of the bus to the
outdoors.
Silicon Surge Suppressors are available many sources, including Clare/General Instruments and
Motorola, For information about this product, in the US contact Lucas Industries Incorporated,
5500 New King Street, Troy, Michigan 48098 (tel: 313 879-1920, fax: 313 552-1020). Spark gaps
are available from Clare. Refer to the vendor's literature for installation details.
In extreme situations, such as totally-isolated power systems, additional protection against
lightning damage should be provided by adding surge suppressors for groups of I/O blocks. Such
suppressors should be installed from incoming power leads to ground (enclosure baseplate/block
case where leads enter the enclosure).
A device specifically designed to protect the Genius bus is available from CONTROL
TECHNOLOGY, 835 Hwy 90, Hancock Square Suite 10 (P. O. Box 2908), Bay Saint Louis, MS
39520. (tel 601 466-4550, fax 601 466-4553). Contact them for application information. The
devide must be used in combination with power line suppression to fully protect the system.