Rockwell Automation 1761-HHP-B30 MicroLogix 1000 with Hand-Held Programmer (HHP) - Manual (Page 294)

Chapter 15
Using Communication Protocols
15–10
Step 4 is not shown in the timing diagram.
 If you do not receive an ACK, step 3 does not occur. Instead a NAK (no
acknowledge) is received. When this happens, the ST bit remains clear. A
NAK indicates:
•
the Target Node is too busy, or
•
it received a MSG packet with a bad checksum.
No response indicates:
•
either the Target Node is not there, or
•
it does not respond because the MSG packet was corrupted in
transmission.
When a NAK occurs, the EW bit is cleared. (Note that the NR bit will only
be set for DH-485 and NAK conditions. An crror code 02H, Target Node is
busy, is received which causes the NR bit to be set.) The ER bit is also set
which indicates that the MSG instruction failed.
Monitor the NR bit. If it is set, indicating that the Target Node is busy, you
may want to initiate some other process (e.g., an alarm or a retry later). The
NR bit is cleared when the rung logic preceding the MSG changes from false
to true.
 When an ACK occurs, the Target Node sends one of three responses shown
in Step 6.
 Following the successful receipt of the packet, the Target Node sends a reply
packet. The reply packet will contain one of the following responses:
•
I have successfully performed your write request.
•
I have successfully performed your read request, and here is your data.
•
I have not performed your request because of an error.
At the next end of scan, following the Target Node’s reply, the MicroLogix
1000 controller examines the MSG packet from the target device. If the
reply contains “I have successfully performed your write request,” the DN bit
is set and the ST bit is cleared. The MSG instruction 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 “I have successfully performed your read request, and
here is your data,” the data is written to the appropriate 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 “I have not performed your request, 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.