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DIGITAL STORAGE LOGIC PROBE for Windows 98 Design by Trent Jackson Words by Trent Jackson and Ross Tester Here’s another reason not to throw out that old computer. This fully functional Digital Storage Logic Probe is driven by a Windows-based PC. With it you can view and record valid TTL and CMOS logic levels via 32-bit Windows software. And we even supply the software! I f you have ever needed to design, service or troubleshoot digital equipment, you’ll know just how valuable a logic probe can be. Well, this one goes one step further: connect it to your PC’s parallel port 22  Silicon Chip running Win98 and you can not only view logic states, you can record them, save them for more analysis or comparison, print them and more. You can also locate and store high or low-going pulses via software latching and even disable unwanted logic highs or lows (via software). Unlike most conventional DSOs (Digital Storage Oscilloscopes) and similar devices, this device records true bit values – 0s and 1s – not waveforms or voltages. www.siliconchip.com.au You can switch between TTL and CMOS circuitry. In TTL circuits, which always operate from a 5V supply, any voltage less than 0.8V is considered to be a logic “low” and any voltage greater than 2.0V is considered to be a logic “high”. Intermediate voltages are not valid. In CMOS circuits, which can operate anywhere between about 3V and 15V, it’s not quite so simple. Any voltage less than 26% of the supply voltage (Vcc) is considered logic low, while any voltage higher than 73% of Vcc is considered logic high. How does the logic probe know what the Vcc is? Simple – it takes its power from the circuit under test! The vast majority of CMOS circuits operate with levels between 5V and 15V (3-5V operation is rare) so for simplicity, the Logic Probe has been designed to work with 5-15V levels. Virtually any PC which can handle Windows 98 can be used, though a Pentium-class is recommended. The parallel port is used, optically isolated from the logic probe to prevent damage to the port (and possibly the PC) should a worst-case scenario occur. We all know that Murphy’s law says that any scenario which does occur will be worst-case! The probe is connected to the inverting input of one of IC1’s two comparators and to the non-inverting input of the other. IC1 is an LM393, a dual precision comparator. The two elements are connected to form a standard window comparator, one gate (IC1a) detecting TTL/CMOS switch. In TTL position (which assumes a 5V supply), the divider selected ensures that 2.0V is applied to one comparator and 0.8V to the other, thus giving us the required TTL logic state conditions. In the CMOS position (which can have a wide Vcc range), the other divider puts 73% Vcc on one comparator and 26% on the other – thus achieving the CMOS logic state conditions. Full optical isolation from parallel port The open-collector outputs Fully TTL & CMOS co mpatible of both the comparators are Probe over-voltage pr connected to optocoup-lers otection VCC reverse-polarity OPTO 1 & 2, the outputs of protection which in turn connect to Low cost and very ea sy to build printer port pins 10 and 11. 32-bit Windows 98 ba A third optocoupler sed View and record logi (OPTO3) connects to pin 12 c levels – its purpose is solely to let Save and open record ed data the software know that there Print out recorded da ta is VCC present. All three optos have 10Ω suppressor resistors between them and the printer valid high logic voltages (above its port. They are low in value due to the reference voltage) and IC1b detecting fact that the parallel port has its own valid low logic voltages (below its pull-up resistors. reference voltage). While higher values would be deThe reference voltages are provided sirable, they cannot work in this case by two voltage dividers across the because there would be too much voltsupply rail. These connect to IC1’s age drop across them – and they could other inputs. The reference voltages also slow the operation of the port. vary depending on the setting of the The six diodes connected to the Features: • • • • • • • • • How it works Starting at the probe, we can see a 4.7kΩ isolating resistor and then a pair of signal diodes and a zener diode. The signal diodes will clip any negative or positive-going spike which may be present when measuring, while the zener will clamp any high voltage to a safe level. The .01µF capacitor provides not only high frequency roll-off but also gives a small amount of hysteresis to the circuit. It will also tend to integrate square wave inputs to some degree and while this is undesirable, experience has shown that the overall performance of the probe is largely unaffected. The probe is held at a nominal 39% Vcc by the 560kΩ/360kΩ voltage divider across the supply. This keeps the unconnected probe in “no man’s land”, ie, indeterminate logic state, to avoid false conclusions when reading. www.siliconchip.com.au Looking at the rear of the case, showing the 26-way IDE cable which connects to your PC’s parallel port. You will probably have to make this cable yourself. August 2002  23 24  Silicon Chip www.siliconchip.com.au K D3 1N914 A K A K + 10F K 1N914 ZD1 15V 1W 0.1F A 360k 32.5% Vcc 0.1F DIGITAL STORAGE LOGIC PROBE A 1N4004 .01F 4.7k K D2 1N914 0.1F A D1 1N4004 Fig.1: the complete circuit of the logic probe shows just how few parts there are in it. Basically, it’s just two comparators, some opto-couplers and a few LEDs! 2002 SC GND PROBE GND VCC 3-18V MAX! S2 100k 180k 560k 4.7k 100k + 26.7% Vcc TTL 100k CMOS TTL 150k CMOS ZD1 0.8V 73.3% Vcc 2.0V 15k 360k S1b S1a 6 5 2 3 A E K LEDS 1 & 2 4 IC1b IC1: LM393 IC1a 8 B C BC548 LED2 7 LED1 1 1k A RED C K K LED3  A K  A B E Q1 BC548 A GRN 2.2F 4.7k K   2 1 2 1 2 K K K K K K 150  OPTO3 4N25 47  OPTO2 4N25 47  LED3 TRI COLOUR A GRN A RED 300 300 D4-9: 1N914 10F 4.7k 1 OPTO1 4N25 4 5 4 5 4 5 A D4 A D5 A D6 A D9 A D8 A D7 10 10 10 10 18-25 7 6 5 4 3 2 13 12 11 10 CON5 TO PRINTER PORT data lines of the parallel port (pins 2-7) form two “OR” gates (D4-D6 form one, D7-D9 form the other). These two gates have their outputs connected, via current limiting resistors, to the anodes of a bicolour LED (LED3). This method has been used to obtain reasonable brightness from the LED by effectively paralleling the currents from the data lines. The LED shows the high (red) or low (green) logic levels. However, it can also show whether the probe is floating (flashing green) or no Vcc (flashing red). We haven’t yet mentioned Q1, the 1kΩ resistor and LEDs 1 and 2. They form a 2.5V regulated supply for the three optocouplers. This is essential due to the fact that the supply voltage can be anywhere from 3–18V. The LEDs are not used for their light emission (in fact, they’re sealed inside the box!). Rather, they are used for the fact that when forward biased, each will have a constant voltage across them (about 1.5V). Therefore Q1’s base is held at a constant nominal 3V. With about half a volt or so drop across Q1’s base/ emitter junction, the emitter voltage remains at a constant 2.5V, give or take. And speaking of supply, as we mentioned before this is taken from the circuit under test (by means of cables with mini crocodile or IC clips). The CMOS VCC can be anywhere from Everything mounts on the one PC board except the banana sockets, bicolour LED and the two switches. Construction is quite straightforward. 3- 18V. D1 isolates the supply and provides reverse-polarity protection; the 10µF and 0.1µF capacitors provide some smoothing and bypassing. Connecting cables The connection between the probe 150k 100k 560k 360k 0.1F 0.1F 4.7k 914 D2 Fn01 15V ZD1 401 LM393 12080340 2.2F GND VCC GND S2 POWER S1 TTL/CMOS 100k 0.1F 914 4.7k D3 4.7k 100k 300 PROBE Fu01 LED2 .01F + 47 LED1 Fu2.2 47 150 300 1 1 1k OPTO2 OPTO3 4N25 4N25 1 15k 1 360k 10 10 OPT01 4N25 1 914 914 914 914 914 914 D7 D8 D9 D6 D5 D4 Q1 D1 1N4001 1Q 4.7k 10 + 401 1 1 10F 401 BICOLOUR LED 10F Fu01 + 10 180k (IDC PLUG AND CABLE TO PC PRINTER PORT) CON3 hardware and computer is via a standard 26-way flat ribbon cable. One end of this cable is fitted with a keyed 26-way IDE female plug (which mates with a 26-way male socket mounted on the PC board); the other end is fitted with a standard Fig.2: you should be able to match this component overlay and wiring diagram very closely to the photo above to make construction simple! www.siliconchip.com.au August 2002  25 A close-up of the inside of the box to help you with the 15-way rainbow cable wiring. Use the same colour cable as we did and make life easy on yourself! parallel port (Centronics-type) IDE plug. It is most unlikely that this cable will be an off-the-shelf item so you are going to have to make it up yourself. It is relatively easy to do – while a special tool is normally used to fit IDE plugs to cables, it can be done in a bench vise. IDE plugs are not soldered – tiny, sharp “fingers” pierce each wire in the cable and make connection. A clip holds the whole thing together when assembled. Have a look at our close-up photo of the cable and you’ll see that at both ends, the cable loops through the plug and then turns back on itself. The loop takes the strain off the connection itself. You may also see a tiny arrow moulded into the PC board-end plug. This shows pin 1 and is usually the pin which the red stripe on the cable connects to. In our case, though, the red stripe goes to the opposite end. At the parallel port plug, when you hold the plug with pins towards you so that you are looking at a letter “D”, the red stripe goes to the bottom. The other cables you will need include a set of power cables and probe cables. A collection of these is shown in the main photograph and at the end of this article – all are fitted with banana plugs at one end to go into matching sockets on the probe case. The other ends can be multimeter-type probes, small crocodile clips, IC connecting clips, and so on. The choices depend on the way you want to use the probe. Construction The project is mounted in a medium sized (130 x 67 x 40mm) jiffy/zippy box and, with the exception of the switches, bicolour LED and four input sockets, all components mount on a single-sided PC board measuring 95 x 57mm and coded 04308021. And here’s the fully-opened-out project, completed and ready to close up. Notice the thin cut-out in the case (top right) for the IDE cable to pass through. 26  Silicon Chip We printed this little label to go on the case to show what the cable went to... www.siliconchip.com.au Before you start PC board construction, use it (or a photocopy of the PC board artwork in Fig. 4) as a template to drill four mounting holes in the lid of the case. Locate the board centrally and drill four 3mm holes in line with the four holes at the corners of the PC board. After checking the board for defects, start construction by soldering in the resistors, 15 PC stakes and four wire links. You might have to scrounge a 30mm length of tinned copper wire for the longest link because it will probably be too long for the usual source of link wire, cut-off resistor pigtails. Next, solder in the capacitors, diodes, on-board LEDs and the transistor (remember almost all those components are polarised). Likewise, all the ICs are polarised so you not only have to get them in the right spots, you have to get them the right way around! The last “component” to go on the PC board is the 26-pin parallel port cable connector. You will note that one side of this connector has a notch cut in it. This notch goes to the outside of the PC board. Leaving the board for a moment, it is now time to drill the case for the terminals, LED and switches. Photocopy the drilling diagram and temporarily sticky-tape it to the bottom of the case (the bottom of the case actually becomes the top!). Use this as a template to drill the holes (take note of the various sizes). And while you’re about it, you need to file a very narrow (about 1-1.5mm deep) slot in one edge of the case to allow the parallel port cable to pass through without being guillotined when you screw the case and lid together. Parts List – Digital Storage Logic Probe 1 PC board coded 04308021, 95 x 57mm 1 plastic utility case, 130 x 67 x 44mm 1 front panel label, 124 x 63mm 1 DPDT toggle switch (S1) 1 SPDT toggle switch (S2) 4 insulated banana sockets (2 red, 2 black) 1 26-way PC-mounting IDC header socket (male) 1 26-way IDC plug (female) 1 25-way D25 male IDC plug 1 150mm length 15-way rainbow ribbon cable 15 PC stakes 4 10mm M3 tapped spacers 8 5mm M3 screws 4 rubber feet Semiconductors 1 LM393 dual comparator (IC1) 3 4N25 optocouplers (OPTO 1,2,3) 1 BC548 or similar NPN transistor 1 15V, 1W zener diode (ZD1) 2 red LEDs, 5mm (LED1, LED2) 1 tricolour LED, 5mm (LED3) 1 1N4004 silicon power diode (D1) 8 1N914 silicon small signal diodes (D2 - D9) Capacitors 2 10µF 25VW PC mounting electrolytic 1 2.2µF 16VW PC mounting electrolytic 3 0.1µF 50VW MKT polyester (code 104 or 100n) 1 .01µF 50VW MKT polyester (code 103 or 10n) Resistors (1%, 0.25W) 1 560kΩ 2 360kΩ 1 180kΩ 1 150kΩ 3 100kΩ 1 15kΩ 4 4.7kΩ 1 1kΩ 2 300Ω 1 150Ω 2 47Ω 4 10Ω 4-band   OR VGS2 Graphics Splitter NEW! 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And that’s it! All we have to do now is look at the software and the operation of your probe. The software The software, DSLP.exe, operates under Windows 98 and has the standard “look and feel” of your other Windows programs. When you open DSLP, you’ll find a window with a number of panes. Top left is a measurement box which indicates standard logic conditions at a glance, with a time-delayed bar graph immediately underneath. Next down is a settings box which enables you to toggle common settings on and off – it is used to enable and disable various parameters such as logic high and low, whether the pulses latch and so on. The probe sensitivity slider sets the sampling rate and hysteresis levels. On the right top side of the window is the system box – the heart and soul of the software. This pane enables you to set the parallel port address (three most common ports shown) and also gives you the status of the port, whether hardware is connected or not and whether or not power is connected. Clicking on the reset binary digit data buffer box will clear all current data in the recorder box. For good measure, there is a realtime 24-hour system clock readout. Finally, across the bottom of the window is a binary data recorder, where incoming data is recorded in a true bit fashion. All of these settings and controls will become self-explanatory as you Fig.3: this is the window which should greet you when you run the DSLP.EXE file. The various panes are quite self-explanatory. 10uF 10uF 1 104 104 Q1 1 1 1 28  Silicon Chip Now it’s time for final assembly. First of all, mount the PC board on the lid using 10mm tapped stand-offs. If you want to save a couple of bob, you could just use some screws through the lid with a nut both sides of the PC board. Plug the parallel port connector cable into its socket on the PC board (remember that keyway) and place the lid/PC board assembly down into the box so the parallel port cable lies in the slot you filed in the edge of the case. Screw the case and lid together and fix four rubber feet to the lid so the 2.2uF 10nF 104 This slot needs to be just wide enough to accommodate the cable (about 34mm) and ours was about 25mm from the end of the case. Before you mount the LED and input sockets through the bottom of the case, the front panel needs to be fitted. It can be either glued on or stuck on with (thin!) double-sided adhesive tape. Take care not to mark the panel from here on. Use the diagrams and photos to locate the various bits. When all (including the bicolour LED) are in place, you can connect the PC board to the case with a length of 15-way rainbow cable (it’s a lot easier to follow using rainbow cable than ordinary IDE cable!). If you use the same colours as we did, you can use the photos and drawings to ensure the right wire goes to the right PC stake. When soldering to the bicolour LED, take careful note as to which leads are which: the cathode (K) is the centre lead while the green anode is closest to the tab on the side of the LED. Therefore, the red anode is closest to the flat side. All three leads should be shortened considerably to avoid the chance of shorting – ours were cut to about three or four millimetres long. 1 04308021 Fig.4: full size artwork for the PC board. Even if you don’t make your own board, a photocopy is always handy as a drilling template. www.siliconchip.com.au use the probe. Interfacing the hardware and software This is extremely straightforward. As long as you are using a Pentium-based PC (or equivalent) and running Windows 98 (and of course your hardware is assembled correctly and you have loaded the software on your computer!), you should not have any problems. Plug ’er in and away she goes... The software, dslp.zip, can be downloaded from www.siliconchip. com.au It is a 2MB file so be patient! Once downloaded and unzipped, run “setup” and it will install automatically. When you run the unzipped dslp. exe file, you should be greeted with a window as shown in Fig.3. From there, it’s just a matter of selecting your parameters and using the probe. A selection of the connector cables you could need for this project. At left is a “curly cord” multimeter probe which is ideal as a data probe; the other cords have various types of clips for connecting to the circuit under test. All have banana plugs on one end. Operation The software basically works like this: assuming a valid high level voltage is detected by the probe (and therefore present on pins 2 and 5 of IC1,) pin 1 of IC1a will go low, forward biasing OPTO1’s LED and causing its transistor to conduct. This pulls pin 10 on the parallel port low. The software reads this and processes it accordingly. It will also write a data value of 56 decimal to the parallel port, taking pins 5,6 and 7 high – in turn, lighting up the green LED in bicolour 8 8 LED3. Detecting and processing 6.5 6.5 6 63 a valid low level voltage is achieved in exactly 18 the same way, except 8 8 that IC1b, OPTO2 and pin 11 are involved. 29 Similarly, the soft18 ware writes a value of 7 decimal to the port, sending pins 2, 3 and 4 high, lighting the red 18 18 18 21 22 LED in LED3. 125 If the LED is flashing, (either colour) you have either of the two “error” Figs. 5 & 6: 1:1 artwork for the front panel and a drilling template for the case. The panel artstates as shown on the work, along with the PC board pattern, can be downloaded from www.siliconchip.com.au SC front panel. www.siliconchip.com.au August 2002  29