The Tektronix TWD120



The Tektronix TWD120 is an oscilloscope that uses a PC as a display. Technically it is known as a “Digitizer” since it does not have the typical oscilloscope affordances such as knobs, buttons or a display. It's capabilities are similar to the TDS320.

The TWD120 is said to have been invented by Tektronix for General Electric Medical for field servicing of GE's medical diagnostic equipment. About 4000 of these devices were made. Near the end of their life, all of the TWD120s were sent back to Tektronix for calibration and then immediately obsoleted and sent out for disposal. Originally the TWD120 had a list price of about $US3900 and now are commonly found on the used market for $US400. They show up on eBay for as low as $100. Spectra Test Equipment ( had some in stock for $400, but now they just seem to come up on eBay for $350 as a Buy It Now price. They typically come with probes, manuals, software, SCSI terminator, and a carrying case. You have to provide your own PC and SCSI board.


The TWD120 uses a SCSI bus to communicate with a host PC and a lot of people have issues with this. SCSI is very straight forward, it was used by Apple in all of their Macintosh computers until market forces prompted them to change to the typical (slower, cheaper) IDE disk drive communications. True, there are some simple rules that you have to follow to terminate the SCSI bus, very much like people had to do in the early days of PC disk drives, but if you can't follow simple rules like “only put a terminator at the ends of the bus”, ask an old Mac guy to help you.

More importantly, there seem to be two types of SCSI controller boards available, the cheap ones that were put together to run Zip drives that cause the system to hang on startup, and the rest. Boards that are known to work are:

SIIG AP-10 PCI card

Adaptec SlimSCSI 1480B PCMCIA card

Adaptec AIC-7880 controller in a Dell C-Dock

Adaptec APA-2940

Setting up the SCSI devices

When Windows first detects the scope, it notifies you that a new device is found with a type of “TEK”. It will not have a driver and you should follow the installation process and declare it as an “Other” device of type “Unsupported device”. You will have to do this 8 times (!) but it won't ask again after that. If you get to this point, you are very close to getting it all going.

On some versions of Windows if you go into the Device Manager control panel, under Other devices, click on one of the Unsupported Devices and notice the Bus Number, you will need it below.

Getting it going

The program to control the TWD120 (oddly called TWD120) makes an assumption that causes problems – it assumes that the digitizer will be on the first SCSI bus (0). In general this is not a big deal, except that Microsoft made an odd decision in Windows NT and some of the successors, all of the disk drives and CD-ROM drives use a SCSI driver to talk to the IDE disk subsystem. Your IDE boot disk will be on SCSI bus zero. Your CD-ROM is on the second IDE bus, but also SCSI bus one. If you install a SCSI controller, it becomes the second or third SCSI bus in the system, but the TWD120 software only looks at the first SCSI bus.

To get around this problem, Regis Crelier ( wrote a piece of code that acts as a “shim”. It goes between the TWD120 program and the winaspi.dll program which then talks to the SCSI board. Regis' shim replaces the SCSI driver file winaspi.dll and takes TWD120's SCSI function calls for bus 0 and simply calls the same function on bus 1, 2, or 3 (depending on which version of the shim you installed) in the file winaspi0.dll.

Here is what you do to install the shim:

Install winaspi software from Adaptec. It may come on your SCSI board driver disk.

Run TWD120, it just might work right out of the box. (if your digitizer ended up on bus 0 when you checked the Device Manager).

If not, find the file winaspi.dll (it should be in your windows directory)

Make a copy winaspi.dll and name it winaspi0.dll

Copy the file winaspi.dll from the directory “ID#1” into your TEK directory.

Run TWD120. If it doesn't find your scope, try the winaspi.dll file in directory “ID#2”, and then “ID#3”.

This shim works so well that I have run my TWD120 on Windows 3.1, 95, 98, SE, NT, 2000 and XP.


Through a game of chance, I was able to secure a copy of the calibration disk for the TWD120s. I haven't tried it, but here it is. No guarantees, no warranty, yadda, yadda.


Expanding your display window too much causes the program to crash. Remember, this device came out when VGA (640*480) was king. I believe 1024*768 is about maximum. Just restart it and away you go.

The print function gives useless results, unreadable black on white. Use <ALT><PRNT SCRN> to paste the image into your favourite word processor.

There is a parallel and serial port on the back of the digitizer. There is no apparent function for either. They could be for calibration.

You can put more than one digitizer on a system, but the software will only talk to the first one it finds. Not useful.

Saving a waveform gives a file that can be viewed in the viewer program, but the format is undocumented.