Rockwell Automation 1771-KA2 Communication Adapter Module - Manual (Page 76)

Command Initiation, Execution, and
Monitoring
Chapter 7
7-4
Recall that the START bit is program-controlled. The REMOTE/LOCAL
FAULT bits, meanwhile, are controlled by the 1771-KA2 module. The
programmer must keep this relationship in mind when planning START
bit control and FAULT bit monitoring in the ladder-diagram program.
Note from Figure 7.2 that the FAULT bit, once ON, remains ON until the
START bit is reset (turned OFF). Only after it has detected that the
program-controlled START bit is OFF does the communication adapter
module then turn the FAULT bit OFF. Note also that the DONE bit is not
set ON in the event of a fault condition.
NOTE: In the special case where the START bit is turned OFF by the
program before the KA2 sets a DONE or FAULT bit, attempts to send that
command are terminated. The LOCAL FAULT bit is pulsed ON for
approximately 60 msec in this instance. This type of situation may occur,
for example, if some event is programmed to UNLATCH the START bit
before command completion.
The user program controls the START bit: setting it ON to initiate
command execution, turning it OFF after command completion or after a
fault is detected. Normally, the START bit is turned ON by a LATCH
instruction, OFF by an UNLATCH instruction. The use of retentive
LATCH/UNLATCH instructions is best suited for the timing relationship
of START, DONE, and FAULT bits and helps to keep programming
simple and straightforward.
To LATCH the START bit, the program examines application conditions.
These may include input/output device data, values, or other information
from the controlled process. For the most part, an application condition
used to initiate a command is one of these general types:
The ON or OFF status of some I/O device
Transition of some I/O device
Timed condition
To UNLATCH the START bit, the program examines the response from
the communication adapter module. This response may be one of the
following:
DONE bit
REMOTE FAULT bit
LOCAL FAULT bit
Controlling The Start Bit