Keithley Instruments
2009
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Keithley Instruments
IEEE 1588 and LXI in Your Next T&M System Design
According to Keithley Instruments, the IEEE 1588 Precision Time Protocol (PTP) is a high-precision timing protocol for synchronization used in measurement and control systems residing on a local area network (LAN). By following this protocol in test and measurement (T&M) system design, timing accuracy is in the sub-microsecond range can be implemented at a relatively low cost. Although the IEEE 1588 specification's flexibility has fostered its adoption in multiple industries, it can reduce interoperability and ease of use in some instances.
Fortunately, the LXI (LAN-based eXtensions for Instrumentation) Standard builds on many of the time synchronization features specified in IEEE 1588 in ways that are useful to T&M system integrators, particularly those building automated test systems. The LXI Standard focuses on Ethernet-based LAN systems for measurement and data acquisition instrumentation. Its base-level Class C instrument specification defines a consistent LAN implementation and a Web browser interface for instrument setup, control, and data access.
Additional capabilities are available in instruments designed according to LXI Class A and B specifications. Such instruments possess a common sense of time by using IEEE 1588 and peer-to-peer LAN messaging. Therefore, these instruments provide additional capabilities that can optimize their use in configuring T&M systems. These capabilities include:
- Measurement Data Time Stamping – When all of a T&M system's instruments share a common sense of time, data can be reliably correlated simply by comparing and sequencing time stamps.
- Synchronizing Measurement Triggers – LXI Class A and B instruments can initiate measurements or other actions at a specific time; they can also synchronize actions using peer-to-peer LAN messages. These synchronization capabilities are particularly valuable when system components are widely separated.
- Reducing or Avoiding System Latency Effects – Such instruments can achieve better real-time trigger performance than hard-wired trigger systems by using time-based triggers to avoid external latencies and compensate for internal latencies.
One of the biggest challenges system builders face is integrating the time-based functions that IEEE 1588 enables with other system functions. One simple but effective approach is to treat time-based triggers just like any other trigger source. This means any action that can be triggered by a command sent via Ethernet, or a hard-wired trigger cable, can also be triggered at a given time or in response to a LAN trigger message. Similarly, any internal function that can generate a trigger output, such as measurement complete or output settled, should also be able to generate an appropriate LAN message.
One recently developed instrument illustrates the advantages IEEE 1588 and LXI Class B capabilities can offer T&M system builders. Keithley's Model 3706 Switch System/Multimeter (DMM) for electrical testing features an on-board Test Script Processor (TSP®) that allows loading short programs, or scripts, onto the instrument itself to perform tests independent of a separate controller. This instrument's flexible event system allows a TSP script to run in response to a trigger and perform any desired combination of instrument functions. Both LAN-delivered trigger messages and time-based triggers are treated like any other triggers in the event system.
TSP scripts can also send Class B LAN messages, so any instrument event can initiate a LAN message to trigger or control other Class B instruments. The Model 3706 firmware includes standard Class B Web pages for configuring LAN networking and IEEE 1588 functions, as well as pages for instrument control and script editing.
Perhaps most important from a system integration perspective, the use of IEEE 1588 and Class B capabilities with test script processing boosts throughput significantly by eliminating controller and communications latencies. The instrument's TSP provides a general-purpose scripting language with computation and program flow control capabilities. These can be extended with Instrument Control Library (ICL) commands for executing instrument functions. ICL commands can be sent to the instrument individually from a controller, or multiple commands can be grouped into a script that's executed with a single command.
About the Author
Paul Franklin is Project Manager for Sensitive, Switching, and DMM Products within Keithly Instruments, http://www.keithley.com. He chaired the LXI Consortium's Technical Committee from 2005-2007. Before joining Keithley Instruments in 2000, he gained more than 20 years of measurement and control industry experience as an engineer and a manager with electronic controls and industrial automation firms. Mr. Franklin earned B.S.E.E. and M.S.E. degrees at Case Western Reserve University and is a member of IEEE, the IEEE-Computer Society, the IEEE-Instrumentation and Measurement Society, and the Association for Computing Machinery.
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