Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 155)

Data Encoding and Addressing
Chapter 6
6-18
The symbolic address field can be from 1 to 8 bytes long. The first byte
contains the ASCII code for the first character in the symbol name, the
second byte contains the ASCII code for the second character, and so on.
If the symbol name is more than 8 characters long, encode only the first
8 characters.
To use a symbolic address in a command message, encode the symbol in
the field labeled “ASCII symbol” in the command message formats
(Chapter 5). Note that the message formats show a byte of value zero (0)
before and after the symbolic address field. You must include these zero
bytes because they act as delimiters to distinguish the symbolic address
from other fields in the message.
Important: PLC-3 controllers can also transmit commands that contain
symbolic addresses. If you plan to transmit this type of command
message to your computer from a PLC-3 station, then you must write
computer application programs that are capable of accepting these
commands and interpreting the symbolic addresses.
Logical ASCII Addressing
Logical ASCII addressing is supported only by PLC-5 controllers. ASCII
addressing allows you to specify an address in the same form that you use
at the programming station, and lets any device communicate with any
other device, without knowing the internal memory structure of the target
device.
A logical ASCII address starts with an ASCII NULL (0) character and a
dollar sign ($) to differentiate it from the symbolic form. The rest of the
address then follows, as a string of ASCII characters. The string is
terminated with another NULL character. See the example in Figure 6.9
in this chapter.