Hex Display: A Debugging Tool

Testing Hex Display

Testing Hex Display

Before I start building TLC-MBC I need to get some tools for debugging any hardware problems I might come up against. The first is Hex Display, it is used to show the state of the address bus while single stepping the processor. Hex Display was inspired by Quinn’s Hex Out.


The stars of the show here are the TIL311s (data sheet), they have a built-in hex decoder, latching, blanking and separate power inputs for the leds and logic. I had six of them for a project that was long forgotten, so I used four of them here, in sockets naturally. TIL311s get quite hot if they are on for a long time, to counteract that you could pulse pin 8 the blanking input. As I do not foresee them being on for long I am not going to bother with adding an oscillator.Hex Display Schematic

I also had a couple of 74LS373s, octal transparent latches, so I used them to go between the address bus and the TIL311s, but really just about any octal driver/buffer/latch chip could be used.

Building Hex Display

At some point, I want to start making my own PCBs, so I used Eagle to lay out the schematics, but for now I am firmly in the land of Vero Board and jumpers… I used sockets for the chips and displays so that I could re-use them. For the connector I used right-angled header pins so that I could use a couple of Dupont cables to connect Hex Display to the bread board. I spent the afternoon soldering this up, testing connections with an ohm-meter before plugging in the silicon.

Testing Hex Display

As you can see in the picture above, I connected Hex Display to a bread board. The orange jumpers act as “switches” for each input line. I then toggled each input between five volts and ground and observed the display, everything behaved as expected.

I have to admit, I do like these hex displays, the letters A-F are in uppercase. It was one thing I disliked with the Sym-1, it used 7-segment leds and so the A-F was in lowercase and to me it just didn’t look right, looked amateurish… cheap… I don’t know where I got it from but even when I write hex numbers, the letters are uppercase only.


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