Precision Time Protocol Industry Profile
Industrial automation systems consisting of several distributed controllers need a precise synchronization for commands, events and process data. For instance, motors for newspaper printing are synchronized within some 5 microseconds to ensure that the color pixels in the different cylinders come within 0.1 mm at a paper speed of some 20 m/s. Similar requirements exist in high-power semiconductors (e.g. for converting between AC and DC grids) and in drive-by-wire vehicles (e.g. cars with no mechanical steering wheel). This synchronisation is provided by the communication network, in most cases Industrial Ethernet. Many  ad-hoc synchronization schemes exist, so IEEE published a standard Precision Time Protocol IEEE 1588 or "PTP", which allows sub-microsecond synchronization of clocks. PTP is formulated generally, so concrete applications need a stricter profile. In particular, PTP does not specify how the clocks should operate when the network is duplicated for better resilience to failures. The PTP Industrial Profile (PIP) is a standard of the IEC 62439-3 that specifies in its Annex C two Precision Time Protocol IEEE 1588 / IEC 61588 profiles, L3E2E and L2P2P, to synchronize network clocks with an accuracy of 1 μs and provide fault -tolerance against clock failures. The IEC 62439-3 PTP profiles are applicable to most Industrial Ethernet networks, for synchronized drives, robotics, vehicular technology and other applications that require precise time distribution, not necessarily using redundant networks. The IEC 62439-3 profile L2P2P has been adopted as IEC/IEEE 61850-9-3 by the power utility industry to support precise time stamping of voltage and current measurement for differential protection, wide area monitoring and protection, busbar protection and event recording. The IEC 62439-3 PTP profiles can be used to ensure deterministic operation of critical functions in the automation system itself , for instance precise starting of tasks, resource reservation and deadline supervision. The IEC 62439-3 Annexes belongs to the Parallel Redundancy Protocol and High-availability Seamless Redundancy standard suite for high availability automation networks. However, this specification also applies to networks that have no redundancy and do not use PRP or HSR. Topology. The PIP relies on the IEEE 1588 topology, consisting of grandmaster clocks (GC), ordinary clocks (OC), boundary clocks (BC), transparent clocks (TC) and hybrid clocks (HC = TC&OC). For redundancy, a PIP network contains several clocks that are master-capable. Normally, the best master clock ensures that  only one grandmaster broadcasts the time. In redundant networks, and especially in PRP, several masters can be active at the same time, the slave then chooses its master. Main features. IEC 62439-3 Annex C uses the following IEEE Std 1588 options: Performance. IEC 62439-3 Annex C aims at an accuracy of better than 1 μs after crossing 15 bridges with transparent clocks. It assumes that all network elements (bridges, routers, media converters, links) support PTP with a given performance: By relying on these guaranteed values, the network engineer can calculate the time inaccuracy at different nodes of the  network and place the clocks, especially the grandmaster clocks suitably. IEC TR 61850-90-4 (Network engineering guidelines) gives advice on the use of IEC/IEEE 61850-9-3 in substation automation networks. IEEE 1588 settings. IEC 62439-3 Annex C restricts the parameters of IEEE Std 1588 to the following values: Additions to IEEE Std 1588. IEC 62439-3 Annex C specifies requirements in addition to IEEE 1588: Standard owners. This protocol has been developed by the IEC SC65C WG15 in the framework of IEC 62439, which applies to all IEC industrial networks. To avoid parallel standards in IEC  and IEEE in the field of grid automation, the L2PTP profile specific to grid automation previous IEC 62439-3 Annex B has been placed under the umbrella of the IEC&IEEE Joint Development 61850-9-3. Technical responsibility rests with IEC SC65C WG15, which is committed to keep the IEC 62439-3 profile L2P2P and IEC/IEEE 61850-9-3 aligned.