Technical data

Format of controller inputs
and outputs
Internally, the PID control algorithm uses the floating point format for
numerical representation and can be supplied with floating point
values. You can also supply the PID controller algorithm using the
normalized fixed point format (see bits 6 and 7 in the control word
STEU). In this case, the controller automatically converts the words to
the floating point format with every call.
Adaptation of words from the input and output modules in the STEP 5
program is faster if you use the normalized fixed point format (see
table at the end of this section).
Inputs You can input W, YH, X, Z and XZ as floating point or normalized
fixed point numbers. Different memory cells are reserved for each
variable in the data transfer block.
Input as normalized fixed
point numbers
(For an explanation of the normalized fixed point numbers, see the
table at the end of this section).
Note
While keeping within the nominal input ranges of the analog
input modules, do not forget that the bit pattern for a certain input
value is different from when you use the full input range. This is
particularly important when you adjust the setpoint. Otherwise, it
is possible that a setpoint input at the PG cannot be reached
although the actual value is far higher than the desired value.
If your analog-to-digital converter supplies negative numbers as a
number and sign, the 2’s complement of this number must be formed
before it is transferred to the controller DB. Following this, the binary
digit 15 must be set to 1.
If the number -0 is possible as a number and sign in the following
format:
1000000000000000
in your analog-to-digital converter, the 2’s complement must not be
formed. The number must be transferred to the controller DB as +0:
0000000000000000
Output The controller output YA exists in the DB as a normalized fixed point
number and a floating point number. Taking into account the input
and output modules used (analog-to-digital converter,
digital-to-analog converter) the format must be converted for
normalized fixed point inputs and outputs before and after the
controller is called in the STEP 5 user program before values are
transferred to or from the controller DB.
OB 250/251: Closed-Loop Control/ PID Algorithm
CPU 928B Programming Guide
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