Rockwell Automation 1775-S5_SR5,D17756.5.5 User Manual PLC-3 FAMILY I/0 - Manual (Page 66)

DH and DH+ Communication
Chapter 4
4-6
Ladder Program
The first link in the communication process is your ladder program. You
send a DH/DH+ command message by means of the message (MSG)
instruction (Figure 4.2).
When the rung becomes true, the processor informs the scanner to begin
sending command(s) across the link. At the same time, bits in a control
file word change their state (Table 4.B) to reflect the status of the
message instruction.
Important: Once enabled, even if the rung becomes false, the scanner
continues to send commands across the DH/DH+ network until the
message is done or errors.
Table 4.B
The Status of Bits in a Control File Word
When the: The processor:
message instruction is true sets the enable bit (16) and the latchĆenable
bit (12)
scanner receives the message instruction sets the request bit (17)
scanner begins operation sets the busy bit (14)
scanner completes operation resets the busy bit (14) and sets either the
done bit (15) or the error bit (13)
rung becomes false resets the request bit (17), busy bit (140),
enable bit (16), and the latchĆenable bit (12)
rung becomes true a second time resets the done bit (15) or the error bit (13)
message errors appear sets the error bit (13) and records the error
code in bits 00 thru 07
You can program the message instruction with either:
a single message command (see chapter 6) that can be up to 76
characters long.
the name of a DH/DH+ message procedure that contains a group of
commands and is stored in the message area of PLC-3 or
PLC-3/10 memory.