Allen-Bradley MicroLogix 1200 Reference Manual page 325

4. Step 4 is not shown in the timing diagram. If you do not receive an
ACK, step 3 does not occur. Instead, either no response or a negative
acknowledge (NAK) is received. When this happens, the ST bit
remains clear (0).
No response may be caused by:
• the target node is not there
• the message became corrupted in transmission
• the response was corrupted in response transmission
A NAK may be caused by:
• target node is busy
• target node received a corrupt message
• the message is too large
When a NAK occurs, the EW bit is cleared (0), and the ER bit is set
(1), indicating that the message instruction failed.
5. Following the successful receipt of the packet, the target node sends a
reply packet. The reply packet contains one of the following
responses:
• successful write request.
• successful read request with data
• failure with error code
At the next end of scan, REF, or SVC instruction, following the target
node's reply, the controller examines the message from the target
device. If the reply is successful, the DN bit is set (1), and the ST bit is
cleared (0). If it is a successful read request, the data is written to the
data table. The message instruction function is complete.
If the reply is a failure with an error code, the ER bit is set (1), and the
ST bit is cleared (0). The message instruction function is complete.
6. If the DN or ER bit is set (1) and the MSG rung is false, the EN bit is
cleared (0) the next time the message instruction is scanned.
See MSG Instruction Ladder Logic on page 21-28 for examples using the
message instruction.
Communications Instructions
Publication 1762-RM001C-EN-P
21-25