Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 86)

Message Packet Formats
Chapter 5
5-7
ADDR
The ADDR (address) field contains the address of a memory location in
the command executor. ADDR specifies the address where the command
is to begin executing. For example, if the command is to read data from
the command executor, ADDR specifies the address of the first byte of
data to be read.
Note that the ADDR field specifies a byte address, not a word address as
in PC programming. Chapter 6 explains how to convert PC word
addresses to byte addresses.
SIZE
The SIZE byte specifies the number of data bytes to be transferred by a
message. This field appears in read commands, where it specifies the
number of data bytes that the responding station must return in its reply
message. The allowed value for SIZE will vary with the type of
command, as indicated in the command formats below.
DATA
The DATA field contains binary data from the application program. The
number of data bytes in a message depends on the command or function
being executed, as indicated in “Message Formats.” Chapter 6 gives some
details on the format for the data field.
This section presents the detailed message formats for each type of
command and reply message. PLC-5 processors cannot be interfaced
directly with the Data Highway; they work only on Peer Communication
Link (PCL) networks. To access a PLC-5 from a Data Highway station,
join the two networks with a 1785-KA PLC-5/Data Highway adapter
module. In this manual, the command message formats are discussed in
the same order as Table 5.A: Message Formats