Rockwell Automation 1775-S5_SR5,D17756.5.5 User Manual PLC-3 FAMILY I/0 - Manual (Page 65)

DH and DH+ Communication
Chapter 4
4-5
In read operations, the command message requests the data transfer, but the
corresponding reply message actually contains the data being transferred.
In write operations, the command message contains the data being
transferred, and the reply message merely reports the status (receipt or
non-receipt) of the transfer.
The PLC-3 processor must be free to control its own processes at the same
time that the scanner is communicating over a DH or DH+. For this
reason, both the processor and the scanner have their own programs and
programming languages. Figure 4.2 illustrates how these two
programming levels (processor and scanner) interrelate.
Figure 4.2
Levels of Programming in DH or DH+ Communication
STAT
MSG
MESSAGE TYPE 1 STAT
10012
01
STAT
CTL = FB200:0000=200
CHANNEL: E2.3.1
EN
12
DN
15
ER
13
2) 1775ĆS5 Module
@PROC_A
Data Highway Message Procedure
PROC_A
#H024$B12:37 = 15
Message procedure command
to transmit a message to data
highway station number 24
Message instruction to
execute message procedure
PROC_A
1) PLCĆ3 Processor
Ladder Diagram Program
3) Data Highway
DST SRC CMD STS TNSW ADDR SIZE
DATA
(OPTIONAL)
Command message transmitted to station 24
Levels of Programming