Plc 3 Logical Addressing - Allen-Bradley 1779-KFMR User Manual

PLC 3 Logical Addressing

Use the PLC-3 logical addressing format to send these PLC-3-type
commands:
Write Block
Read Block
Write Bit
Write File
Read File
All Time-Critical Commands
All Remote Command Control Commands
You can use this format to specify up to six levels of PLC-3 extended
addressing. Figure B.1 shows an example of the logical addressing
format for addressing a word in the PLC-3 data table.
The first field in the format contains a set of bit flags. Each flag is
associated with one of the levels of a PLC-3 extended address. If a flag
bit is set to one, there must be an address specification for the
corresponding level in the address fields that follow. If a flag bit is zero,
the address fields that follow should not contain an address specification
for that level; instead, a default value is assumed.
For Level:
You must always specify the value for the lowest level of the desired
extended address, even if it is the default value. Since the present PLC-3
recognizes a maximum of 7 levels of extended addressing, you cannot
specify more than 7 levels with the logical addressing format.
If the address fields can be specified in one byte each, then you can code
the values directly. If it takes two bytes to specify an address field, then
you must use a delimiter byte of value FF hex before each two-byte field.
Any two-byte fields should be coded low byte first.
In Figure B.1, the first byte contains the bit flags to indicate which
addressing levels are specified. In this example, only Levels 3, 4, and 6
are specified; default values are used for the other levels. This format
reduces the total number of bytes needed to specify a PLC-3 logical
address in a command message.
Appendix B
The Default Address Is: