Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 22)

Communication Concepts
Chapter 2
2-10
required. Using an excessive number of high priority commands defeats
the purpose of this feature and could delay or inhibit the transmission of
normal priority messages.
Command Structures
There are four basis types of command on a Data Highway network or a
standalone link:
Read
Write
Diagnostic
Mode Select
Reads
There are two types of read:
Physical
Unprotected
A physical read allows you to read any area of PC memory at a remote
station. However, a PC processor cannot originate a physical read
command; only a computer can originate a physical read.
An unprotected read can access only the data table area of PC memory.
Both computers and PCs can initiate unprotected reads.
Any read can request up to 122 16-bit words of contiguous data from PC
memory.
Writes
We can classify write commands both by their application and by their
level of memory access.
As an application issue, writes are divided between bit writes and
word writes. Bit writes allow the local station to control bits in the
data table of a remote station.
Word writes allow the local station to write up to 121 contiguous words of
data into the remote station’s memory, provided you abide by the
restrictions on memory access discussed next.