Spyder Compare Block

[mechmike2]

I have noticed that output goes to 255 instead of 1 or 0 when input 1 is within the hysteresis of input 2. I would expect it to stay either 1 or 0 depending on way the block is set up.

[digo]

I have noticed that output goes to 255 instead of 1 or 0 when input 1 is within the hysteresis of input 2. I would expect it to stay either 1 or 0 depending on way the block is set up.

According to this:
local:|module://docHoneywellSpyder/doc/FunctionBlocks/Analog/COMPARE.html

Output should either be true or false.

You might wanna send what you got to tech support.

[lwarren]

I noticed this issue back in 2010 and sent it in to the Honeywell Engineers. Here is there response.

After investigating we came to know that it is an expected output according to the design and working of Spyder control but for the user it will be an unexpected output.

“Greater Than” operation is selected in the Compare block and in this operation, compare works as said below:

Compare Greater Than
Case 1: Output is set TRUE if input1 > input2 + onHyst.
Case 2: Output is set FALSE if input1 <= input2 - offHyst.
Case 3: Output does not change if input1 <= input2 + onHyst && input1 > input2 - offHyst.

After download Spyder controller is rebooted and during this functional block outputs are set Invalid. Since Compare output is of “Digital” , 255 is the invalid value so initially compare block output will be 255. After wards it works according to the application.

Here Input1 = 53, Input2 = 61, onHyst = 0, offHyst = 10

Let us see each case:

Case 1: Output is set TRUE if input1 > input2 + onHyst.
Input2 + onHyst (61 + 0) = 61
53 is not greater than 61 hence this case is ruled out.

Case 2: Output is set FALSE if input1 <= input2 - offHyst.
Input2 - offHyst (61 - 10) = 51
53 is not lessthan or equal to 51 hence this case is ruled out.

Case 3: Output does not change if input1 <= input2 + onHyst && input1 > input2 - offHyst.
Input2 + onHyst (61+0) = 61
53 is lessthan 61
Input2 - offHyst (61 - 10) = 51
53 is greater than 51

Hence this case works; hence output does not change i.e. value remains 255

Workaround:

1. offHyst value has to be decreased or
2. Compare block should be executed before the functional blocks whose output are connected as input to the compare block. ( i.e. execution order of compare block should be greater than the functional blocks whose output are connected as input to the compare block)

We are investing the functional blocks of the application and if any unusual is noticed, we will inform you.

[CB_Matt]

I have witnessed this many times. It has happened to me on more that just compare blocks. This is why i have started to add override blocks before any compare blocks. There are others that i have witnessed this on also.

Basically any network points that i have go through an override block so that i can pick a default value for the ddc and then i have no issues.

Actually setting the execution order on these is very important to me also. I always reorder the sequences after i get them all on the wire sheet.