GE PACSystems RX7i, Cpu Programmer'S Reference Manual

Page: 162 / 469

Chapter 4. Ladder Diagram (LD) Programming
GFK-2950C February 2018 147
BUS Read Modify Write
The BUS_RMW function updates one byte, word, or double word of
data on the bus. This function locks the bus while performing the
read-modify-write operation.
When the BUS_RMW function receives power flow through its
enable input, the function reads a dword, word or byte of data from
the module at the specified rack (R), slot (S), subslot (SS) and
optional address region (RGN) and offset (OFF). The original value is
stored in parameter (OV).
The function combines the data with the data mask (MSK). The
operation performed (AND / OR) is selected with the OP parameter.
The mask value is dword data. When operating on a word of data,
only the lower 16 bits are used. When operating on a byte of data,
only the lower 8 bits of the mask data are used. The result is then
written back to the same address from which it was read.
The BUS_RMW function passes power to the right when its
operation is successful, and returns a status value to the ST output. Other mnemonic:
BUS_RMW_WORD