Industrial PC offers deterministic "slot PLC"

Siemens has tackled the dilemma of how to provide deterministic control on PCs by offering a slot-in hardware PC card containing a PLC.

The attraction of using a PC as a "soft PLC" is that PCs are usually faster than PLCs and can manipulate large amounts of control data. The drawback is that most soft PLCs rely on Windows which is not a "hard real-time" operating system such as those found on PLCs. In other words, Windows is "non-deterministic."

Some soft PLC manufacturers have written modifications to Windows to provide hard real-time facilities, but many shun this approach because it leaves them dependant on Microsoft. Others write a real-time operating system (RTOS) that loads up before Windows, and allows Windows to run only as a task under its control.

An alternative to writing software, which is expensive, is to provide a dedicated hardware PLC with its own RTOS. Siemens has opted for this approach with the launch of its latest Simatic industrial PCs.

One new model is the Simatic Box PC 620 which incorporates Profibus-DP and Ethernet interfaces, along with two spare slots for user expansion. The PC, which offers the option of either an Intel mobile Celeron or Pentium II processor, can be installed in control desks or directly onto machines.

Another new model, the Simatic PC R145 is fitted with a Pentium III processor as standard. In addition to Profibus and Ethernet communications, five PCI and three ISA slots allow for further connection to plant devices.

A third arrival, the Simatic Panel PC 670, incorporates Pentium II or Celeron processors and has Ethernet and Profibus interfaces integrated into the motherboard. It allows data rates of up to 12 Mbps on the Profibus-DP/MPI interface and either 10 or 100 Mbps on the Ethernet alternative.