Installation manual

Publication 1747-RM001G-EN-P - November 2008
SLC Communication Instructions 12-39
Next End of Scan
At the next end of scan or SVC, the SLC processor determines if it should
examine the MSG queue for “something to do.” The processor bases its
decision on the state of bits S:2/15, S:33/7, S:33/5, S:33/6, network
communication requests from other nodes, and if previous MSG instructions
are already in progress. If the SLC processor determines that it should not
access the queue, the MSG instruction remains as it was. (Either the EN and
EW bits remain set, or only the EN bit is set, or only the WQ bit is set until the
next end of scan or SVC. If only the WQ bit is set, the MSG instruction must
be re-scanned later with true rung conditions.)
If the SLC processor determines that it has something to do, it unloads the
MSG queue entries into the MSG buffers until all four MSG buffers are full.
Each MSG buffer contains a valid network packet. If a packet cannot be
successfully built from the MSG queue, the ER bit is set and a code is placed
in the MSG block to inform you of an error. When a MSG instruction is
loaded into a MSG buffer, the EN and EW bits are set.
The SLC processor then exits the end of scan or SVC portion of the scan. The
processor’s background communication function sends the packets to the
Target Nodes that you specified in your MSG instruction. Depending on the
state of bits S:2/15, S:33/7, S:33/5, and S:33/6 you can have up to eight MSG
instructions (four from each channel) active at any given time.
Successful Receipt of Packet
If the Target Node successfully receives the packet, it sends back an ACK
(acknowledge). The ACK causes the processor to clear the EW bit and set the
ST bit. The Target Node has not yet examined the packet, to see if it
understands your request. Note that the Target Node is not required to
respond within any given time frame.
For broadcast write commands, Ethernet and DF1 Radio Modem
communication, there is no ACK/NAK mechanism. For the SLC 5/05, the
ST bit is set when the Ethernet daughterboard internally indicates it has
received the command from the main processor and will send it out. For
broadcast write commands and DF1 Radio Modem, the ST bit is set when the
command has successfully been transmitted. Skip step 4 for Ethernet and DF1
Radio Modem communications.
TIP
If the Target Node faults or power cycles during this time
frame of a MSG transaction, you will not receive a reply. As
a result, use a MSG Timeout value in your MSG instruction.