Allen-Bradley 1779-KFMR User Manual page 120

Chapter 8
Command Format:
Reply Format:
Protected Write Bit
Use this command to set or reset specified bits within specified bytes
within limited areas of data table memory in a programmable controller.
Your access can be limited by memory access rungs in the communication
zone of the controller's ladder-diagram program.
The fields following the TNS field consist of up to 61 four-byte blocks,
each of which contains a two-byte logical-address field, a one-byte
set-mask field, and a one-byte reset-mask field. Use the address field to
specify the logical byte address. Each 1 in a set-mask field sets the
corresponding bit to 1. Each 1 in a reset-mask field resets the
corresponding bit to 0. Those bits within the byte, but not designated by a
1 in either the set or reset mask, remain unchanged.
The interface at the receiving node executes this command by first
making a copy of the addressed controller byte. It sets the bits specified
in the set mask, then resets the bits specified in the reset 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.
Unless otherwise stated, values shown are in hexadecimal, and each block
shown represents one byte.