Allen-Bradley 1779-KFMR User Manual page 148

Chapter 9
Write Bit With Reply
Use this command to set or reset specified bits within specified words in
the data table of the destination host's memory. The fields following the
CMD field consist of three-field blocks, each of which contains a
logical-address field, followed by a two-byte AND-mask field and then a
two-byte OR-mask field. Each 0 in an AND-mask field resets the
corresponding bit to 0. Each 1 in an OR-mask field sets the
corresponding bit to 1. Those bits within the word but not designated by
either the OR mask or AND mask remain unchanged. The total length of
the CMD, ADDR, AND-mask, and OR-mask fields is limited to 40 bytes
maximum.
The interface at the receiving node executes this command by first
making a copy of the addressed controller word. It resets the bits
specified in the AND mask, then sets the bits specified in the OR mask,
and then writes the byte back into controller memory. At the same time,
the controller can be changing the states of the original bits in memory.
Because of this, some data bits may unintentionally be overwritten. To
help prevent this, we suggest you use this command to write into the
storage area of a programmable controller's data table, and have the
controller only read the byte, not control it.
Command Format:
In the reply, the STS field indicates a successful transaction completion
(00) or an error code.