DIY LPT Logic Analyzer with Old Switch Case + Power

While ago, as an electronic hobbyist, I want a logic analyzer. Being a diy fan, I constructed a simple but efficient logic analyzer. I have old PIII laptop and which has one LPT port. So, I searched for simple design and found these.

1) The Fabulous Logic analyzer

This is original design. But I prefer this site’s design.


Hans add some 1K resistors at buffer IC output and 100 ohm at buffer input as series. And 10K resistors at buffer IC input to protect unwanted noises and to keep pins low unusing.

The circuit is very simple and not need to explain. It is just a buffer circuit for LPT port. I used 74LS244 ic. This is my circuit design. I add LED at  each buffer output to visualize data signal.

After testing circuit, I find for a suitable casing. I have some bad Dlink 5 port switches. I notice it’s casing, some LEDs and good wall-watt adapter.

First, I open the case and check a look. Suddenly, I change my mind to replace old circuit. It has good LED and power supply. Again, I am good in salvaging, recycling :P.

Second, checked power supply circuit. As expected, it is 3.3 V supply. No problem, it can be hacked easily to get +5V output.

The power IC is AP34063, universal DC to DC converter IC. From data sheet, I check possible basic circuit. In circuit diagram, variable resistor is used to adjust output voltage.

In circuit, I checked that R27 is connected to pin-5 as data sheet’s design. I am a lazy bone. I remove R27 and connect a 10k variable resistor and adjust until output voltage is reached 5V. Checked that required resistor value is 570ohm (simply series 100+470 ohm). I solder these resistors to circuit. I don’t have SMD parts and used ugly 1/4 watt resistors.

After fixing 5V supply, I removed all active components (IC, main chip, crystal, matching transformers) with a hot air gun to protect unwanted power drain and recycling.

Then, drilled the hole and attached buffer circuit over main board. Soldered, connected all power, input, output and LED. For LED connection, each LED circuit connection on old  board are traced by using multimeter. All LED are connected from removed IC pins. By stretching these pins by multimeter with 1X ohm line, LED pins can be found. I solder LPT cable directly on buffer. I soldered 2 more LEDs, one for power and one for logic channel since the original circuit contains only 7 LEDs.

I want to use old RJ-45 jack as input connector (it is a good 8 pins connector). So, I solder input pins of buffer circuit to RJ-45 connector pins. Some old circuit board connections are required to removed with a utility knife.

Last, I cramp flat 8 pins cable with RJ45 connector and attached with corcodile clicks to used as input cable. One drawback is I can get only 7 channels because have to use one pin for ground connection.

This is final design,

and running in action.

Finally, there is a new logic analyzer in my bench. I even stored it with original box.

Final thought,

  • It can run up to 1MHz sample rate (means 500kHz may be the highest for input frequency range)
  • It requires giveio software to run in windows paltform (for XP). I found this package and nice instruction here
  • It supports some protocols decoding such as I2C, SPI, UART etc..
  • The timing seem depend on the hardware (PC) you used

Anyway, this analyzer is incredible and useful for me and thanks to developers.


2 thoughts on “DIY LPT Logic Analyzer with Old Switch Case + Power

  1. Pingback: Open Bench Logic Sniffer Arrived! | Oakkar7, another Blog

  2. I see quite a few sites which appear interesting and really worth a read. There is absolutely nothing worse than sifting through limitless blah blah sites simply to find a couple that hold your attention. Thanks. Great job!

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