Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 47)

Data Highway Communication
Chapter 3
3Ć2
A command message either gives (writes) data to, or requests (reads)
data from, one station to another. A reply message is a station’s response
to a command message.
Command messages are generated by message procedures that you
program into the 1775–KA module. Execution of a message procedure is
controlled by the message (MSG) instruction in the PLC–3 ladder
diagram program. When a 1775–KA module receives a command
message from another station, the module automatically generates the
appropriate reply message.
As points of reference, we can talk about local and remote stations. The
local station is the one currently initiating some action, or the one we are
currently doing something with. All other stations are then remote.
We can also describe stations in terms of their relationship to a message.
The transmitting station is the one sending the message, and the
receiving station is the one that gets the message. A station that transmits
a command message is called a command station, and a station that
transmits a reply message is called a reply station.
You can send either:
a single message procedure command (Chapter 6) that may be up to 76
characters long.
the name of a Data Highway message procedure which contains a
group of commands and is stored in the 1775–KA module
You specify the station that will receive the command with a PLC–3
extended address. This address always takes the form:
E2.5.nn
where
E2 specifies that this command addresses the module status
area of PLC–3 memory
5 specifies that you are sending the message instruction
through the 1775–KA module
nn is replaced with the thumbwheel setting on your particular
1775–Ka module