Rockwell Automation 1771-KA2 Communication Adapter Module - Manual (Page 59)

Communication Zone Rungs
Chapter 5
5-7
For practical reasons, do not exceed the display area of the programming
terminal when entering these rungs. You can program more than one
memory access rung if needed. Note, however, that should multiple
memory access rungs be required, you must enter them in succession in
the communication zone, immediately following the header rung and
before any command rung.
The command rungs direct the data transfer operations of the
communication adapter module. The command rung lists the type of
command and the memory areas affected and allows command execution
to be initiated in the user program.
There are 2 basic command rung formats that differ only in terms of the
unit of memory which they control. The basic command rung formats
are:
Word command format
Bit command format
Use the word command format for commands that transfer one or more
data table words between stations. These are unprotected write, read, and
protected write commands.
Use the bit command format for commands that control, from one station,
one or more data table bits at another station processor. These are
unprotected bit write and protected bit write commands.
In both formats the command rung begins in a similar manner. (Refer to
Figure 5.5). The first rung element is an EXAMINE ON instruction,
addressing the START bit. The second rung element, termed the
command code, tells the remote station number, type, and priority of the
command. (Section titled “Message Priority,” chapter 8, describes
command priority.) For most commands, normal priority is preferred.
Command Rungs