Emerson FM-3 User 32-bit Bit Register and User Bit Masking, Bit Mask Result stored in each bit

Setting Up Parameters

User 32-bit Bit Register and User Bit Masking

When using different communications protocols (i.e. DeviceNet, Profibus, Modbus), it is often desirable to access groups of User Bits in a single parameter, rather than having to access them individually. In the FM-3 module it is possible to access 32 User Bits in a single parameter. This parameter is named BitRegister.#.Value. Because some of the 32 User Bits may be used by the program, and should not be modified from the network communications, it is possible to “Mask Off” certain bits. Masking bits prevents them from being modified in the program when the 32-bit parameter is written to.

When a User Bit Register (group of 32 User Bits) is written to, the value is then logic-AND’ed with the mask to determine the resulting state of each of the 32 individual bits. If the individual bit value of the 32-bit mask is “1”, then the corresponding bit from the written 32- bit parameter is passed through, and the resulting value stored in the specific bit will be the written bit value. If the bit value of the 32-bit mask is “0”, then that particular bit is blocked (or masked), and the resulting bit value does not change, (Original Value AND NOT 32-Bit Mask) or (Value Written over Network AND 32-Bit Mask). An example of this is shown below.

Original	Bit #31																	15															0
Value		1	0	0	0	0	0	0	1	0	1	0	0	0	0	0	0	1	0	0	0	0	0	1	0	0	0	0	0	0	0	0	0

Value written over network

0 1 1 0 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1

1 1 0 1 0 1 0 1 1 0

32-bit Mask

Result stored in each bit

0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

1 1 0 0 0 0 0 0 0 1 0 1 0 0 0 0 1 1 0 1 0 1 1 1 0 1 0 1 0 1 0 0

Figure 42: Writing to the User Bit Register

The Mask is only used when WRITING to the 32-bit parameter, BitRegister.#.Value. When reading the 32-bit value, all bits are read regardless of the mask.

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