Siemens S5-135U/155U appendix Operator functions, Faults are indicated by five small red LEDs

Multiprocessor Operation/Coordinators

Design	The COR 923C is designed as a plug-in PCB in double Eurocard format.
	Two 48-way Series 2 blade connectors serve to connect the module to the S5
	bus in the subrack.
	The front plate width is 1 1/3 standard plug-in stations.
	A mode switch with three settings is fitted in the front plate for other
	operator functions.
	Faults are indicated by five small red LEDs.
	There is a recess with cover in the upper third of the front plate. By removing
	the cover, you can operate the DIL switches to set parameters for the module.
	You can connect the COR 923C via a 15-pin front connector to a
	programmer, OP, operator control panel or the CP 530 and CP 143.

Principle of	The COR 923C cyclically allocates a bus enable signal to each of the two to
Operation	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 923C, you set the number of CPUs with DIL
	switches. The enable time for access to the S5 bus is preset at 2 ms 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 6-7)

System Manual	6-19
C79000-G8576-C199-06