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

Communications Instructions 405An easier method is to use the message timeout variable described onpage 421, because it simplifies the user program. This built-in timeoutcontrol is in effect whenever the message timeout is non-zero. It defaultsto 5 seconds for channel 0, so unless you change it, the internal timeoutcontrol is automatically enabled.When the internal timeout is used and communications are interrupted,the MSG instruction will timeout and error after the set period of timeexpires. This allows the control program to retry the same message ortake other action, if desired.To disable the internal timeout control, enter zero for the MSG instructiontimeout parameter. If communications are interrupted, the processor waitsindefinitely for a reply. If an acknowledge (ACK) is received, indicated bythe ST bit being set, but the reply is not received, the MSG instructionappears to be locked up, although it is actually waiting for a reply fromthe target device.Enable (EN)Address Data Format Range Type User Program AccessMG11:0/EN Binary On or Off Control Read / WriteThe Enable Bit (EN) is set when rung conditions go true and the MSG isenabled. The MSG is enabled when the command packet is built and putinto one of the MSG buffers, or the request is put in the MSG queue. Itremains set until the message transmission is completed and the runggoes false. You may clear this bit when either the ER or DN bit is set inorder to re-trigger a MSG instruction with true rung conditions on the nextscan.IMPORTANTDo not set this bit from the control program.Enabled and Waiting (EW)Address Data Format Range Type User Program AccessMG11:0/EW Binary On or Off Status Read OnlyThe Enabled and Waiting Bit (EW) is set after the enable bit is set and themessage is in the buffer (not in the queue) and waiting to be sent. TheEW bit is cleared after the message has been sent and the processorreceives acknowledgement (ACK) from the target device. This is beforethe target device has processed the message and sent a reply.Publication 1766-RM001A-EN-P - October 2008