Logic Analyzers Fit Small Budgets

A look at low-cost logic analyzers shows good things really do come in small packages. These PC-based instruments offer the standard inputs found on their larger bench-instrument cousins, and several include digital-output and analog I/O capabilities. Outputs can stimulate a system under test while an analog input lets designers correlate analog and digital signals. And the small instruments can accompany a laptop into the field.



The utility of these instruments, or modules, goes beyond traditional logic analysis. Todd Schreibman, vice president of sales at Link Instruments reported, “An engineer in a hydroelectric facility needed to determine when 120V relays opened and closed. Rather than analyzing fast logic signals, he had to check the slow sequencing of about 80 relays.”



At the other end of the application spectrum, Hal Newcomb, president of NCI explained, “Engineers have brought out the data and address lines in cell phones, and they capture data with our instruments as they drive around their coverage area. When they compare the acquired data with expected results, they can see if they have problems in the network or in a phone.”



These modules take advantage of the memory, disk drive, display and peripherals that come with desktop or lab PCs. So, low-cost modules need to provide only the circuits necessary to capture signals and send data to a host computer. Software—available free at vendors’ Web sites—lets engineers display, analyze and share data.



The following short descriptions of several logic-analyzer modules illustrate their capabilities. (Check manufacturers’ Web sites for complete sample-rate, buffer-size, resolution and other specifications.)



Tech Tools

The DigiView analyzer ($499) from Tech Tools (www.techtools) provides 18 digital inputs, and it compresses data and streams it to a host PC’s memory so an acquisition can run from 128K to as many as 30G samples. “Because we use dynamic data compression, users get 10-nsec resolution and long timespans without having to trade one for the other,” explained Jerry Merrill, Tech Tool’s owner. The DigiView instrument’s transitional-analysis mode saves data only when a change occurs.







When it comes to triggering, users can select a logic level or a don’t-care condition for any combination of inputs, or users can trigger on an edge at any input. Triggers also can combine signal patterns and edges. The DigiView lacks sequencing or duration qualifications for triggering, but a new instrument, available this summer, will provide those capabilities.



“People spend more time analyzing data than acquiring it,” stressed Merrill. “So they should evaluate analysis software

to determine how well it searches, pans and zooms through data. The best hardware is useless if you cannot easily analyze the data.”



Because DigiView data can expand to several-hundred gigabytes, Merrill recommended against exporting it to a program such as Excel. Instead, users can export a subset of the data as defined by time limits or cursor positions. Engineers also can write their own programs to manipulate the incremental-time and logic-signal data.



Link Instruments

(www.linkinstruments .com) offers several instruments from the LA-2124 ($800) that provides 24 digital channels to the LA-55160 ($7,500) that supplies up to 160 digital channels. The digital lines also can serve as outputs. “Users can acquire data with the logic analyzer, edit the data and then output it as patterns,” noted John Yeh, vice president of engineering at Link. The LA-5000 Series also lets developers write programs in LabVIEW or C/C++ to acquire and manipulate data.







The LA-2124 provides a single 24-bit-wide trigger, but the more expensive 5000 Series instruments offer 16- level sequential triggering across all inputs. In effect, condition A will arm condition B, which will arm condition C and so on for 16 levels. Or as part of a sequence, the trigger logic can remain in a loop until a condition occurs for a preset number of times.



Yeh added, “People often think they need a large data buffer, but if they capture too much data, it takes a lot of time to go through it and extract useful results. When they choose better trigger conditions, they will get the data they need without requiring a big buffer.”



Engineers can “cross trigger” a Link Instruments analyzer and the company’s digital storage oscilloscope (DSO) module. Then they can trigger the analyzer from the scope or vice versa. Engineers also can use the analyzer as an alternative to bed-ofnails testing while they wait to use expensive automatic test equipment (ATE).



NCI USA

The GoLogic module from NCI (www.nci-usa.com) provides as many as 72 digital channels, and the unit can operate in a transitional-timing mode that saves data only when the module senses a state change. Thus, according to Jeff Newcomb, Hardware Engineer at NCI, engineers can “store” hours’ worth of data taken in high-speed bursts, but only when changes occur.



“For a new user, triggering often causes confusion,” said Newcomb. So NCI provides a database of common triggers

such as pattern, edge and pattern-followed-by-an-edge. Users select a trigger condition and fill in the time or pattern

information. The software also lets users control “low-level” triggering, should they need this capability.



Newcomb explained that many users work with a serial bus such as I2C, so the GoLogic hardware can convert a serial

stream into a parallel format. Then, users can trigger on an address or on data as they would normally express them. Plugin software lets users view I2C data, address, start condition and other information.



When developers need a mixed-signal capability, they can use NCI’s Scope Link software that uploads data from a DSO, and displays it in the GoLogic waveform window. “Typically, engineers use the analyzer to trigger a DSO and they then upload the analog information so they can view it aligned with the digital waveforms,” noted Newcomb.



Dynon Instruments



Although the Dynon Instruments ELAB-080 provides only 16 digital inputs, it includes a two-channel DSO, an arbitrary waveform generator (ARB), a clock generator and a power source. Ian Jordan, Lead Design Engineer at Dynon Instruments (www.dynoninstruments.com), said that these days engineers must have a mixed-signal tester. They might, for example, create an analog signal, view it as it passes through a filter and then monitor digital lines to see how a DSP chip responds to the filtered signal.



Before engineers can use a logic analyzer productively, they need a sense of the patterns they want to look for. “A deep sample memory lets users look through many samples to find an event they want,” explained Jordan. “The first tests may fill the buffer memory. After the users have a better idea of what they want to find, they can set more restrictive trigger conditions and look at a specific event without having to fill—and search through—the entire buffer.”



Dynon’s analysis software saves data in a tab-delimited format, so that users can export it to a spreadsheet or database. They also can use MATLAB or Excel to generate waveform data for the built-in arbitrary waveform generator.



BitScope

BitScope Designs (www.bitscope.com) sells two basic products, the Pocket Analyzer (BS50U; $295) and the USB BitScope 310 (BS310U; $495). “If an application requires extra inputs, users can configure several instruments—other than the BS50U—to talk with one PC via Ethernet or USB,” said Bruce Tulloch, director of software development at BitScope.







They daisy-chain the trigger signal from instrument to instrument. This connection does not synchronize the clocks, however, but the clocks have accuracy sufficient for most needs.



Some BitScope software resides in the instrument, and developers can use ASCII scripts to program it. Data decimation, filtering and compression take place in the instrument prior to data transfer to a host PC. However, the spectrum-analysis software resides in the PC. “If you want to zoom in and zoom out, you need to connect the BitScope because it handles those and other functions,” explained Tulloch.



Triggering varies by product type, but engineers can set up analog triggers for edges and levels, define hold-off or hold-on periods and other conditions. Logic triggers let users set a bit pattern of up to eight bits, and digital inputs can trigger analog inputs and vice versa.



The front-panel connector on the BitScope instruments supplies eight logic inputs, two analog inputs, power, ground and serial-in and serial-out lines. Each analog or digital input has a 100 kΩ input impedance, which will not exert a large load on the circuits engineers must monitor. A separate BNC connector for the scope input operates like a standard buffered oscilloscope input with a load of 1 MΩ and 20 pF. Users can connect an X10 or an X100 scope probe to the BNC connector, just as if it were part of a bench scope.



The larger instruments in the BitScope family each provide a DB-25 connector and a cable that connects to a pod. Each pod breaks out individual signals, and the LP103W pod, for example, supplies active circuits and buffered inputs for 5V TTL and CMOS logic.







CWAV

The four logic analyzers from CWAV (www.usbee.com) range from the basic USBee SX ($295) to the USBee ZX ($895). The SX “pod” provides logic-analyzer and signal-generator functions, and the ZX pod includes those functions and others that include an SPI monitor and an I2C controller/monitor.



“When engineers need to run a test, they do not have to relearn how to use our instruments,” said Tim Harvey, owner of CWAV. “The pods stream data to a host PC, so if users choose to, they can set the buffer to the size of their PC’s main memory. That lets them save data over long periods and then later they can search for specific events. Users do not have to trade off sample rate, sample period and buffer depth.”





When a logic analyzer or other “application” runs, it loads its code into the pod which then becomes the desired

instrument. Because “soft coding” establishes operating conditions pod by pod, several pods can run independently on one PC. If required, developers can use the CWAV Tool Builder software to write their own code for a ZX module.



“We simplified the triggering so it does not scare people,” said Harvey. “They can select up to four sequential events, say, trigger on the first rising edge after a falling edge on another signal.” Four levels seem sufficient because people can save a lot of data and look for specific information after it gets transferred to their PC’s memory.



CWAV has a new product, a small plug-in logic analyzer that works with BASIC Stamp 2 (BS-2) microcontroller modules from Parallax (www.parallax.com). Priced under $100, the new USB-based analyzer provides students and technicians with a means to monitor and view the logic signals from a 24-pin BS- 2 module. The analysis software is almost identical to that in CWAV’s standard package.