Message Priority - Allen-Bradley 1779-KFMR User Manual

Chapter 2
Introducing the 1779 KFM Interface

Message Priority

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The two categories of message priority are:
Supervisory
Time-Critical (Time-critical commands are a subset of supervisory
commands.)
A write verified command always has supervisory priority. A command
to read an immediate-access block always has time-critical priority. For
each of the other types of commands, you can specify them to have either
time-critical priority or supervisory priority.
Every message except an immediate-access block reply has to wait until
the node gets the token before it can be transmitted. Furthermore, token
passing occurs on a regularly scheduled basis; it is not influenced by the
priority of the messages waiting to be transmitted. However, if two
messages are waiting to be transmitted when the node gets the token, a
time-critical message is given priority over supervisory messages at that
node. Although time-critical messages are given priority, a minimum
percentage of time is regularly set aside for supervisory messages so that
they get to their destination regardless of the number of time-critical
messages to be transmitted.
Time-critical messages can only be sent to other nodes within the same
Data Highway II link. However, supervisory messages can be sent to any
node in any link in a network (Figure 2.5).
In Figure 2.5, the PLC-3 processor and the computer are on a separate
link; also, the PLC-2/15 processor, the PLC-2/30 processor, the 8200
CNC, and the dumb terminal are on a second separate link. The PLC-3
processor and computer can send time-critical messages to each other but
can send only supervisory messages to other nodes on other links.
Similarly, the PLC-2/15 and PLC-2/30 processors can send time-critical
messages to each other but can send only supervisory messages to other
nodes on other links.
You are limited to transferring 21 words maximum using a read
immediate-access block command and limited to transferring less using
other time-critical commands. However, the size of a supervisory
message can be much greater, depending on the type of command. For
example, in Figure 2.5, the computer could upload the memory of any
PLC processor in blocks of 244 bytes.