Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 82)

Message Procedure Commands
Chapter 6
6Ć4
Normal
Priority
If you use the less–than sign (<) with the assignment command, the
command will generate a priority Data Highway message. Without the
less–than sign, the assignment command will generate a normal Data
Highway message.
For example, the statement
#H027$I15:4<=$I12:24
transmits a priority message to Data Highway station 27 (octal).
The priority modifier can be used with either type of assignment (=or==).
Important: Stations with high priority messages are given priority over
stations with normal priority messages throughout the command/reply
cycle. For this reason, a command should be given a high priority
designation only when special handling of specific data is required. Using
an excessive number of high priority commands defeats the purpose of
this feature and could delay or inhibit the transmission of normal priority
messages.
Command Message Type
Command messages are of two types:
protected
unprotected
As explained in section titled Data Transfers, (chapter 3), protected
commands can access only specified areas of data table memory at a
PLC/PLC–2 station. You will need to send a protected write command
only if a switch at the remote PLC/PLC–2 prohibits other stations from
sending unprotected write commands. Unprotected commands can access
any area of the data table.
By default, command messages generated by the assignment command in
PLC–3 message procedures are of the protected type. To generate an
unprotected command message, use a blank space and the modifier U
after the assignment command.