Page 65 of 82
Date: 27.01.06
8.2.2
Acyclic communication (DP-V1)
8.2.2.1
General information
With PROFIBUS DPV1, a master can use acyclical bus access to access
individual device parameters for reading or writing new values for the parameter.
To do this, it must be possible to address these parameters precisely so that the
master knows how it can access a particular parameter. This information is
contained in the slot-index directory, which is a table showing where the various
device parameters are located.
The individual device parameters are addressed in two stages: via the
slot
(
≈
chapter) and the
index
(
≈
entry in this chapter). Another simple analogy is street
and house number. The street name on a letter is used to fix the city area and
approximate neighborhood, the house number can be used to find exactly the
house concerned. The destination can be identified by combining these two
pieces of information.
Exactly the same process applies with PROFIBUS DPV1. For example, the entry
"Input range of instrumentation amplifier" is located in slot 1 / index 1, This
information can be used to access this device parameter. The value of the
parameter can be read or its value changed by writing a new value to it. More
information is still required in order to be able to read and interpret the addressed
value properly:
What sort of variable type is this value?
(
how many bytes must be read, how
should these bytes be interpreted? (There is no point e.g. reading just 3 bytes of
a 4-byte floating point number, because the numerical value cannot be computed
from this)).
How many bytes can be read from/written to at this address?
How can this value be accessed?
(
rw
(read&write): the value can be read, and
a new value can also be written,
ro
(read only): the value can only be read,
wo
(write only): the value can only
be written).
What are valid values for this entry and what do they mean?
(e.g. for slot1/index2
"Excitation" a value of 8 cannot be entered because this excitation voltage does
not exist. Even if the value '3' is read, this by itself is of no use: one also needs to
know that this '3' means “Excitation = 10 V”)
Additional information:
For a slot/index labeled “
EVENT!
”, the action specified in the table is initiated by a
write access of any U8-byte to this slot (or index).
Values are transferred in Motorola format (Most Significant Byte first) in hexadecimal
notation.
Should this byte order need changing, one simply needs to set the LSB of the 4th
byte of the user-param to 1 at parameterization.
Floating-point numbers (float) are transferred as 32-bit floats (4 bytes) as per
IEEE-754. They are referred to below as REAL32. Appendix A contains a key to
the coding of this number format and a quick guide to converting the 4 bytes into
a floating-point number by bit manipulations.
