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

408 Communications Instructions2. At the next end of scan, REF, or SVC instruction, the controllerdetermines if it should examine the communications queue foranother instruction. The controller bases its decision on the state ofthe channel’s Communication Servicing Selection (CSS) and MessageServicing Selection (MSS) bits, the network communication requestsfrom other nodes, and whether previous message instructions arealready in progress. If the controller determines that it should notaccess the queue, the message instruction remains as it was. Eitherthe EN and EW bits remain set (1) or only the EN bit is set (1) untilthe next end of scan, REF, or SVC instruction.If the controller determines that it has an instruction in the queue, itunloads the communications queue entries into the message buffersuntil all four message buffers are full. If an invalid message isunloaded from the communications queue, the ER bit in the MG fileis set (1), and a code is placed in the MG file to inform you of anerror. When a valid message instruction is loaded into a messagebuffer, the EN and EW bits for this message are set (1).The controller then exits the end of scan, REF, or SVC portion of thescan. The controller’s background communication function sendsthe messages to the target nodes specified in the messageinstruction. Depending on the state of the CSS and MSS bits, you canservice up to four active message instructions per channel at anygiven time.3. If the target node successfully receives the message, it sends back anacknowledge (ACK). The ACK causes the processor to clear (0) theEW bit and set (1) the ST bit. The target node has not yet examinedthe packet to see if it understands your request.Once the ST bit is set (1), the controller waits for a reply from thetarget node. The target node is not required to respond within anygiven time frame.TIPIf the Target Node faults or power cycles during the messagetransaction, you will never receive a reply. This is why you should usea Message Timeout value in your MSG instruction.Publication 1766-RM001A-EN-P - October 2008