Principle Of Operation - Siemens S5-155U System Manual

923A Coordinator Module

Principle of operation

Bus Enable for:
CPU 1
2us
CPU 2
CPU 3
CPU 4
Bus Lock
Reset
Figure 6-5 Timing Sequences of the Bus Control Signals
6 - 18
Bus arbitration
The COR 923A cyclically allocates a bus enable signal to each of the
two to four CPUs in the S5-135U/155U PLC. Only during this time
can the relevant CPU utilize the common S5 bus.
The assignment of bus enable signals takes place in time-division
multiplex operation. On the COR 923A, you set the number of CPUs
with jumpers. The enable time for access to the S5 bus is preset at 2 µs
for all CPUs. If the bus enable signal has been emitted by the
coordinator for a CPU, this CPU can extend the enable time with the
bus lock signal. However, the user has no influence on this.
The bus enable assignment sequence begins with CPU 1 after the
Reset signal is removed by the power supply, and enables the CPUs
in the following order according to the preset number of CPUs:
CPU 1, CPU 2, CPU 3, CPU 4, CPU 1, CPU 2 etc. (see Figure 6-5)
2us +
Bus Lock
CPUs in Operation
Time
System Manual
C79000-B8576-C199-03