Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 53)

Data Highway Communication
Chapter 3
3Ć8
Access privileges
Not every Data Highway station can read or write to every other station.
In general, read and write access privileges depend on two factors:
type of processor at the transmitting and receiving stations
protections set at the receiving station
The rest of this section explains how these access privileges vary
according to the above factors.
PLC–3 Stations
A PLC–3 station can always read data from any major area of another
PLC–3’s memory. However, one PLC–3 station can write only to the data
table area of another PLC–3 station.
In addition, a local PLC–3 station can prevent remote PLC–3 stations
from writing to the local station’s data table by setting a memory
protection switch. At the local station, the memory protect switch can be
overridden by selecting option 4 in the Module Options Menu (section
titled Accept Writes, chapter 2) at the local station.
PLC/PLC–2 Stations
For communication with a PLC or a PLC–2 station, read, and write access
privileges depend on switch settings at that station. For an explanation of
how to set the switches for read and write access, refer to the
Communication Adapter Module User’s Manual (publications 1771–6.5.1
and 1774–6.5.8).
Accessing a PLC/PLC–2 Station
Access to a PLC/PLC–2 station also depends on the type of command
transmitted to that station. There are two types of commands:
protected write commands
unprotected read and write commands
Protected write commands can only write to specified sections of the data
table in a PLC/PLC–2 processor. Memory access rungs in the PLC/PLC–2
ladder diagram program specify where in the data table the PLC–3 can
write data.
Unprotected commands, on the other hand, can read or write to any
section of the data table at a PLC/PLC–2 station. (Again, refer to
publication 1771–801 or 1774–819 for an explanation of protected and
unprotected commands and memory access rungs.)