1766-RM001A-EN-P MicroLogix 1400 Programmable Controllers ...

Communications Instructions 4094. Step 4 is not shown in the timing diagram. If you do not receive anACK, step 3 does not occur. Instead, either no response or anegative acknowledge (NAK) is received. When this happens, theST 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 transmissionA NAK may be caused by:• target node is busy• target node received a corrupt message• the message is too largeWhen 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 sendsa reply packet. The reply packet contains one of the followingresponses:• successful write request.• successful read request with data• failure with error codeAt the next end of scan, REF, or SVC instruction, following the targetnode’s reply, the controller examines the message from the targetdevice. If the reply is successful, the DN bit is set (1), and the ST bitis cleared (0). If it is a successful read request, the data is written tothe data table. The message instruction function is complete.If the reply is a failure with an error code, the ER bit is set (1), andthe ST bit is cleared (0). The message instruction function iscomplete.6. If the DN or ER bit is set (1) and the MSG rung is false, the EN bit iscleared (0) the next time the message instruction is scanned.See MSG Instruction Ladder Logic on page 411 for examples using themessage instruction.Publication 1766-RM001A-EN-P - October 2008