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Data Encoding and Addressing

Chapter 6

6-9

addressing in a PLC’s ladder diagram program to allow it to access its

own data table memory.

Due to the differences in PLC memory organization, the logical

addressing scheme varies for each controller type.

PLC/PLC-2

PLC and PLC-2 Family controllers access their data tables by using an

octal word address. In PLC/PLC-2 command messages, this type of

logical word address must be represented as an equivalent byte address.

This byte address appears in the 2-byte field labeled ADDR in the

message block formats (Chapter 5).

To encode a logical PLC/PLC-2 address, convert the octal word address to

whatever number system you are using in your computer application

programs. Next, double this converted word address to get the

corresponding byte address. Place the result in the ADDR field, low byte

first.

For example, to address PLC Word 020, you would first convert the octal

value 20 to the desired base. In this example, let’s use hexadecimal

values. Octal 20 is 10 hex. Doubling this value gives 20 hex for the byte

address. You would then code the value 0020 hex in the ADDR field of

the message, low byte first. In binary format, ADDR would look like:

00100000 00000000

First Byte Second Byte

Low Byte (Value 20 Hex) High Byte (Value 00 Hex)

ADDR

Important: PLC and PLC-2 Family controllers use this same logical

addressing format when they transmit command messages to another

station. If you plan to transmit a command message to your computer

from one of these PCs, you should set up a buffer space in your computer

to simulate PC memory. You would then have to write computer

application programs to accept and execute commands from the PC

stations and to translate the ADDR value into the corresponding address

in the simulated PC memory.

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Plus RS-232 Manuel d'utilisateur Page 146