PROFIBUS DP overview
Page 14
Version FELD-V2002.02
PROFIBUS manual
4.4 Data
conversion
4.4.1 Description of the data formats in this manual
Data transfer for the various modes is described below. The terms PLC inputs
and PLC outputs refer to the DIGIFORCE
®
9310 unit. These terms are
reversed when referred to the master.
The function of the PLC-In / PLC-Out bits is identical to the parallel PLC I/O
ports on the unit itself and can be found from the DIGIFORCE
®
9310
operating manual.
The floating-point numbers (“float”) mentioned are 4 bytes long (32 bits) and
are based on the IEEE-754 standard (see appendix A).
Numbers that are not specifically labeled or are labeled with “d” or “dec” are
decimal numbers
. (Example: 1234, 1234dec, dec1234, 1234d.
Numbers that are labeled with “0x” or “hex” are
hexadecimal numbers
.
(Example: 0x1234, hex1234, 1234hex, 1234h)
Numbers that are labeled with “b” or “bin” are
binary
numbers
. (Example:
b1100, bin1100, 1100b, 1100bin)
4.4.2 Handling problems that arise when reading floating-point numbers
This only concerns cases in which floating-point numbers need to be read from
the DIGIFORCE
®
9310 unit (in the cyclical protocol with Profimode >1).
Floating-point numbers (data type REAL) are encoded as 4 bytes for transfer
(see chapter "PROFIBUS transfer modes cyclical protocol" on page <>). This
may create problems depending on the type of PLC used:
Cause
In the DIGIFORCE
®
9310-PROFIBUS, the sign byte is transferred first. A
Siemens S7, for example, expects this byte in the highest of the four addresses
not in the lowest address. This inevitably leads to misinterpretation of the
numeric value.
Remedial action
If a completely implausible result is obtained on decoding a floating-point
number, the problem can be overcome by changing the order of the four bytes.
Please see the hardware setting information in appendix B on page <>.
High
address
Low
address
ACT
UAL
Byte 4
Byte 3
Byte 2
Byte 1
REQ
UIR
ED
Byte 4
Byte 3
Byte 2
Byte 1
High
address
Low
address