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GFK-0825F Chapter 1 Introduction 1-11
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Using Field Control in a Genius Bus Redundancy System
In Genius bus redundancy, there are two bus cables each connected to a Bus Controller. I/O
devices may be connected to either one bus of the pair, or to both. However, a device that is
connected to both busses actually communicates on only one bus at a time. Before the alternate
bus can be used for communications, a bus switchover must occur and the device must “log in”
with the Bus Controller(s) on the alternate bus.
The Bus Interface Unit Terminal Block contains a built-in bus switching relay that is used to switch
busses in a dual bus system. Other types of devices with this capability are dedicated Bus
Switching Modules and Series 90-70 Remote I/O Scanner modules. These are the only types of
devices that can be directly connected to both redundant bus cables.
A Bus Interface Unit cannot be used as the BSM Controller for a bus stub. Other devices cannot be
located on a stub downstream of a BIU.
Also, the Bus Interface Unit should not be connected to an external Bus Switching Module.
Redundant Bus Configurations
Many different redundant bus configurations are possible. Three basic ways of using a Bus
Interface Unit with a redundant bus are described below.
A Bus Interface Unit can be installed directly on both cables of the dual bus pair. The
Bus Interface Unit is configured to operate as a bus switching device in addition to performing
its normal functions. Here, two Field Control stations are installed on a dual bus. Each Bus
Interface Unit would be set up as a bus switching device.
Bus A
Bus B
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A Bus Interface Unit can be located on just one bus of a redundant bus pair, if bus
redundancy is not needed for the modules in that station. In this example, the Bus Interface
Unit on the left is connected to both Bus A and Bus B and is configured as a bus switching
device. The Bus Interface Unit on the right, which serves non-critical I/O modules, is
connected to Bus A only, and is not configured as a bus switching device.
Bus A
Bus B
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