Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 217)

13–2 PLC Addressing
Publication 1770Ć6.5.16 - October 1996
PLC-2 and 1774-PLC processors support logical and physical
addressing:
For information on See page
PLCĆ2/1774ĆPLC logical addressing 13-2
PLCĆ2 physical addressing 13-3
1774ĆPLC physical addressing 13-3
PLCĆ2/1774ĆPLC Logical Addressing
PLC-2 and 1774-PLC processors access their data tables by using an
octal word address. For PLC-2/1774-PLC command messages,
you must put the equivalent of this address in the 2-byte message
packet field labeled ADDR. To encode a logical PLC-2/1774-PLC
address:
1. Convert the octal word address to the number system you are
using in your computer application program.
2. Double this converted word address to get the corresponding byte
address.
3. Place the result in the ADDR field, low byte first.
For example, to address PLC-2 word 020, you must first convert the
octal value 20 to the desired base. In this example, we use
hexadecimal values. Octal 20 is 10 hex. Doubling this value gives
us a value of 20 hex for the byte address. You then code the value
0020 hex in the ADDR field of the message, low byte first.
In binary format, ADDR looks like this:
0 0 0 1 0 0 0 0
First Byte
0 0 0 0 0 0 0 0
Second Byte
Low byte
(value 20 hex) High byte
(value 00 hex)
Important: 1774-PLC and PLC-2 family controllers use this same
logical addressing format when they transmit command
messages to another node. If you want to transmit a
command message to your computer from one of these
PLC processors, set up a buffer space in your computer
to simulate PLC-2/1774-PLC memory. You must then
write computer application programs to accept and
execute commands from these nodes and to translate the
ADDR value into the corresponding address in the
simulated memory.
PLCĆ2/1774ĆPLC
Addressing