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LA-CRO C’est Magnifique It’s a CRO, signal generator, frequency analyser, logic analyser, chart recorder and more, all in one compact package. By PETER RADCLIFFE* Electronics project work can be very frustrating for many students due to a lack of test equipment. Unfortunately, the simple solution of buying lots of equipment is just too expensive for many institutions, so that’s not the answer. And even where equipment is readily available, students usually cannot take it home to “play” with in their own time. At the Department of Computer Systems Engineering at RMIT (Melbourne), we graduate some 250 engineering Peter Radcliffe is a Senior Lecturer at the School of Electrical and Computer Systems Engineering at RMIT. 62  Silicon Chip students each year who must be competent in electronics design, communications sys­tems design, and of course computer systems and software design. And in common with many other institutions, we have found it a struggle to keep our high-quality project work going, in part because equipment is so expensive. More importantly, we have found that students learn much more when they can do laboratory and project work at home. They can fiddle around and try things, come back to it later if they are stuck, and spend as much time on the equipment as they wish – things that are impossible in a 2-hour lab session. Unfortunate­ly, very few students can Fig.1: this screen grab shows an AM signal from the inbuilt signal generator (green trace) and the resulting demod­ulated signal (pink trace) obtained using a diode, resistor and capacitor. Fig.2: the frequency spectrum for the AM signal (green) and the demodulated signal (pink). Note the carrier and sidebands around 10 harmonics, and the DC offset and demodulated signal around the first harmonic. afford the equipment necessary to work at home – until now, that is. My solution to this dilemma has been to create “LACRO”. Basically, the aim was to keep it as inexpensive as possible while providing many useful test instruments, all in one easily-carried package. Of course, it’s speed is limited (the sampling rates are just 1µs digital signal and 1.2µs for analog) but it’s affordable and is sufficient for most purposes. Physically, the device is built into a metal diecast case which makes for a very rugged assembly indeed. In fact, it’s virtually bullet-proof! The actual dimensions are 160 x 150 x 45mm (W x D x H), so it slots easily into a briefcase or carry bag. Power comes from a 12VAC plugpack supply. (4) A signal generator which includes both AM and FM modu­lation facilities. There are two outputs: ±1V and ±10mA. (5) A 2-channel chart recorder that saves to disk as it samples. Measurements include average, peak, RMS, frequency and period. The logic functions include: (1) A logic analyser with trigger and sampling rates up to about 1MHz. (2) A logic chart recorder. (3) A logic tester in which eight channels of data can be edited. The eight outputs can drive the circuit under test and the eight inputs captured. All 16 waveforms can then be displayed on screen. Where’s the PC? Other features As you’ve no doubt gathered by now, LA-CRO isn’t a standa­lone device. Instead, it plugs into the parallel port of a PC and works with an accompanying software program that runs under anything from Windows 3.1 to Windows 2000. In operation, the software generates virtual instrument panels direct­ly on the PC’s monitor. You don’t need fancy hardware to run the LA-CRO software and an old PC can be pressed into service if you have one spare. The minimum system requirements are a 33MHz 486 and 3MB of hard disk space. As for the RAM required, well that depends on the operating system that you’re running. The software has been designed to wring as much as possible from the hardware. The result is a unit that, although modest in speed, has a remarkable range of functions (both analog and digital). We’ll take a look at the analog functions first. These include: (1) A dual-channel digital storage CRO with a maximum reso­lution of about 1.2µs. (2) A frequency analyser, with frequency spectrum of the signal displayed in terms of harmonics of the fundamental fre­quency. (3) Frequency response analysis from about 25Hz to 100kHz. LA-CRO’s talents don’t end with the features listed above, though. There are lots of other features, as follows: (1) LA-CRO can supply +5V and -5V rails at 300mA to power circuits under test. (2) An output called “GO” goes high, from 0V to +5V, The LA-CRO circuit board is mounted on a metal diecast base, with a cover then fitted over the top. It connect to the PC via a standard printer cable. SEPTEMBER 2000  63 Fig.3: the chart recorder has been used here to capture the turn-on transients of an amplifier. Fig.5: eight user-defined outputs can be used to drive a circuit and eight inputs captured using the Logic Tester. Fig.4: this relay operation was uncovered using the oneshot storage capability of the CRO and the GO output for driving the relay. The green trace shows that the relay switched some 8ms after being energised. The coil current (red trace) took about 3ms to build and has some ripple as the armature closes against the coil. Fig.6: the digital chart recorder can set any of the eight digital outputs and record the responses of the 12 digital inputs and the X & Y inputs. Notice that the X and Y threshold voltages can be individually set by entering in the desired values, and you can set the element width and sampling rate. three samples after any CRO sampling starts. This can be used to drive devices at currents up to 300mA, including relays and small light bulbs. GO can be extremely useful when it comes to stimu­lating transient events. (3) Except for the AC/DC selection, all controls, including gain, operate from the screen. (4) In single-cycle mode, the CRO automatically sets the timebase so that exactly one cycle is displayed. (5) The data on most screens can be saved as a .CSV file that can subsequently be loaded into any spreadsheet. (6) The eight digital outputs and 12 digital inputs can be directly controlled from the LA-CRO software or from C and C++ programs that you write. (7) The signal generator can load an arbitrary signal shape from a text file. For instance, one of the examples provided with LA-CRO spells out “MUM” on the CRO! By the way, the LA-CRO Help file is quite extensive and is context sensitive. Each control is fully explained and its limitations stated. The analog portion of LA-CRO is accurate to about seven bits which is adequate for most purposes. A close inspection of the sinewave will show a little noise but the distortion is under 1%. Of course, the modest sampling speeds (1µs digital and 1.2µs analog) mean that you won’t be debugging your Pentium PC or RF circuits with LACRO. On the other hand, these sampling speeds are more than adequate for audio work and digital inter­faces like serial EEPROMs, data links and slower microprocessors. In many cases, LA-CRO may be the only test instrument you need as it can power a circuit, provide analog and digital in­puts, and then measure the analog or digital outputs and display the results on the monitor. 64  Silicon Chip External connections LA-CRO connects to external circuits using three 34way IDC connectors. The CRO inputs and signal generator outputs are most easily connected by test clips (which come with the full package). Test clips are less expensive than BNC connectors and being small, they can often connect to parts of the circuit Fig.7: the ±10mA output and the XY mode are used here to show the voltage-current characteristics of a zener diode. that would be would be impossible to reach with standard BNC leads. On the other hand, the digital inputs and outputs are prob­ably best connected using 34-way IDC cables. These can be fitted with connectors so that they simply plug into the IDC header pins, or you can solder the relevant leads to the pins. Although 34-way IDC cables can be purchased new, you can save money by purchasing then secondhand. These are the same as the floppy disk drive cables used in PCs and can often be found in the “disposals” boxes at electronics and computer stores. Alternatively, you can scrounge them for nothing from junked PCs. If you need to make up your own leads, IDC connectors are quite cheap and can be purchased in both solder and wire-wrap versions. Driving LA-CRO Connecting LA-CRO to your computer and installing the soft­ware is dead simple. The software comes on two floppy disks and is installed by double-clicking the setup icon (the procedure is a bit more complicated for Windows 3.1 but it’s all explained in the instructions). A standard printer cable is used to connect the hardware to the PC’s parallel port. Note, however, that most PCs have several EPP (enhanced parallel port) modes and not all these will work with LA-CRO. This means that, in some cases, it might be necessary to enter the BIOS setup and change the printer port setting. When you load the software, a display window appears with buttons for all the instruments. After that, you can go to the required test in­strument by clicking its button. The various screen grabs (Figs.1-9) show just some of the virtual instrument panel displays under actual measurement conditions. One interesting feature is that either an HC240 or HCT240 logic input buffer can be used in LA-CRO, to cater for CMOS or TTL voltage thresholds respectively. An HC240 chip is supplied and this (or an HCT240) is installed in socket U1, adjacent to the voltage regulator. The digital portions of LA-CRO can be driven directly from C or C++ programs. This raises some interesting possibilities. Windows NT and Windows 2000 block any Fig.8: there are lots of options available for the signal generator, including AM and FM modulation. Fig.9: the “Help” file is context sensitive. Click on the Help button for any of the instruments and the Help file shows you how to use it. attempt to use IO port access so the LA-CRO program must be running to enable IO access. Conversely, under Windows 3.1, 95 or 98, there is no need to run the LA-CRO software to access the IO ports. Availability Two versions of LA-CRO are available: a student version consisting of a fully-assembled PC board for $220.00 and a full package with case, plugpack, test clips and software on floppy disk for $330.00. You can find out more, including how to order, by pointing your web browser to www. techno-centre.com In fact, the Techno-Centre website is worth visiting in its own right as it has lots of good information on hardware, soft­ware and business issues. Take SC a look for yourself. Find Out More About RMIT LA-CRO is now used very successfully in the School of Elec­trical and Computer Systems Engineering at RMIT (Royal Melbourne Institute of Technology). To find out more about RMIT, visit their website at www.rmit.edu. au and check out their web-based “Open Day” for more projects and details of the university courses. SEPTEMBER 2000  65