Real-time systems are defined as systems in which the correctness of the system depends not only on the logical correctness of the computation, but also on the time at which the result is produced. Honeywell's choice of the Time Triggered Protocol (TTP) for a low cost avionics bus in 2001, exemplifies the industry trend to using Commercial Off The Shelf (COTS) components to implement real-time systems. The driving factors behind the trend are the introduction of SoC devices (Systems on a Chip) that contain processors, memory, network access, I/O interface, system and application software, and smart MEMS transducers which contain the sensing element, the signal conditioning logic and network access logic on a single silicon die. In this paper, we compare and contrast TTP, FlexRay and MIL-STD-1553 architectures in terms of the basic requirements for a systems bus, namely, network architecture, bus access, message formats, clock synchronization, fault tolerance, error detection and tool support.