Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 223)

13–8 PLC Addressing
Publication 1770Ć6.5.16 - October 1996
PLCĆ3 Symbolic Addressing
Symbolic addressing uses ASCII symbols to represent a logical
address. Before using a symbolic address in a message, you must
first define the symbol at the PLC-3 processor that is to receive the
message. (For more information, refer to your PLC-3 user manual.)
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.
This module Accepts
1775ĆKA ASCII symbols
1775ĆS5, ĆSR5 • ASCII symbols
• logical ASCII
All revision levels of 1775ĆS5, ĆSR5 accept PLCĆ5 type reads and type
writes. 1775ĆS5, ĆSR5 series A, revision E or later can also use logical
ASCII with a PLCĆ3 word range read or word range write.
Example of logical ASCII:
hex byte hex
byte
ASCI
I
00 $N0:0 00
To use a symbolic address in a command message, encode the
symbol in the field labelled ASCII symbol in the command message
formats. (See Chapter 7, “Communication Commands.”)
The message formats show a byte of value zero 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: Only PLC-3 controllers can transmit commands that
contain symbolic addresses. If you plan to transmit this
type of command message to your computer from a
PLC-3 node, you must write computer application
programs that are capable of accepting these commands
and interpreting the symbolic addresses.