Rockwell Automation 1771-KA2 Communication Adapter Module - Manual (Page 66)

Status Words
Chapter 6
6-2
Any accessible data table words can be used as status words in the station
processor. Note that the same recommendations for data table control
given in section titled “Accessible Data Table Locations - PLC-2
Processors,” chapter 4, should be followed when selecting status words.
That is, processor work areas, input image table words and word 027
should not be used as status words.
The START/DONE word is the first of a pair of adjacent status words for
communication adapter module use. This word stores a START bit and
corresponding DONE bit for each of up to 8 commands. (NOTE: should
more than 8 command rungs be programmed at a station, additional pairs
of START/DONE and REMOTE/LOCAL FAULT words can be used.)
The START bit for any command is always in the upper byte (bits 10-17)
of the START/DONE word. The DONE bit for this command is then the
corresponding bit in the lower byte (bits 00-07) of the same word.
In Figure 6.1.A the sample command rung examines bit 03210 as its
START bit. As Figure 6.1.B shows, the corresponding DONE bit is bit
03200. Note that the 5-digit address of START/DONE bits for each
command differs only in the fourth digit: the START bit always has a “1”
in the fourth digit, the DONE bit always has a “0.”
As its name implies, the START bit initiates command execution. This bit,
controlled by the user program, is set ON to initiate the sending of a
command. The communication adapter module monitors the status of
START bits and executes the corresponding command when its START
bit is set ON.
The DONE bit as its name implies, indicates command completion. This
bit, controlled by the KA2, is set ON when a command is executed.
Each command rung examines a unique START bit. Thus, one pair of
START/DONE and REMOTE/LOCAL FAULT words has enough bits for
up to 8 command rungs. Should more command rungs be programmed,
select additional word pairs as necessary.
To optimize memory use and minimize the time required by the
communication adapter module to scan START bits, use all 8 START bits
in one word pair before using another START/DONE word. For the same
reason, when programming communication zone rungs, group in
sequential fashion command rungs that use the same START/DONE
word.
START/DONE Word