1.4 The signal quality corresponding with OPC
Every signal (input, output, parameter) in the REXYGEN system has the so-called quality flags in addition to its own value of corresponding type (table 1.1). The quality flags in the REXYGEN system correspond with the OPC (OLE for Process Control) specification [1]. They can be represented by one byte, whose structure is explained in the table 1.2.
| Bit number | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Bit weight | 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
| Bit field | Quality | Limits
| ||||||
| Q | Q | S | S | S | S | L | L |
|
| BAD | 0 | 0 | S | S | S | S | L | L |
| UNCERTAIN | 0 | 1 | S | S | S | S | L | L |
| not used in OPC | 1 | 0 | S | S | S | S | L | L |
| GOOD | 1 | 1 | S | S | S | S | L | L |
The basic quality type is determined by the QQ flags in the two most important bits. Based on these the quality is distinguished between GOOD, UNCERTAIN and BAD. The four SSSS bits provide more detailed information about the signal. They have different meaning for each basic quality. The two least significant bits LL inform whether the value exceeded its limits or if it is constant. Additional details and the meaning of all bits can be found in [1], chapter 6.8.
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