JWA Systems - DigiTrace

A logic analyzer using the PC's parallel port

Introduction.

A logic analyzer is useful in electronic development and debugging, especially where fast logic circuits are involved with lots of signals whose relations have to be verified or examined. A logic analyzer is a like a recorder for digital signals. During a certain (small) period of time, the state of a few digital lines can be recorded to a file. An event can be specified to signal the start of the recording, i.e. line 1 toggling from 0 to 1. This recording can be viewed afterwards, allowing for zooming and scrolling in the time domain. This is a homebuilt logic analyzer with software that can be downloaded from JWA Systems.

Features

This logic analyzer can:

Run on Win95 and Win98 and ME using non-interrupted burst acquisition.

Run on Win2000 NT XP with interrupted acquisition, using the allowio driver.

Record up to 8 channels.

Use any parallel port.

Sample at up to 1 million samples per second, depending on your hardware.

Record 32768 samples.

Save and load recordings to/from disk

Use an "Advanced Trigger" scheme. It waits for a channel to remain stable for a given time (Delay), and then it starts sampling after the first change of that channel.

Ironsides used the DigiTrace to determine the left, mid and right sticks values in msecs as shown below.

User interface.

The following graph shows a screenshot of the user interface. Channel 1 has been nominated as the trigger.  Once triggered, Channel 2 traces the pulses input from that probe.  The vertical red lines are set by left clicking (Mark 1)and right clicking (mark 2)  the rising or falling edges - the Delta, or difference in time, is automatically calculated - in this case 26.18 microseconds.  The screen shot shows that 150 microseconds of data are displayed from the left to the right edges of the screen.  The 'width' of data displayed can be adjusted by hitting the PLUS + and MINUS - signs.  Equally, the slider can move recorded data from downstream into the screen shot.

Notes by TugBoat

Logic analysers and Cathode Ray Oscilloscopes (CRO) can normally run in two modes 'free running' or 'triggered'. 'Free running' mode is where the device just monitors input and shows it on the display, 'triggered' mode is where the devices waits for a particular trigger condition and then starts displaying.

The 'triggered' mode makes the waveform stable - on a CRO things appear to be 'steady', rather than just leaping around. With a 'free running' display the chances are you won't see the event you are looking for.

The PC based tools are normally recording devices, not display at the same time as record. For recording devices the 'triggered' mode makes the best sense.
To use the logic analyser you nominate one channel as the trigger.  The software then waits for the channel to be 'stable' (ie. at the same logic level for a period) and then waits for the next transition of the trigger channel. This transition starts the recording process.

This is a really simple trigger, commercial analysers will permit you much more complex triggers, including sequential triggers (ie. a sequence of events that must occur in sequence).

For example,  you could hook up Channel 1 to the servo signal and set Channel 1 as the trigger.

In the buffered circuit, the 74HC245 chip is a CMOS part with very high input impedance and it will 'see' almost anything as noise. You will need to tie the unused Channels to ground to reduce the confusion.

While the downloaded software will run correctly on Windows 98, there is an issue with Windows XP/2000, in that these systems do not let the software access the hardware directly.   In Windows XP when you try to run DigiTrace, a 'privileged instruction' trap error message occurs.  To overcome this situation, you need some software to sit between the hardware and your program. 

The JWA Systems web site addresses this issue with the use of 'PortTalk' and 'allowio'.  For more information, see the JWA Systems web site. The inexperienced user is advised to be very careful before trying to manipulate Input/Output modifications to systems software.

But, for those brave enough to try it.  The procedure is as follows:

   1. Download the DigiTrace code and install the 3 files that were in the ZIP package into a directory.

   2. Download the the 'porttalk22.zip' package and then extract 'allowio.exe' and put it into the DigiTrace directory.

   3. Extract 'porttalk.sys' and install it into 'Windows/System32/drivers'.

   4. Create a batch file (in the DigiTrace directory) You can do this using the text editor Notepad.  Put in:

                            allowio digitrac /a

   5. Save the file in the DigiTrace directory calling it:                    

                            digitrace .bat    

   6. Run the batch file to get DigiTrace running, and  no faults should be reported.

   7. Create a shortcut of the batch file to put on your desk top to launch DigiTrace each time.
 

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Last modified: March 18, 2004