Publication 1747-RM001G-EN-P - November 2008
12-40 SLC Communication Instructions
Step 4 is not shown in the timing diagram.
An ACK is Not Received
If you do not receive an ACK, step 3 does not occur. Instead, either no
response or a NAK (no acknowledge) is received. When this happens, the ST
bit remains clear.
These conditions can cause no response.
• The target node is not there.
• The target node does not respond because the packet became too
corrupted in transmission to be properly received.
• The response was corrupted in transmission back.
These conditions can cause a NAK.
• Target node is too busy.
• Target node received a corrupt packet.
Target Node Sends a Reply Packet
Following the successful receipt of the packet, the Target Node sends a reply
packet. The reply packet will contain one of the following responses.
• The write request was successfully performed.
• The read request was successfully performed and here is the data.
• The request was not performed because of an error.
At the next end of scan or SVC, following the Target Node’s reply, the SLC
processor examines the packet from the target device. If the reply contains
successfully performed the write request, the DN bit is set and the ST bit is
cleared. The MSG instruction function is complete. If the MSG rung is false,
the EN bit is cleared the next time the MSG instruction is scanned.
If the reply contains successfully performed the read request, and here is the
data, the data is written to the data table, the DN bit is set and the ST bit is
cleared. The MSG instruction function is complete. If the MSG rung is false,
the EN bit is cleared the next time the MSG instruction is scanned.
If the reply contains the request was not performed because of an error, the
ER bit is set and the ST bit is cleared. The MSG instruction function is
complete. If the MSG rung is false, the EN bit is cleared the next time the
MSG instruction is scanned.
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