A soft PLC that is not so soft
February 2003
PLCs, DCSs & Controllers
The subject of soft PLCs in the automation and control industry usually raises, or lowers a few eyebrows, depending on what the particular eyes have seen happen with soft PLCs. The one lobby claims that soft PLC is the only way to do large scale PLC automation these days, especially in conjunction with scada and other third party software that can directly communicate with the PLC within the PC.
The anti-lobbyists will point out the particularly undesirable reliance of the soft PLC on the stability of the PC, specifically the operating system (and there are apparently some not so stable ones out there). Is there a solution? Could one have the great features a soft PLC has to offer without the risk of the whole plant going down at the first 'fatal exception' error?
Bosch Automation Technology believes it has found just the right solution. It is a middle path of sorts, a soft PLC with a hard shell, so to say. It is a PCI card, with a Unix processor onboard that runs VxWorks, looking like a simple network card to Windows, but actually using the PC's processor and controlling the PC's resources. VxWorks is a widely adopted realtime operating system (RTOS) in the embedded industry, see www.windriver.com/products/vxworks5/#
This means that even in the case of a total breakdown of Windows NT, the safety-relevant control function remains fully operational. In addition, the PCL incorporates such PLC features as Watchdog, remnant data areas, forcing and peripheral monitoring, features which up to now were restricted to the classical PLC.
OPC, Profibus-DP, InterBus-S, CANopen, DeviceNet and ethernet TCP/IP are all possible forms of network and inter software communication. The PCL, the software-PLC from Bosch, combines the reliability and functionality of the PLC with the flexibility and open characteristics of the PC. PCL can also be programmed using the high-level language C for special assignments, such as process data processing (PDP). With up to 8192 byte I/O, 60 ms/1 k instructions processing speed and 16 KB of flags, its manufacturers say that it is unlikely that any user would reach these limits.
For more information contact Pascal Schmitz, Flexible Electronic Systems, 082 506 8312, [email protected]
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