Mode Of Operation - Siemens SIMATIC S5 Manual

1.3 Mode of Operation

As shown in the block diagram (Fig. 1.3/1), the analog input signals are switched to an analog
to digital converter (ADC) by a multiplexer. With a maximum conversion time of 80 ms, the ADC
digitalizes the input voltage using the dual slope technique. The 13 control loops (8 with Pt 100
sensors) are processed cyclically. Before the voltage is digitalized, the signal lines are checked
for line breaks and any detected faults are signalled.
A line break is recognized when the total line resistance is greater than 1 kohm or when the
transmitter (thermocouple or resistance-type sensor) has a contact resistance greater than 1
kohm compared with the reference potential. If voltage dividers or shunt resistors are used to
adapt the measuring range, no line break can be signalled. When delivered, no line break signal
is possible for channels 13 and 14.
The parameters and control commands transferred from the CPU via the data bus are stored in
a 2048 byte RAM area which is divided into 64 messages each 32 bytes long. The module
occupies 32 bytes in the address area of the PLC.
A microprocessor controls the functions of the controller module according to the firmware
which is stored in a 64 Kbyte EPROM.
The calculated manipulated variables are output in digital form (pulse-duration modulated) to
output drivers via an output register. If the 5 V power supply fails, the module is reset.
If the 24 V load voltage fails, the module continues to operate; the digital outputs can, however,
no longer be activated. The NAU (power failure) signal is not evaluated by the module.
The digital outputs can be disabled with the BASP signal, the content of the registers is retained
(see Section 3.4.4 BASP Evaluation).
The controller's integrator values are buffered if the IP 244 controller module is inserted in a slot
with battery back-up (UBATT). The presence of battery back-up is detected by the module
automatically.
IP244
C79000–B8576–C859–02
Technical Description
2–5