Rockwell Automation 1771-KA2 Communication Adapter Module - Manual (Page 77)

Command Initiation, Execution, and
Monitoring
Chapter 7
7-5
Thus, the rung used to UNLATCH the START bit examines these three
possible responses in parallel branches of the ladder diagram program.
Section titled “ON/OFF Input Status,” “Transition,” and “Timed” describe
the most commonly used forms of START bit control. Each section
describes a different method for initiating command execution; all
examples show how the program uses both DONE and FAULT bits to
UNLATCH the START bit.
An important assumption underlies the examples outlined in sections
titled “ON/OFF Input Status,” “Transition,” and “Timed” and shown in
Figures 7.3 through 7.5. Here, the program automatically re-tries
transmission of a command in the event of a fault. Only the DONE bit
terminates attempts at command execution; a LOCAL or REMOTE
FAULT bit response causes the program to re-initiate command execution.
(This type of programmed re-try is not to be confused with the re-try
procedure of the 1771-KA2 module itself; transparent to user
programming, the module automatically attempts several re-tries of a
message before it sets a FAULT bit. Section titled “Floating Master,” in
chapter 8, describes these automatic re-tries of the module.)
Programmed re-try has distinct advantages. As the communication
adapter module continuously tries to send the command message, any
data content of the message (for a write command) is continuously
updated. As soon as the fault condition is corrected, the message is sent,
with the latest data. In addition, once the command is completed, the
program automatically UNLATCHES the START bit. This eliminates the
need for a manual reset of the START bit when a fault condition is
corrected.
ON/OFF Input Status
The ON or OFF status of an input device can be used to LATCH the
START bit. Figure 7.3 shows rungs for this type of command initiation.