Rockwell Automation 1761-HHP-B30 MicroLogix 1000 with Hand-Held Programmer (HHP) - Manual (Page 293)

Chapter 15
Using Communication Protocols
15–9
The following section illustrates a successful timing diagram for a MSG
instruction in a Series D or later MicroLogix 1000 discrete controller or a
MicroLogix 1000 analog controller.
Rung goes True.
Target node
receives packet.
Target node processes packet
successfully and returns data
(read) or writes data (success).
Bit 15 EN
Enabled
Bit 10 EW
Enabled and Waiting
Bit 14 ST
Start
1
0
1
0
1
0
Bit 13 DN
Done
1
0
Bit 9 NR
Negative Response
1
0
Bit 8 TO
Time Out
1
0

 
Target node
sent reply.
Control Block Status Bits

Bit 12 ER
Error
1
0

The EW bit is set (1) and the ST, DN, NR, and TO flags are cleared. If the
transmit buffer is not available, the EN flag remains false (0).
 When rung conditions go true and the transmit buffer becomes available, the
EN flag goes true (1). The EN bit remains set until either the DN, ER, or TO
bit is set. The TO bit has no effect unless the ST bit has first been set.
 If the Target Node successfully receives the MSG packet, it sends back an
ACK (acknowledge). The ACK causes the processor to clear bit S:2/7. (Bit
S:2/7 is valid for Series C discrete only). Note that the Target Node has not
yet examined the MSG packet to see if it understands your request. It is
replying to the initial connection.
At the next end of scan, the EW bit is cleared (0) and the ST bit is set (1).
Once the ST bit is set, the processor waits indefinitely for a reply from the
Target Node. The Target Node is not required to respond within any given
time frame. At this time, no other MSG instruction is serviced.
Note: If the Target Node faults or power cycles during the time frame after
the ST bit is set and before the reply is returned, you will never receive a
reply. No other MSG instructions are able to be serviced unless this MSG is
terminated in error using the TO bit. This is why it is recommended you use
a timer in conjunction with the TO bit to clear any pending instructions.
(When the TO bit is set [1] it clears pending messages.) Typically message
transactions are completed within a couple of seconds. It is up to the
programmer to determine how long to wait before clearing the buffer and
then re-transmitting.
Timing Diagram for a
Successful MSG Instruction
Programming