FC 96 - Redundant Analog Input

The redundant analog input (REDAI) function code is a transfer between a pair of redundant analog signals. An external logic select input <S3> controls the transfer. The output value is equal to the input value selected with <S3>.  Function code 96 checks the difference between the two inputs and the rate of change of the selected input. Output quality is bad if the quality of the selected input is bad, if the rate of change exceeds the rate limit specified by S5, or if the difference between the two inputs is larger than S4. To test the quality of the signal, include a function code 31 (test quality) block in the configuration. The quality of the point cannot be used as an input to any other type of block. However, the output value can be used as an input to other blocks.  Refer to Appendix H, for a definition of point quality.  

 

Outputs:

Blk

Type

Description

N

R

Output N = <S1> if <S3> = 0

Output N = <S2> if <S3> = 1

 

 

Specifications:

Spec

Tune

Default

Type

Range

Description

S1

N

5

I

Note 1

Block address of first input

S2

N

5

I

Note 1

Block address of second input

S3

N

0

I

Note 1

Block address of select input; 0 = <S1>, 1 = <S2>

S4

Y

9.2E18

R

Full

Deviation limit (same units as <S1> and <S2>)

S5

Y

9.2E18

R

Full

Rate limit (same units as <S1> and <S2> per sec)

S6

Y

9.2E18

R

Full

Rate error deadband (same units as <S1> and <S2>)

 

NOTES:

1. Maximum values are: 9,998 for the BRC-100, IMMFP11/12 and 31,998 for the HAC

 

 

 

96.1 Explanation

 

96.1.1 Specifications

 

S1 - X1

Block address of first input.

 

S2 - X2

Block address of second input.

 

S3 - SELCT

Block address of select input. The value in this block determines which input (<S1> or <S2>) is the output.

0 = output <S1>

1 = output <S2>

 

S4 - DEV

Maximum deviation permitted between <S1> and <S2>. This applies when both <S1> and <S2> are good quality.

 

S5 - RATE

Maximum allowable rate of change per second for the selected input (same units as <S1> and <S2> per second). If the rate

of change minus S6 exceeds S5, output quality is bad.

 

S6 - DBAND

Defines the deadband for the rate limit. When the rate exceeds the rate limit plus S6, output quality is bad. The deadband

prevents excessive bad quality readings when the rate is hovering around the limit.

 

 

96.2 Logic Flow

 

If <S3> = 0; output value and quality = <S1> with quality

If <S3> = 1; output value and quality = <S2> with quality

If <S1> and <S2> quality = good, and <S1> – <S2> > S4; output quality = 1 (bad)

If rate of change of output > S5 + S6; output quality = 1 (bad)

 

where:

<S1> = Value of first input.

<S2> = Value of second input.

<S3> = Value of select input.

       0 = <S1>.

       1 = <S2>.

    S4 = Deviation limit.

    S5 = Rate limit.

    S6 = Rate deadband

 

 

96.3 Applications

 

Function code 96 relies on external logic to select and output one of two redundant analog inputs. Figure 96-1 shows the configuration for the output of a function code 98 controlling the selection. When the active module in the slave select block changes from module one to module two, the active input to the redundant analog input block switches from the first input <S1> to the second input <S2>. Configuring the blocks so that inputs one and two are received from modules one and two, respectively, insures that if one module goes bad, the redundant analog input automatically switches to the good module.