Shige's Coffee Break

My fourth essay is about an original Yokogawa technology – the pair & spare controller architecture in a field control station (FCS). This is the foundation for the seven-9s (99.99999%) availability of our CENTUM VP integrated production control system. Look at the architecture of an FCS:

The yellow areas show two redundant FCS processor modules. Each of these processor modules has two CPUs, for a total of four. This is the core of our pair & spare technology.

When the processor module on the left (CPU 1 and 2) is in operation, the one on the right (CPU 3 and 4) is on standby. CPU 1 and 2 simultaneously receive input signals, carry out computations and compare the resulting data, and output this back to the control buses and the field I/O bus if their computation results are identical. The CPUs carry out these operations continuously. But should the computation results from the two CPUs not match, the left module classifies it as a failure and its operation is handed over to the right module. It is significant that CPU 3 and 4 in the right module have also been receiving the same signals from the control buses and the field I/O bus while on standby. In other words, all four CPUs are carrying out exactly the same operations at all times. The switchover of operation from the master to the standby module is done without delay, and the switchover time has been reduced to almost a negligible level.

Furthermore, as you can see in the diagram, our FCS has no single point of failure. The dual redundant configuration applies not only to the processor modules but also to the power units, control bus interfaces, and field I/O interfaces. This design philosophy is applied throughout our system, including the human machine interfaces and fieldbuses.

The pair & spare architecture is also used in the ProSafe-RS safety instrumented system. When it was first released to the global market, ProSafe-RS already had TÜV safety certification. It created quite a stir as no company had ever obtained TÜV safety certification as planned and in time for the release of a completely new system. In so doing, Yokogawa defied what had been established wisdom in the safety business world. And this field-proven pair & spare technology is the key to this remarkable achievement.