Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 49)

Data Highway Communication
Chapter 3
3Ć4
2. Press the message instruction key.
3. Specify message type 1.
4. Choose a control file word where status information about the
message command can be stored. In our Figure 3.1, we used binary
file 200, word 200.
Data transfers can be either solicited or unsolicited, depending on whether
they are initiated by the local or a remote station, respectively. Either type
of station initiates the data transfer by issuing a command message. If the
local station issues the command message, the corresponding reply
message is said to be solicited because the local station has solicited, or
requested, the data contained in the reply message. If a remote station
issues the command message, that message is said to be unsolicited.
For solicited messages, a local station receives data from a remote station
during a read operation. The local station sends data to a remote station
during a write operation.
For unsolicited messages, a local station receives data from a remote
station during a write operation. A local station sends data to a remote
station during a read operation.
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 1775–KA module is communicating over the Data
Highway. For this reason, both the processor and the module have their
own programs and programming languages. Figure 3.1 illustrates how
these two programming levels (processor and module) interrelate.
PLC-3 Program
The first link in the communication process is your PLC–3 ladder diagram
program. You send a Data Highway command message by means of the
message (MSG) instruction. Figure 3.1 shows a typical MSG instruction.
Levels of Programming