Rockwell Automation 1761-HHP-B30 MicroLogix 1000 with Hand-Held Programmer (HHP) - Manual (Page 289)

Chapter 15
Using Communication Protocols
15–5
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Done Bit DN (bit 13) is set when the message is transmitted
successfully. The DN bit is reset (cleared) the next time the rung goes
from false to true.
•
Start Bit ST (bit 14) is set when the processor receives
acknowledgement from the target device. This identifies that the target
device has started to process the MSG request. The ST bit is reset when
the DN, ER, or TO bit is set or on a false-to-true rung transition.
•
Enable Bit EN (bit 15) is set only if the transmit buffer is available. If
the transmit buffer is not available, the EN flag remains false. When the
transmit buffer becomes available, the EN flag goes true. It remains set
until the next false rung execution after the MSG completes (DN bit set)
or an error occurs (ER bit set).
Important: The operation of the EN bit has changed since Series C.
The operation associated with a message read or write
instruction is performed when you enable the instruction.
Replies are processed at the end of the scan.