Non-Intrusive Bus Analysis

This paper discusses the importance of non‐intrusive 1394 bus analysis tools for usage in deterministic data networks. The dynamic bus arbitration scheme in 1394 results in a changing bus architecture whenever a conventional bus/protocol analyzer is added to the bus. While certain communication protocols (SAE AS5643) have been developed specifically to address this issue the fundamental challenge still remains: How can suitable test instrumentation gather data from the bus under test without affecting the system under test itself?

As any other 1394 device, bus analyzers rely on a dual stage approach for its bus interface, i.e. physical layer device (PHY) and a link layer controller (LLC). Especially the PHY is normally realized as a standard off‐the‐shelf component. So when used within a test instrument this device will participate like any other device in the tree/identification process and therefore change the bus wide device enumeration. An example for this is shown in the left hand graphics (click to enlarge). Notice how the bus enumeration changes when the additional device (for this argumentation an analyzer) is added to the bus.

If not handled appropriately this new enumeration can have significant effects on the asynchronous packet transmissions. Due to the absolute addressing scheme of this transmit method the packets would be sent to completely other nodes of the bus. This is illustrated in the right-hand graphic. Note that once an additional device is added only transmission T1 will reach the intended device whereas T2 and T3 would be sent to different bus nodes.

For commercial 1394 applications typical plug‐and‐play device discovery schemes have been deployed based on information contained in Configuration ROMS that can be queried remotely from any bus participant. But such schemes are often considered too elaborate, costly and unnecessary in dedicated and enclosed bus architectures as often found in aerospace and other industrial applications. Under normal operations the concept of “deterministic” nodeID assignments and therefore never changing packet addressing holds true. However, that concept falls apart and changes immediately as a new device like a bus analysis tool is added.

To overcome above challenges DapTechnology has developed a unique solution (InvisiPHYTM) that allows monitoring 1394b data transactions without actively participating in the bus arbitration. This solution has been successfully used as a front end interface (the physical layer) in data analysis instruments like bus analyzers (FireStealth®). Equally the same solution can be to build non‐intrusive flight recording equipment as often required in aerospace (and others) industries. Without actually being “connected” to the bus such an equipped instrument can be used to completely passively monitor bus activities without interfering with the bus topology and other devices being aware of its existence.