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The Dedicated Systems Encyclopaedia offers several definitions of "real time," some of them contradictory, and there doesn't seem to be 100% agreement over the terminology. The basic definition it offers is that "a real-time system is one in which the correctness of the computations not only depends on the logical correctness of the computation, but also on the time at which the result is produced. If the timing constraints of the system aren't met, system failure is said to have occurred." The truth is in the timing constraints.
"Hence, it's essential that the timing constraints of the system are guaranteed to be met. Guaranteeing timing behavior requires that the system be predictable. It's also desirable that the system attain a high degree of utilization, while satisfying the timing constraints of the system."
Various levels of real-time operations also separate certain functionality from one RTOS to another. There is hard real time, which is essential in all mission-critical applications where missing an event (external stimulus) could be catastrophic. Soft real-time events can be missed and be recovered later. The latter might result in a loss of machine production or reduced product quality, but is not catastrophic.
Automation Solution Center at IL Solutions, Kalamazoo, Mich., provides machine-control solutions, and recommends Phoenix Contact's Steeplechase Visual Logic Control (VLC) to many machine builders. "Where the application calls for deterministic servo control, synchronized execution of control algorithms with servo position updates, dedicated closed-loop process variable control with high accuracy, real-time diagnostics, alarming and operational information, and in-process data storage, retrieval and manipulation, then a RTOS must be a key component of the solution," stresses Wayne McNeil, automation specialist at IL Solutions. An application McNeil points to is Lyle Industries' thermoformer machines. With multiple events happening in both motion and process control, an RTOS is deemed the only solution. IL Solutions selected a PC-based control platform with VLC software installed. "This solution allowed Lyle to be independent of the hardware platform from machine to machine," says McNeil. Steeplechase implements INtime RTOS from TenAsys. The INtime kernel operates underneath a Windows platform, but runs independently on the PC CPU.
An easier choice for Lyle might have been a single-brand control solution, but its system then could be inexorably tied to the pace the vendor chooses for upgrades and fixes. "It's our customers that set the pace for the future requirements of our machines," says Gary Sowden, Lyle's operations VP. "We're obligated to meet those requirements without restriction or reservation."
National Instruments offers different types of RTOS depending on hardware platforms. "We use Phar Lap EGS for x86 platforms and VxWorks for RISC-based platforms," states Jeff Meisel, NI's product manager for LabView Real Time. "Real time is essential in most event-driven applications, especially in machine control, but when the ultimate, real-time system is essential for extremely mission-critical control, we recommend field-programmable-gate-array (FPGA) technology. A FPGA platform is much easier to get certified by regulatory agencies than any intensive software platform." That's an interesting take on what truly is a hard, real-time system.