Exploring Tektronix TDS 340 100MHz digital storage oscilloscope

Out of curiosity, I purchased a Tektronix TDS 340 100 MHz 2-channel digital storage oscilloscope from eBay. It passes self-test at startup and is able to show the 1kHz square wave calibration signal on both channels nicely:

The first thing on this oscilloscope that captures my attention was the faceplate on the top right of the device. This is for the 1.44MB 3.5″ floppy disk drive, featured in the TDS 340A, which probably shares the same enclosure design with the TDS 340. However, don’t expect to remove the faceplate and connect a PC floppy drive, or even a floppy drive from another TDS 340A, into the TDS 340 because the floppy drive interface board is not present on the TDS 340.

This oscilloscope supports up to 500MS/s sampling rate and has a memory of 1000 samples. Two internal memory slots, REF1 and REF2, are available for user to store waveform data for manipulation. It also supports external trigger, delayed timebase and some math operations on the input signal. Overall, it is good enough for most of my hobbyist micro-controller and analog projects.

The only thing I do not like about this oscilloscope is the AUTOSET feature. Among other things, it will set the acquisition mode back to 16-point average, instead of sampling. This causes a slower signal display, which is apparent when signal is removed from the probe – it will take a few seconds for the trace to update completely as the data points from the previous signal are still in the oscilloscope memory.

Fast Fourier Transform (FFT)

You can also perform FFT on the input signal and see the frequency components using the MATH menu. The following is the FFT of the 1kHz calibration signal:

Using the CURSOR menu, I am able to measure the first FFT peak of 1kHz, the fundamental frequency of the signal, and the harmonics frequencies. A good revision on FFT and signal theory which I learned back in my university days.

As I noticed, the FFT output on this oscilloscope is pretty noisy, compared to the smoother FFT waveform generated by the Rigol DS1052E of the same 1kHz signal:

Why is this the case? I leave this as an exercise for the reader. A hint is that the TDS 340 supports only average acquisition mode when FFT is used whereas the Rigol supports all acquisition modes (normal, average, peak detection) even with FFT enabled and has other menus to configure FFT window and sampling options.

Installing the Option 14 interface card

To be able to transfer the captured signal data to a computer for manipulation, I decided to purchase an Option 14 interface card from eBay and install it in the oscilloscope. This card features a parallel printer port, a male DB9 RS-232 serial port, a 9-pin female VGA port and a GPIB interface port:

The card has a 50-pin IDC male port used for communication with the oscilloscope and a 6-pin cable for video output. Although Option 14 boards can be safely interchanged between TDS340A, TDS340 and other similar models, some cards, especially those meant for the TDS 340A and later generations, also have another power cable that must be connected to a dedicated socket in the oscilloscope power supply. This is to provide power for some supported portable printers as seen in the photo below (notice the power socket):

If these boards are used in the TDS 340, the printer power supply cable should be left unconnected.

When purchasing a used Option 14 card, make sure it comes with the necessary cables. Mine came with the video cable but not the 50-pin cable. Luckily my 50-pin SCSI single-drive IDC female-to-female cable worked just fine. If you have to use a SCSI cable like what I did, make sure that the cable is single-drive and has no built-in terminators or other circuits to set the SCSI device ID, which may interfere with the communication and cause unexpected problems. The cable will need to be at least 40cm long to connect to the mainboard.

The edges of the GPIB port make it impossible to fit the card into the back of the oscilloscope that has horizontal metal bars. I need to cut the bars for the card to fit in:

Although the IDC cable is keyed to prevent wrong insertion, the video cable is not. However, during my experiment, inserting the video cable in the wrong direction will simply stop the VGA port from outputting video without any other long-term effects – all I need to do is reinserting the cable correctly. Obviously, don’t leave the cable incorrectly connected for extended period. The pictures below show how to correctly connect the video cable (notice the colors of the individual wires) and the IDC cable:

VGA output from the Option 14 card

After installation, reassemble the oscilloscope, power it on to make sure that it still passes self-test and press the HARD COPY button. If the card is detected, after a short while, you will see an error message “Hardcopy device not responding” instead of the usual information on how to install the hard copy interface:

The next test is to see if the VGA port on the card is working well. Page 144 of the TDS 340 technical reference provides the pinout for the DB9 VGA port:

This is the pinout for the more common 15-pin VGA port, used in most modern devices:

I made an adapter with the following pin configuration to be able to feed it into a standard VGA monitor:

• Pin 2 of Option 14 DB9 VGA port (Video) connected to pin 2 (Green video) of VGA connector.
• Pin 1 (Red video) and pin 3 (blue video) of VGA connector must be grounded.
• Pin 4 of DB9 VGA port (Horizontal Sync) connected to pin 13 of VGA connector.
• Pin 5 of DB9 VGA port (Vertical Sync) connected to pin 14 of VGA connector.
• Pin 6,7,8 of DB9 VGA port (Ground) connected to pin 5,6,7,8,10 of VGA connector.
• Pin 11 (Monitor ID) of VGA connector must be grounded. This indicates a 640×480 low-resolution VGA output and reduces the need to send monitor information via I2C using the DDC SDA and DDC SCL pins.

This is the back of the oscilloscope with the Option 14 card installed and the VGA adapter connected:

This is the VGA output on my 24-inch LCD monitor:

Using the hard-copy feature

With the Option 14 card installed, the oscilloscope supports sending a capture of the current oscilloscope display, known as hard copy, via any of the following methods:

• Printing to common parallel/serial printers at the time
• Sending raw image data via RS232 or GPIB

You can configure the output file format in the Hcp Format menu. The TDS 340 supports BMP, TIFF, PCX, PostScript (PS) and Interleaf image formats. Except for Interleaf which I couldn’t find a viewer for, the rest of the file formats are readable on the PC by most modern document viewers.

In my tests, I use Realterm to capture the serial data sent once the HARDCOPY button is pressed. At the maximum RS232 speed of 19200bps on the TDS 340 (the TDS 340A supports a higher speed of 38400bps), it takes almost 20 seconds to transfer the 37.5KB 640×480 monochrome bitmap image produced by the oscilloscope. The image size reduces to approximately 18KB if PCX is used. The following is a monochrome bitmap produced by the hardcopy feature, optimized for printing:

Downloads for TDS 340A, TDS 360 & TDS 380 oscilloscopes:

User Manual
Technical Reference
Programmer Manual
Service Manual 

See also:

Programming the Tektronix TDS 340 100MHz digital storage oscilloscope
Calibration and acquisition problems on Tektronix TDS 340 oscilloscope
Rigol DS1054Z, GW-INSTEK GDS1054B and Hantek DSO4084C digital oscilloscopes