Siemens PROFIBUS DP-V1, Operating Instructions Manual

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Page 20 SmartLinx PROFIBUS DP-V1 – Operating Instructions A5E36197302
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Data types
The Siemens device parameters take on many values in various formats, as discussed in
the Siemens device operating instructions. For the convenience of the programmer, those
values are converted to and from 16-bit integer numbers, since those are easily handled
by most PLCs.
Integer
Integer parameter values are by far the most common. For example, parameter
3.2.6.2.
Reading (see the Parameters chapter of your Siemens device operating instructions for
details) returns a number representing the current reading (either Level or Volume,
depending on the Siemens device configuration).
Numeric values must be in the range –20,000 to +20,000 to be valid. If a parameter is
requested and its value is more than +20,000, the number 32,767 is returned; if it is less
than –20,000, the number –32,768 is returned. If this happens, increase the number of
decimal places for that parameter.
If a parameter cannot be expressed in terms of percent (example: Span), or has no
meaningful value, the special number 22,222 is returned. Try requesting the parameter in
units, or refer to the Siemens device operating instructions to understand the format and
use of the requested parameter.
Bit values
Bits are packed into registers in groups of 16 bits (1 word). In this operating instructions,
bits are numbered from 00 to 15, with bit 00 referring to the least significant bit and bit 15
referring to the most significant bit.
Floating point number
The IEEE single precision floating point standard representation requires a 32-bit word,
which is represented as numbers from 0 to 31, left to right. The first bit is sign bit ’S’, the
next eight bits are exponent bits ’E’, and the final 23 bits is fraction ’F’:
Unsigned 32-bit Integer (UINT32)
Large numbers are put into unsigned 32-bit integers. By default, they are set up so that
the first word (register) is the most significant word (MSW) and the second word
(register) is the least significant word (LSW) depending on the setting of the word order
bit.
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
MSB LSB
F F F F F F F F F F F F F F F F F F F F F F F E E E E E E E E S
31 - 9
8 - 1 0