Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 147)

Data Encoding and Addressing
Chapter 6
6-10
PLC-3 and PLC-5
PLC-3 and PLC-5 controllers use a form of logical addressing known as
“extended addressing.” With extended addressing, you specify the
address for each level (or subdivision) of PLC-3 or PLC-5 memory, down
to the smallest subdivision you want to access.
With a PLC-3, you can use this method to specify up to 6 levels of
extended addressing, which is enough to address any particular word in
PLC-3 memory. With a PLC-5, you can specify up to 4 levels of extended
addressing, which is enough to address any word in PLC-5 memory.
To send a command message to a PLC-3 station, you would put the
extended address in the field labeled “PLC-3 Extended ADDR” in the
message block formats (Chapter 5). Figure 6.7 shows an example of how
to enter a PLC-3 extended address in logical binary format in this message
field.
The first byte in the extended address field is a set of bit flags that indicate
which addressing levels are specified in the bytes following the bit flags.
If a flag bit is set to 1, the address bytes must contain a specification for
the corresponding level of the extended address. If a flag bit is zero, the
address bytes should not contain a specification for that addressing level;
instead, a default value is assumed.
For Level: The Default Address Is:
1
2
All Others 3 (Data Table)
Current Context
0
You must always specify the value for the last level of the desired
extended address, even if it is the default value.
If the address values can be specified in one byte each, then you can code
the values directly. If it takes two bytes to specify an address, then you
must use a delimiter byte of value FF hex before each 2-byte address.
Any 2-byte value should be encoded low byte first.
In Figure 6.7, the first byte contains the bit flags to indicate which
addressing levels are specified. In this example, only Levels 3, 5, and 6
are specified; default values are used for the other levels.