Allen-Bradley MicroLogix 1200 Reference Manual page 315

Enabled and Waiting (EW)
Address Data Format
MG11:0/EW Binary
The Enabled and Waiting Bit (EW) is set after the enable bit is set and the
message is in the buffer (not in the queue) and waiting to be sent. The
EW bit is cleared after the message has been sent and the processor
receives acknowledgement (ACK) from the target device. This is before
the target device has processed the message and sent a reply.
Error (ER)
Address Data Format
MG11:0/ER Binary
The Error Bit (ER) is set when message transmission has failed. An error
code is written to the MSG File. The ER bit and the error code are cleared
the next time the associated rung goes from false to true.
Done (DN)
Address Data Format
MG11:0/DN Binary
The Done Bit (DN) is set when the message is transmitted successfully.
The DN bit is cleared the next time the associated rung goes from false to
true.
Start (ST)
Address Data Format
MG11:0/ST Binary
The Start Bit (ST) is set when the processor receives acknowledgment
(ACK) from the target device. The ST bit is cleared when the DN, ER, or
TO bit is set.
Communications Instructions
Range Type User Program Access
On or Off Status Read Only
Range Type User Program Access
On or Off Status Read Only
Range Type User Program Access
On or Off Status Read Only
Range Type User Program Access
On or Off Status Read Only
Publication 1762-RM001C-EN-P
21-15