Rockwell Automation 1775-S5_SR5,D17756.5.5 User Manual PLC-3 FAMILY I/0 - Manual (Page 87)

Addressing DH and DH+ Data Transfers
Chapter 5
5-13
Formatting an Assignment Command
The equal sign (=) is the assignment command. The destination is on the
left of the equal sign, and the source is on the right. When the scanner
executes an assignment command, it assigns or copies the source value to
the destination. For example:
$I12:024 = US_5
The scanner copies the value of user symbol US_5 into word 24 (octal) of
input file 12 (see “Using Symbols,” page 6-4).
Available Modifiers
You can add several modifiers to the assignment command to change the:
scope of the assignment
priority level of a DH message
type of command message transmitted
Scope of the Assignment
If the destination for an assignment command is a user symbol, then you
can use a double-equal sign (==) in place of the single equal sign to specify
that the user symbol be interprocedural in scope. The single equal sign
defines a procedural user symbol. For example:
US_2 == 6
The scanner defines US_2 to an interprocedural user symbol and assigns it
the value 6 (see “Using Symbols,” page 6-4).
Message Priority (DH only)
If you want to give priority to a DH message, you can use the less-than
sign with the equal sign (<=). Without the less-than sign, the assignment
command generates a normal message. For example:
#H027$I15:4 <= $I12:24
The scanner transmits a priority message to station 27 (octal).
You can use the priority modifier with either type of assignment (= or ==).
Important: For DH communication, stations with high priority messages
are given priority over stations with no-priority messages throughout the
command/reply cycle. For this reason, you should only give a
high-priority designation to a command 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 no-priority messages.