The number of bits that are allocated to a measuring instrument or control device in the outstation hardware determines the number of possible states for that point:
| Number of bits allocated for the measuring instrument/ control device in the outstation | Number of possible states for the point representing the measuring instrument/ control device |
|---|---|
|
1 |
2 |
|
2 |
3-4 |
|
3 |
5-8 |
Many drivers support 1-bit digital points, which can be in either of two states:
| Bit Position |
0 |
1 |
| State |
0 |
1 |
Some drivers also support 2- and 3-bit digital points.
A 2-bit digital point can be in one of four states:
| Bit Position |
00 |
01 |
10 |
11 |
| State |
0 |
1 |
2 |
3 |
A 3-bit digital point can be in one of eight states:
| Bit Position |
000 |
001 |
010 |
011 |
100 |
101 |
110 |
111 |
| State |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
Example:
A digital input point represents a pump.
The data for the pump is stored in 2 consecutive bits of a digital input card in the outstation.
The digital point that represents the pump can have 4 possible states.
(Each bit allocated to the point can be on (1) or off (0)—as the point has 2 bits, the possible permutations are 0 0, 0 1, 1 0, or 1 1, and as such, four possible states.)
You need to Define the Number of Bits for a Digital Point on the Form for that point. The Number of Bits setting has to be consistent with the number of consecutive bits used in the outstation or PLC, to store the data for the measuring instrument or control device that the point represents.
You can then specify a suitable description for the state that is associated with each bit, and define whether an event is logged, or an alarm is raised upon entry into each state (see Define the Point States for a Digital Point).