Using the Message Instruction
13–9
If the Tar
get Node successfully receives the MSG packet, it sends back an ACK
(an 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 Tar
get 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 will wait indefinitely for a reply from the Target
Node. The Target Node is not required to respond within any given time frame.
During this time, no other MSG instruction will be serviced.
Note If the Tar
get Node faults or power cycles during the time frame after the ST bit is
set and befor
e the r
eply is r
eturned, you will never r
eceive a r
eply
. No other
MSG instructions will be able to be serviced unless this MSG is terminated in
err
or using the
TO
bit. This is why it is r
ecommended 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 ar
e
completed within a couple of seconds. It is up to the pr
ogrammer to determine
how long to wait befor
e clearing the buffer and then r
e-transmitting.
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) or no response at all is received. When this happens, the
ST bit
remains clear
. A NAK indicates:
• the Tar
get 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 at the next end of scan. (Note that
the NR bit will only be set for DH-485 and NAK conditions. An error code
02H, Tar
get 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 Tar
get 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 T
ar
get Node sends one of three responses shown in
Step 6.
Programming
efesotomasyon.com - Allen Bradley,Rockwell,plc,servo,drive
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