Rockwell-automation 1775-KA PLC-3 Communication Adapter Module User Ma Manual de usuario descargar pdf (Pagina 159)

Network and Application Layer Protocols

Chapter 12

1212

For

level:

The default address is:

3 (data table)

1 (Current context)

All others

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.

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Rockwell-automation 1775-KA PLC-3 Communication Adapter Module User Ma Manual de usuario Pagina 159