3

Operation of a 4-Block Output Group

Each GMR output state is sent to four blocks set up in an H-pattern as shown on the opposite page. This type of grouping creates a fault-tolerant system where any single point of failure does not cause the system to lose control of a critical load. This is achieved by:

Houtput voting (which is explained on page 3-3), and

Hthe electrical characteristics of sink and source blocks, and

Hredundant busses.

Electrical Characteristics of Sink and Source Blocks

If a load is wired between a sink and source block, both the sink output and the source output must be active to control the load. If either the sink output or the source output fails On, turning the other Off, turns the load Off. Doubling the number of blocks to four and putting them in an H pattern means that if any single point of failure occurs, the system can still control the load.

The following chart shows how the GMR system uses the 4-block H-pattern output group to maintain control of critical loads following certain types of failures. All operating blocks receive the same I/O data, because within a fault-tolerant 4-block H-pattern group, all four blocks are configured at the same output address. The chart indicates which blocks actually affect the state of the load under different fault scenarios. All operating blocks act on the I/O data received.

 

Other Blocks Used

Other Blocks Used

Fault

To Turn the Load Off

To Turn the Load On

 

 

 

 

 

 

output at block A fails On

turn outputs at block C and D Off

turn output at block C or D On

 

 

 

output at block A fails Off

turn output at block B off

turn output at block B and either C or D On

 

 

 

output at block B fails On

turn outputs at block C and D Off

turn output at block C or D On

 

 

 

output at block B fails Off

turn output at block A off

turn output at block B and either C or D On

 

 

 

output at block C fails On

turn outputs at block A and B Off

turn output at block A or B On

 

 

 

output at block C fails Off

turn output at block D off

turn output at block D and either A or B On

 

 

 

output at block D fails On

turn outputs at block A and B Off

turn output at block A or B On

 

 

 

output at block D fails Off

turn output at block C off

turn output at block C and either A or B On

 

 

 

Bus Redundancy in a 4-Block Output Group

If one of the three busses in an output group is damaged or cut, there is still I/O data communicated to at least one sink output and one source output to control the load. When a block loses communication with all the PLCs, its outputs go to a default state. If the default state is Off, the system is fault-tolerant as shown in the following chart.

 

Fault

To Turn the Load Off or On

 

 

 

 

 

 

 

 

 

 

bus A fails

busses B and C still provide I/O communications to blocks B, C, and D;

 

 

 

 

 

turning outputs at those blocks On or Off turns the load On or Off.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

bus B fails

busses A and C still provide I/O communications to blocks A and C; if the

 

 

 

 

 

block B and D outputs are configured to default Off, turning output at

 

 

 

 

 

blocks A and C On or Off turns the load On or Off.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

bus C fails

busses A and B still provide I/O communications to blocks A, B, and D;

 

 

 

 

 

turning outputs at those blocks On or Off turns the load On or Off.

 

 

 

 

 

 

 

GFK-0787B

Chapter 3 Output Subsystem

3-7

Page 35
Image 35
GE GFK-0787B user manual Operation of a 4-Block Output Group, Electrical Characteristics of Sink and Source Blocks