Synchronization Methods

The PXI and PXI Express chassis feature a dedicated synchronization bus integrated into the backplane. NI-XNET products support use of this bus to synchronize with other NI hardware products such as DAQ, IMAQ, and NI Motion Control. The PXI synchronization bus consists of a flexible interconnect scheme for sharing timing and triggering signals in a system.

Local Time

The module uses PXI_Clk10, a 10 MHz PXI backplane clock, to drive the local time keeper and to synchronize with other modules in the PXI chassis. If the PXI backplane clock is not available, the module uses an internal oscillator.

PXI_Clk10 provides frequency but not date/time information. When an NI-XNET session is created, XNET initializes the date/time information for the local clock using host time.

Network Time

In addition to the local time keeper, the module can maintain network time (IEEE 802.1AS) for each port. When Ethernet frames are received, each packet is timestamped with network time as well as with local time.

When a port acts as a master in an electronic control unit (ECU) network, the network time is initialized from host time and is synchronized to local time.

When a port acts as a slave in an ECU network, local time and network time may eventually drift relative to each other. The date/time information for network time is obtained from the ECU that acts as the grandmaster clock.

Both local and network time can be adjusted using the NI-XNET API.

Host Time

Host time is the clock of the operating system on which the NI-XNET driver is running. The host time can obtain time/date information using a real time clock (RTC) or a network time protocol (NTP) server.

Although host time provides accurate date/time information, the accuracy and resolution of its clock can often be in tens of milliseconds. In contrast, the module provides resolution for local time and network time in nanoseconds. Although local time and network time use host time to initialize their date/time information, they do not use the same physical clock as host time. Therefore, both local time and network time may eventually drift relative to host time.

PXI Triggers

Each of the eight PXI triggers on the module can be simultaneously timestamped by the local time keeper and the network time keeper for each port. PXI triggers can be used to synchronize the module time keepers with trigger events on other PXI modules.