Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 159)

Network and Application Layer Protocols
Chapter 12
12Ć12
For level: The default address is:
1 3 (data table)
2 1 (Current context)
All others 0
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 2–byte field.
Any 2–byte fields should be coded low–byte–first.
In Figure 12.5, 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.
In Figure 12.5, the level–4 address is 260 (decimal), which is too large to
fit in one byte. Therefore, a byte of all 1’s is used to delimit the 2–byte
address field for this level. The value 260 is then coded low–byte–first.
Note that the last level (level 6 in this case) must be specified in the
address field even though it is equal to the default value of zero.