Allen-Bradley 1779-KFMR User Manual page 20

node. The local node interface then verifies that the data it received is
identical to what it had transmitted (Figure 2.8).
Figure 2.8
Write Verified Message Transfer
Command Message - Block of data
to write into host at Node 2
Node
1
Reply Message - Status of whether
the write was successful
Command Message - Read
from host at Node 2
Reply Message - Block of data read
from host at Node 2
A write with no reply takes the least amount of time. A write verified
takes the most time.
All messages are transmitted across the Data Highway II link with
Manchester encoding and CRC-16 error checking to help ensure data
integrity before the interface writes data into its host's memory. A write
verified command may provide added security in applications where the
level of electrical noise is very high. However, you should be aware that
the interface can report a false error in reply to a write verified command.
For example, after the data is written into the remote host's memory, it
could be overwritten by the program in the host or by another Data
Highway II node before it could be sent back for verification. If you use a
write verified command, you must ensure that the data is not overwritten
before it can be sent back for verification.
Chapter 2
Introducing the 1779 KFM Interface
Node
2
2 13