Microelectronics | 16-bit ATX ISA BACKPLANE


I was thinking on restarting experiments with vintage computing hardware. Thus I decided to start working on an 8 slot, 16-bit ISA backplane that will fit either AT or ATX computer cases and will accept power from a modern ATX supply. Well I would've preferred to design it so that it could accept either AT or ATX power supplies. But I just couldn't find an on-line parts supplier for P8 and P9 male connectors to fit on the printed circuit board. Thus ATX it is.

I always wanted to experiment with option and boot ROMs. Back in the very early 1990s when I was routinely using an 80386SX / 25 MHz machine, 27C series UV EPROMs were expensive and difficult to find. These days I have access to a lot of ROM integrated circuits at low prices from the local and online electronics stores. Needless to say that over the time I have recovered quite a few ROM ICs from vintage computer boards. I can erase and reprogram them for my experiments. Thus this backplane implements support for one 64 Kbit option ROM of type 27C64 or 28C64. Address range decoding for the ROM is implemented as well.

Adjacent circuitry is also carried on this backplane. Think reset and ATX power control circuits, and indicator LEDs. I have even made provisions for the case LEDs and cooling fans. If you don't like the noise of a 12 V cooling fan, then just plug it into the 5 V fan header. I decided not to include a 5-pin DIN keyboard connector because this backplane will mostly be used with ATX cases. These don't align the back bracket with the keyboard connector that well. I know there are metal brackets already milled for an AT-style keyboard. But then again, this system backplane will most certainly use the keyboard socket on the CPU ISA card. Thus I omitted it from this design.

This ISA system backplane should handle 8088, 80286, 80386, 80486, and Pentium CPU boards as well as any 8-bit or 16-bit ISA expansion card. So a complete computer can be easily built.

The design of such a backplane is no secret and despite all the many apparently complicated connection points, overall, the circuitry is fairly simple to understand and implement. Regarding the printed circuit board layout, I have manually routed all tracks in order to fulfill the best digital design constraints and -- why not -- looks.

A historical fun fact: Back in the 1990s I was spending hours looking at computer printed circuit boards, observing the wire routing design, the choice of electronic parts, and the overall layout. I was seeing this as some kind of abstract art. In fact I was mesmerized by computer PCBs. I remember a few notable ones: an Adaptec AHA-1542CF SCSI ISA controller, ATI VGA WONDER-16 ISA video controller, an old 1989 CHIPS & TECHNOLOGIES 256K VGA controller, an unknown model NCR MFM ISA controller, a beautiful Western Digital WD-1004A-27X MFM ISA controller, a UNiSYS 80286-10 mainboard, and countless other that I cannot remember the model numbers. Looking at those PCBs I was dreaming about creating my own one day. So here I am, designing my own ISA backplane. I am planning other ISA designs in the upcoming couple of years.

Disclaimer: I reserve the right to change the schematics or the implementation without further notice. This is entirely a hobby do-it-yourself design and I am not responsible for any damage made by any possible mistake in any version of the schematics or PCB layouts.

This project is in its final stage.
Current iteration of ASSY. 2486-SBP-001 is VER. 1.6 REV. A

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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 and circuit boards to the public for educational purposes. I did all this on my expense and in my free time. So if you like my work, please consider a donation.

Schematic Diagram

Fig. 1: Electrical principial schematic.

Printed Circuit Board

Fig. 2: Top silkscreen.

Fig. 3: Top layer printed circuit board.

Fig. 4: Bottom layer printed circuit board.

Fig. 5: Top layer printed circuit board - simulation.

Fig. 6: Bottom layer printed circuit board - simulation.

Bill of Materials (BOM)

The following list contains the parts that are required to assemble this ISA backplane.

IdentifierValueQtyNotesMouser Number
Printed Circuit BoardASSY. 2486-SBP-0011VER. 1.6 REV. AOrder from OSHPark
IC174LS6881Magnitude Comparer595-SN74LS688N
IC228C64164 Kbit ROM556-AT28C64B15PU
IC374LS041Hex Inverter595-SN74LS04N
IC474LS741Dual D Flip-Flop595-SN74LS74AN
IC6UA79051Negative Regulator595-UA7905CKCS
T12N22221Small Signal Transistor610-2N2222
D1-D41N41484Small Signal Diode78-1N4148
C1-C48, C6410 uF / 25 V49Tantalum Capacitor80-T350E106M025AT
C55-C5847 uF / 25 V4Tantalum Capacitor80-T350K476M025AT
C62, C65100 uF / 10 V2Tantalum Capacitor80-T350J107M010
C631 uF / 35 V1Tantalum Capacitor80-T350A105K035AT
C49-C54, C59-C61100nF / 50 V9MLCC80-C322C104M5R-TR
R1-R3, R6-R8, R12470 Ω7Carbon Resistor291-470-RC
R4, R51.2 kΩ2Carbon Resistor291-1.2K-RC
R9, R11, R1310 kΩ3Carbon Resistor291-10K-RC
R10, R141 kΩ2Carbon Resistor291-1K-RC
RN18 x 10 kΩ1Bussed Resistor Network652-4609X-1LF-10K
SW17-position1DIP Switch774-2067
LED15 mm Orange LED1Standby Indicator755-SLR-56DC3F
LED25 mm Red LED1+5 V Indicator755-SLR-56VC3F
LED35 mm Red LED1-5 V Indicator755-SLR-56VC3F
LED45 mm Red LED1+12 V Indicator755-SLR-56VC3F
LED55 mm Red LED1-12 V Indicator755-SLR-56VC3F
LED65 mm Green LED1CPU Activity Indicator755-SLR-56MC3F
IC Socket20-pin1IC1575-11044320
IC Socket28-pin1IC2575-11044628
IC Socket14-pin2IC3, IC4575-199314
IC Socket8-pin1IC5575-144308
J120-pin Connector1ATX Power Connector538-46015-2006
J23-pin Header1Header649-68001-203HLF
J3-J52-pin Header3Header649-68001-202HLF
J6, J722-23-20312Fan Connector538-22-23-2031
JP13-pin Header1Jumper649-68001-203HLF
ISA0-ISA7Card Edge Connectors8ISA Slot587-395-098-520-350

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

Mouser Project: 16-bit ATX ISA BACKPLANE

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 system backplane.

  • 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 so even if it appears weird, please do respect the markings. 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: diodes, resistors, IC sockets, resistor networks, transistors, MLCC capacitors, tantalum capacitors, switches, LEDs, ISA sockets, connectors, jumpers.
  • It is recommended -- but not mandatory -- to use a small aluminum heatsink on the -5 V regulator integrated circuit.
  • At the end, clean any flux residues with isopropyl alcohol.
  • If your power supply does not have a -5 V supply rail then configure JP1 to generate -5 V from the -12 V rail. There aren't that many ISA cards using -5 V but if you happen to have one then you need to make sure it will receive the expected negative voltage.
  • Test the ISA system backplane only with the power supply cord attached. If the power supply rail indicator LEDs light-up properly and the power-on circuit works as expected then you can proceed to the next step: insert CPU board, adjacent ISA cards, and so on. Should the fuse blow on the power supply then it is almost certain that there is a short circuit somewhere.
  • In case you decide to experiment with option ROMs then make sure you configure the ROM address range accordingly. It must not interfere with other ROMs installed in the system.

Interface Connectors Description

Following are described all the interface connectors and their respective pinouts.

J1ATX POWERATX Power Connector1 - NC
2 - NC
3 - Ground
4 - +5 V
5 - Ground
6 - +5 V
7 - Ground
8 - NC
9 - +5 V StandBy
10 - +12 V
11 - NC
12 - -12 V
13 - Ground
14 - Power Supply ON
15 - Ground
16 - Ground
17 - Ground
18 - -5 V
19 - +5 V
20 - +5 V
J2PWR/STB-LPower/StandBy LED1 - Power LED Anode
2 - LED Cathode
3 - StandBy LED Anode
J3PWR-LPower LED1 - LED Anode
2 - LED Cathode
J4STB-LStandBy LED1 - LED Anode
2 - LED Cathode
J5PWRPower Switch1 - Power Signal
2 - Ground
J65V FAN5 V Cooling Fan1 - Ground
2 - +5 V
3 - NC
J712V FAN12 V Cooling Fan1 - Ground
2 - +12 V
3 - NC
JP1-5 V Selector1 - -5 V Rail (local)
2 - -5 V Rail (board)
3 - -5 V Rail (PSU)

The Power On Circuit

ATX power supplies are turned on and off through a momentary push button. This circuit does exactly that: sends a /PS_ON signal to the power supply to turn it either on (active low) or off (active high). This circuit is designed by Sergey Kiselev.

The Standby Circuit

This circuit section will provide some visual feedback through the STDBY LED once the system is powered off. A repetitive breathing-like fade in and out effect will signal that the system is standing by. Once you press the power button to turn the system on, the STDBY LED will remain off.

The CPU Activity Circuit

Once the CPU will start putting addresses on the address bus, A7 signal will provide a visual feedback through the CPUACT LED. This is a spartan indication that the CPU is actively working on something.

This circuit is, again, based on Sergey Kiselev's idea of a CPU activity indicator. I have further adapted it in a way that it is only active when the system is powered on. In the original circuit, there was a big chance that the LED will remain lit once power is removed from the system. This is because 74LS74 is powered through +5VSB rail which is always on. Section A of 74LS74 will store and reflect the previous state of A7 signal. And if this signal was high upon powering the system off, the LED will remain lit indicating false CPU activity.

Option ROM Configuration

In order for the microcode in the option ROM to be actually reachable, a valid start address needs to be supplied to the ROM decoder circuit. This is done by configuring the SW1 switch array. First of all, to enable the option ROM, SW1.1 needs to be set to ON. The 74LS688 magnitude comparator will be active once the CPU has control of the address and data bus. In other words /G signal will be active when AEN is not asserted (low). Then SW1.2 needs to be set to OFF. That means ROM /WE signal is not active. Which is normal since we don't want to provide writing capabilities to a non-writable memory. However this switch is useful for programming empty ROMs directly in-place.

A combination of SW1.3, SW1.4, SW1.5, SW1.6, and SW1.7 will dictate the start address for the option ROM. Some combinations will generate conflicts with System BIOS or other option ROMs that might be present in the system. So double check their actual address range before configuring the on-board option ROM start address.

CombinationSW1.3SW1.4SW1.5SW1.6SW1.7Start AddressEnd AddressConflict

Construction and Pictures

Let's proceed to the actual construction then.

This is the bare 2-layer printed circuit board (PCB).

PCB details.

Soldering the components.

And here is the printed circuit board assembly (PCBA) with all the components soldered.

Solder side. I was fun to solder all these many terminals but it was not so fun at all to clean the flux residues.

Bench testing phase follows quickly. I am using a modern Corsair power supply rated at 450 W. It is absolutely overkill for my project but it is all I could find around with a fair price-tag and from a reputable manufacturer.

All good by now so the PCBA is mounted in the PC case that I prepared for this project.

Note the first ISA card installed: a Tseng Labs ET4000/W32i equipped with 2 Mb RAM.

I got this ISA card from Ted Allen, owner and designer at Micro-Labs, Inc. He is a very nice person and he still has some of these fine cards for sale. Check this eBay link for more details. Ted has other ISA video graphics cards as well so please take a look since it worths if you're into 1990s PCs.


Next comes the testing phase with real hardware. I had used the following ISA cards.

  • Single Board Computer Card based on Intel Pentium 100 MHz with 96 Mb RAM
  • Tseng Labs ET4000/W32i Video Graphics Array with 2 Mb RAM
  • Intel EtherExpress 16TP LAN
  • AZTECH 16 Sound Blaster Clone

And some standard hardware.

  • 3.5" 1.44 Mb Floppy Disk Drive (Drive A:)
  • 5.25" 1.2 Mb Floppy Disk Drive (Drive B:)
  • Quantum Fireball CR 4.2 Gb IDE Hard Disk Drive (Partition C: and D:)

Detailed view of the Intel Pentium 100 MHz CPU. It has a small fan but I am pretty sure it can work without one if a bigger heatsink is used. TDP is about 8 W.

Games run smoothly. If you are too young -- good news for you, you have an entire life ahead -- to recognize the games below, here they are.

  • Doom (id Software, 1993)
  • Duke Nukem 3D (3D Realms, 1996)
  • Quake (id Software, 1996)

I had a lot of fun designing this project and building it. I fulfilled a small part of my dream from 26 years ago. Time to move on to the next project though...

Constructing a Second Backplane

Here I am in June 2020, already thinging into the future about my next project. So, still having some bare PCBs left from the initial order, I decided to build a second backplane. I will use this one with a new 80386 CPU based server. More on this at a later point in time. For now let's concentrate on the build.

Again, the bare 2-layer printed circuit board.

Soldering the components.

Capacitor array.

More capacitors.

I am missing a resistor array since it is backordered with Mouser.

These LEDs are eye-catchy.

This design is already two years old. I can't believe how fast time flies by.

Solder side looks nice. This time I had fun both while soldering all these THD terminals and while cleaning the PCB. Because I used pure acetone -- sic.

The backplane PCB assembly is mounted in an old AT case which I easily converted to ATX by slightly altering the internal mechanism of the power button. Then I soldered wires directly to the button terminals and made a small cable that connects to the backplane Power Switch header.

Versions and Revisions

This section lists the project version and revision history.

VER. 1.6 REV. A

  • Fully functional initial production version

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