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© VTI Instruments Corp. Designing Highly Efficient LXI-based Systems Tom Sarfi, VTI In strumen ts
© VTI Instruments Corp. The Case for LAN in ATE And making it viable in a T&M environment The Case for LAN in ATE Comms TypeSerial Bus Speed 12.5 MB/s (100 base-T) 125 MB/s (Gigabit)* Host Interface (Cost) LAN Port, embedded (NA) Cabling(Cost)CAT5e ($5) Max cable length100M-1kM to limitless Addressing Static IP - Soft programmable AutoIP DHCP Assigned Multi-instrument controlEthernet Switch Max # of Instruments NA Power options PoE capability Ubiquity Level Extremely High/Pervasive The Need for LXI in ATE Interoperability Conformance testing required Software drivers IVI (per specification) SCPI support (vendor dependent) Device Synchronization* IEEE-1588 (PTP) Instr-Instr Handshaking* LAN Events Wired-Trigger Bus
© VTI Instruments Corp. Test Programming Paradigm Communicating between the controller and the ATE GPIB, cabled PCIe, MXI2, Ethernet, USB Application Code C++ LabVIEW JAVA Instrument Driver (API) IVI VXIplug&play SCPI command set Hardware Driver VISA/Device Driver DMA SICL Transmission Mechanism GPIB/Word Serial DMA (PCI-PCI, PCI-VME) TCP/IP High degree of variability
© VTI Instruments Corp. TCP/IP Protocol Overhead Data transmission reliability Allowable Ethernet Transport Layers in LXI TCP – guaranteed data transmission UDP – sometimes referred to as unreliable protocol, possible but unlikely to lose data packets (LXI specification Observation) More baggage required to ensure data delivery using TCP/IP IPV6 has larger address fields IP Header TCP Header TCP Data
© VTI Instruments Corp. LAN Speed Test – Gigabit Link Characterizing the Impact of Overhead TCP Connection: All measured times are roundtrip, (includes both request and reply). Time to open TCP connection is not included. Client – Server setup Client sends a 24 byte data packet containing requested data length of reply. Server returns packet(s) with total data length of 16+requested length. Frame overhead is 54 bytes (not included in any of the above). Both NICs have TCP Offload capability. Laptop – Win7 Switch Desktop - XP Total cable length ~ 100 feet Dual Core 2.53 GHz 4 GB Ram 64 bit OS Dual Core 3.22 GHz 3 GB Ram 32 bit OS
© VTI Instruments Corp. Some LAN Speed Test Results – Gigabit Link Carry as much as you can on a trip Laptop – Win7 Switch Desktop - XP Total cable length ~ 100 feet Dual Core 2.53 GHz 4 GB Ram 64 bit OS Dual Core 3.22 GHz 3 GB Ram 32 bit OS
© VTI Instruments Corp. How an Instrument can Impact Test Time Some measurements take time Digital Multimeters Aperture times (can be 10s of ms for high accuracy measurements) Settling times associated with range and function changes Mechanical switch settling time 3 ms – 50 ms depending on application Simultaneous sampling digitizers # of acquired samples/sample rate Power Supply Settling Times Send CMD E.g. Init Measurement Instrument Action e.g, arm/trig/integrate Protocol Overhead CMD Query operation complete status Send CMD Request Response e.g; Fetch Data Protocol Overhead CMD Instrument Send Data Protocol Overhead Data In many applications, the overhead is a negligible percentage of overall cycle time Shipping more data in one packet also reduces the impact on test time Overly Simplified View of Measurement Cycle Time
© VTI Instruments Corp. Creating a More Efficient Test System Case Study - Developing More Efficient Instrument Drivers Replace a MXI2 slot 0 interface with gigabit LXI-VXI interface Enables VXI to be used in a distributed environment Simplified infrastructure, no PCI plug-in needed Drop-in replacement, no code change needed for compatibility 100 kSa/s digitizer in VXI chassis 16 simultaneous sampling channels, 128 kSa memory depth per channel (4MB total) Use soft front panel to acquire data and download to PC Initial test results were abysmal, 1.28 seconds to acquire the data, minutes to offload Looked at underlying code, VISA viIn16 calls were being used to pull data from memory The data was being sent to the PC one sample at a time! Replaced viIn16 with viMove calls – retrieve data in blocks to reduce protocol hit All data returned in less than 1 second, negligible difference between the two interfaces
© VTI Instruments Corp. Creating a More Efficient Test System Integrate a more powerful infrastructure using extended functions LXI Extended Functions
© VTI Instruments Corp. Case Study – Switch/Measure Applications Temperature/Voltage Monitoring, Continuity/Isolation Test Case Parameters Host PCDell E5520, i5 2.5 GHz Ethernet SwitchSMC8505T 10/100/ Digit DMMVTI EX1262 MultiplexerEX1202/EX (48 ch) Measurements384 DMM Aperture1.67 ms DMM FunctionFixed (DCV) DMM RangeFixed (100 V) Switch Settling5 ms (Electromechanical) 48 Analog Signals 6.5 Digit DMM 48 ch. Mux ENET Switch Host Controller INSTRUMENTATION/APPLICATION VARIABLES: DMM Aperture/Resolution DMM Function and/or range changes (delays) DMM Autozero Switch Settling Time
© VTI Instruments Corp. Case Study – Switch/Measure Software/Application code pacing Send Command Close ChN Wait for Switch Settle Send DMM Measure Command Poll on DMM Measure Complete (or wait) 5 ms 1.67 ms Variable Actual Measured: Protocol overhead carried for every switch measure loop Removing all variables, best case scenario 1 / (1.67 ms + 5 ms) * 384 = 2.56 seconds 150 readings/second Send Command Open ChN Variable
© VTI Instruments Corp. Creating a More Efficient Test System Integrating Intelligent Instruments Integrated Switch/Measure into one instrument Modular DMM and Multiplexer Scanning sequence can be built using graphical interface, or through API Scan list resides in on-box memory On-box processor paces the sequencing The only host intervention that is required is to initiate the scan and retrieve the data Internal analog bus reduces external wiring Actual Measured: 150 rdgs/s There is minimal impact due to protocol overhead
© VTI Instruments Corp. The LXI Wired Trigger Bus (WTB) Extension Emulating modular architecture backplanes Analogous to VXI and PXI trigger bus Familiar hardware-based model for triggering and handshaking between discrete modules Eight (8) mLVDS lines with wired-or operation Supports segmented trigger bus lanes Systems can extend beyond the eight trigger lines Separate scan list in DMM and Switch Sequencing paced by trigger pulses passed across WTB when measurement is complete or switch has settled The only host intervention that is required is to initiate the scan and retrieve the data LXI0 = MeasCompl -> advances to next element in switch scan list LXI1 = SwitchSettle -> triggers DMM to take measurement WTB Termination Actual Measured: 145 rdgs/s Again, there is minimal impact due to protocol overhead
© VTI Instruments Corp. LXIsync LAN Event Triggering Peer-Peer messages over CAT5e LXI LAN Events Peer-peer multicast architecture LAN messages are sent over the wire as triggers DMM sends Measure Complete event LAN0 which the switch uses to advance scan list Switch sends Switch Settled event LAN1 which the DMM uses to initiate a measurement LAN signals are converted to backplane signals within each instrument IEEE-1588 implementation Events are sent with timestamps, each instrument uses the timestamp as the trigger event Synchronization typically around 10s of us Initial results were TERRIBLE! (Lesson learned – l started the test right after powering up the box – did not give the two instruments a chance to become synchronized) Actual Measured: 92 rdgs/s Processing LAN events/implementation impacting efficiency
© VTI Instruments Corp. Comparing Results Choose the best fit for the application Method (Electromechanical, 5 ms settle) Measured Software Pacing63 rdgs/sec Integrated/Internal Scan Engine150 rdgs/sec LXI WTB Handshaking145 rdgs/sec LXI LAN Event Handshaking92 rdgs/sec Theoretical Max150 rdgs/s Method (Solid-State, 0.2 ms ON time) Measured Software Pacing60 rdgs/sec Integrated/Internal Scan Engine535 rdgs/sec LXI WTB Handshaking522 rdgs/sec LXI LAN Event Handshaking123 rdgs/sec Theoretical Max535 readings/s Determinism Software PacingBelow Average Integrated/Internal Scan EngineExcellent LXI WTB HandshakingExcellent LXI LAN Event HandshakingAverage Resolution of WAIT statement
© VTI Instruments Corp. Summary TCP/IP protocol carries overhead with every message sent over the wire Testing shows about 300 us per message Using UDP can reduce this by an order of magnitude, but data delivery not guaranteed For many applications, this can be imperceptible Smart instruments minimize host intervention Instrument driver structure/application code can have a significant impact Incorporate the most efficient programming model available LXI extended functions can be used to overcome the impact of overhead LXI WTB offers backplane-like performance LXI LAN Events can be efficient for distributed applications LXI extended functions can be used to overcome the impact of overhead LXI-based test systems can be architected for efficiency
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