Rockwell Automation 1771-KA2 Communication Adapter Module - Manual (Page 73)

Chapter
7
7-1
Command Initiation, Execution, and Monitoring
This chapter describes the support programming for commands at each
station processor. This programming uses the START/DONE and
REMOTE/LOCAL FAULT bits, described in chapter 6, to initiate and
monitor command execution. Using these recommended techniques, the
programmer coordinates communication zone programming with his
application program.
The START bit that initiates a command is program-controlled; the
DONE bit, which indicates command completion, is communication
adapter module-controlled. The timing relationship of START and DONE
bits is used by the programmer to initiate and terminate commands. The
following sections describe this relationship in normal operation and
describe the automatic responses that result from faulted operation.
Normal Operation
Command execution begins when the user program turns a START bit
ON, normally with a LATCH instruction. The 1771-KA2 communication
adapter module detects the ON state of this bit and then begins the
operations necessary to format and transmit a command message.
When the remote station communication adapter module receives the
command message, it acknowledges it. Then, while normal Data Highway
operation continues, the remote station communication adapter module
executes the command and prepares a reply message. (A reply message is
sent for each type of command.) The remote station communication
adapter module responds to a poll for mastership, then transmits its reply
message to the local (sending) station. (Refer to chapter 8 for a
description of mastership and polling.)
When the local station communication adapter module receives the reply,
it sets the done bit on at the local station processor. The DONE bit, in
turn, is examined in the user program to turn OFF the START bit.
After the START bit has been turned OFF (unlatched), the communication
adapter module resets the DONE bit.
General
START/DONE Bit Timing