Since the failed experiments with the initial version of the 32-bit 80386DX ISA Single Board Microcomputer, I always wanted to build an ISA signal view interface card. And while I don't have a logic analyzer moving the oscilloscope or the logic analyzer next to one of my DIY PCs is insanely difficult, I decided to use LEDs as a spartan form of optical signal analysis. Naturally, this project is a LED eater. Thus, I decided to give up on displaying the states of the ISA address and data bus. I only implemented the rest of the ISA signals.

As a side note, I always wanted to be able to visually observe triggering of the IRQ and DRQ signals. Especially when a sound card is requesting its appropriate interrupt. I am also very curious to see if the VGA vertical retrace interrupt (signal IRQ9) is ever active.

Here are the hardware specifications.

  • Optical Inspection of ISA Bus Control and Arbitration Signals
  • Minimal Glue Logic
  • 16-bit ISA-class printed circuit board construction

This project is very simple and the design of the schematic posed no real issues. However, it took a lot of time to draw. I did the complete manual routing of the PCB tracks in four hours or so. Pretty much a straightforward job.

Let's move on with the project.

Disclaimer: I reserve the right to change the schematic diagram, the PCB layout, or the implementation without further notice. This is an entirely hobby do-it-yourself design and I am not responsible for any damage made by any possible mistake in any version or revision of the schematic diagrams or PCB layouts. Since it is an advanced microelectronics project, it requires very good assembly and debugging skills. In addition, I cannot offer any further technical support other than the contents of this article.

This project is in its final stage.
Current iteration of ASSY. 2486-SVIF-901 is VER. 1.0 REV. B

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Laudatur ab his, culpatur ab illis. This project is provided as-is and is not for commercial purposes. It reflects my experimental work in microcomputer system design and should be treated as such. I release the schematic diagram and circuit board layouts to the public for educational purposes. I did all this at my own expense and in my free time. If you like my work, please consider making a donation. It helps me continue these kind of projects.

Schematic Diagram

Fig. 1: Electrical Schematic Diagram

Printed Circuit Boards

Fig. 2: Top Silkscreen

Fig. 3: Top Layer Printed Circuit Board

Fig. 4: Inner Bottom Layer Printed Circuit Board

Fig. 5: Inner Top Layer Printed Circuit Board

Fig. 6: Bottom Layer Printed Circuit Board

Fig. 7: Top Layer Printed Circuit Board - Simulation

Fig. 8: Bottom Layer Printed Circuit Board - Simulation

Gerber Files

Here are the Gerber files compressed in a .ZIP archive.
Please note that the file naming convention that I used is what OSHPark normally expects.
You can also order the printed circuit board directly from OSHPark by following the link in the bill of materials below.

Compressed Gerber Files: isa-sv-interface.zip

Bill of Materials (BOM)

The following list contains the parts that are required to assemble this ISA signal view interface card.

IdentifierValueQtyNotesMouser Number
Printed Circuit BoardASSY. 2486-SVIF-9011VER. 1.0 REV. BOrder from OSHPark
IC1-IC874F2408Octal 3-state Inverter595-SN74F240N
C1-C8100 nF / 50 V8MLCC80-C322C104M5R-TR
C9-C1710 uF / 25 V8Tantalum Capacitor80-T350E106M025AT
RN1, RN3, RN48 x 470 Ω3Bussed Resistor Network652-4609X-1LF-470
RN2, RN56 x 470 Ω2Bussed Resistor Network652-4607X-1LF-470
RN64 x 470 Ω1Bussed Resistor Network652-4605X-1LF-470
RN67 x 470 Ω1Bussed Resistor Network652-4608X-1LF-470
LED1-LED75 mm Red LED7DMA Acknowledge Signals755-SLR-56VC3F
LED8-LED145 mm Green LED7DMA Request Signals755-SLR-56MC3F
LED26-LED34, LED36, LED38, LED395 mm Red LED12Inverted Signals755-SLR-56VC3F
LED15-LED255 mm Orange LED11IRQ Signals755-SLR-56DC3F
LED35, LED37, LED40-LED42, LED455 mm Green LED6Non-inverted Signals755-SLR-56MC3F
LED43, LED445 mm Yellow LED2Clock Signals755-SLR-56YC3F
IC Socket20-pin8IC1-IC8575-193320

Alternatively you can use the following link to the Mouser project that I created for this ISA card. It should make ordering of parts and removing typing faults pretty easy.


Lately I have observed that Mouser discontinued some of the parts in the list above. If you decide to build this project, then you need to find alternatives. It is out of my scope to maintain the correctness of the parts list above.

Assembly Instructions and Notes

Here is a list of things you need to pay attention to should you decide to build such ISA signal view interface.

  • Inspect the printed circuit board once you receive it. Normally OSHPark produces very good quality boards but one never knows. There must be absolutely no short circuits on the printed tracks. If the PCB is faulty then it can damage other ISA cards that you might install in the system.
  • Carefully observe polarity of the tantalum electrolytic capacitors on the silkscreen. I made sure there is no error on the printout. Tantalum capacitors will violently explode and burst in fire if mounted in reverse, possibly injuring you.
  • Take your time to solder all the components on the board. There are a lot of solder points and if you don't have patience in general, then this project might not be for you.
  • Use a temperature-controlled soldering station and quality solder. Take care not to leave solder bridges as any short circuit will most likely lead to failures.
  • In order to ease-up the PCB assembly, I would suggest mounting parts in the following order: IC sockets, resistor networks, MLCC capacitors, tantalum capacitors, LEDs.
  • At the end, clean any flux residues with isopropyl alcohol.

Principle of Operation

The printed circuit board assembly holds just a bunch of buffers and a lot of LEDs. I used 74F series ICs as buffers because of their small propagation delay. I don't ever expect to detect with the bare eye any high-speed signal switching on and off a certain LED. But why not?

So, in short, the principle of operation is like this: as soon as a certain signal is present on the ISA bus, it passes through a buffer and its respective LED turns on. Some of the LEDs will flicker with such a high frequency that they will appear as always on (think CLK and OSC signals). Some other LEDs will be quasi-permanently off (think IRQ9 or RESET signals).

In addition, active low signals are re-inverted so that their respective LEDs will be lit only when the signal is actually active.

Construction and Pictures

The PCBs have arrived from the factory. Now on to the assembly procedure. As with all large PCBs, the pictures present a certain amount of barrel distortion. I took them with my Nokia 6.1 mobile phone.

This is the bare 4-layer PCB, component side.

And solder side.

I started soldering the passive components.

IC sockets follow next.

And the final assembled product. This card will surely help me debug my x86 system designs. And when I am done with debugging, it will provide nice ambient ligthing inside the case of my 32-bit 80386DX ISA Single Board Microcomputer system.

Versions and Revisions

This section lists the project version and revision history.

VER. 1.0 REV. B

  • Fully functional initial production version

Copyright © 2004- Alexandru Groza
All rights reserved.
VER. 1.0 | REV. A