ATX2AT Smart Converter

While waiting for the first batch of Universal Chip Analyzer being assembled, I took some time to check a lot of old 286/386/486 motherboards. These boards used to be available for some bucks on eBay, but they’re now outrageously expensive. Thanks to the retro-computing gossip! They came in “unknown” condition from a local seller and, after a first (quick) visual check, I tried to power them with a known-good CPU (and some already tested FPM SIMMs).

P8/P9 Connectors

Back in these days (from the original IBM PC circa 1981 up to the mid/late ’90), “AT” boards were powered via the infamous P8/P9 connectors. Like many other old-junk addicts, I use a standard passive ATX to AT adapter (available everywhere for ~$10) with a modern ATX PSU. At first sight, the only real drawback is their lack of -5V. ATX power-supply doesn’t output that negative voltage, but it’s not really a big deal: -5V was only used for some ISA boards. The famous Sound Blaster 2.0 (CT-1350B) from Creative Labs, as well as Roland LAPC-I and 3Com Etherlink 16 TP, being the most popular ISA extension boards that require -5V.

Passive ATX->AT Adapter

You can find two kinds of ATX to AT adapters: the cheapest ones have a built-in strap to ground the PWR_ON signal. As soon as you plug the adapter, no matter what’s connected or not, it will start the ATX PSU. To power on/off the whole platform, you must use the switch on the PSU. Other adapters have a (usually bulky) switch to trigger the PWR_ON signal. That’s better because if something goes wrong, the current from the PSU can be stopped much faster. If you just turn off the main switch, the big primary capacitors inside the power supply will continue to deliver their energy for quite some time.


But let’s go back to my old motherboards. The first one was a nice 486 PCI board, powered by a SiS chipset. I connected everything for the first run and flipped the switch on my ATX to AT adapter. After about 3-4 seconds, I heard a sinister crackling noise and immediately switched off everything. Some magic smoke came out from a tantalum cap. I removed everything and carefully checked the damage. The board was FUBAR: two caps almost totally destroyed and, much more serious issue, I had two burnt PCB traces on the back side. I would realize later that a voltage regulator was also destroyed in the process. Unfortunately, my 486 DX4-100 died. It seems the 3.45V regulator was bypassed and the CPU received 5V for some seconds. Both SIMMs survived, but I raged quite a bit and it was time to do something about that.

Burnt PCB Trace

The root cause of all these damages was the time needed to react. When you ear cracking noise, it’s often too late. The problem got even worse because of the tremendous power that a modern ATX PSU can deliver: up to 20A on each rail, and sometimes much more. And you can’t count on the integrated overcurrent limitation. The power drawn by an average 486 motherboard is less than 2.5A. If something fails and creates a short with a low resistance (let’s say 0.25 Ohm, that’s really small), the PSU will continue to deliver 20A without shutting down on overcurrent condition. Enough to burn small PCB traces in less than 2 seconds.

Adding a couple fast-acting fuses on the +5V/+12V power rails, directly on the cables, could be a good solution … but it’s not. First because the “fast-acting” is not really THAT fast, then because you will burn a lot of fuses if you need to fix a board, and finally because there is a big gap in power consumption between a 386SX motherboard and a Socket 4 (Pentium 60) AT board. You don’t want to spend your time messing with the fuses, right? Right.


Please welcome the “ATX2AT Smart Converter” !

 The ATX2AT Smart Converter plugs on a standard 24-pin ATX connector and acts as a protection device for your good ol’ retro-hardware. It can be seen as a couple of programmable fuses with monitoring capabilities. You can set the current limits and define how quickly it will react to an overpower condition.


Here is a bare description of the integrated features:

  • +5V/+12V Programmable Over-Current Protection: Almost all pre-Socket 5 motherboard drawn 90% of their power on +5V. But I also see some dramatic failures on the +12V, finding its way to the +5V and burning everything. The ATX2AT Smart Converter monitors the current drawn on both rail in real time. 8 different limits can be set with the dip switches. For example, my 486 boards run fine with the default settings: 4.00A max for +5V and 500 mA (0.5A) for +12V. But you can also choose 1.50A/0.20A or up to 9A/5A.
  • Configurable reaction time: both integrated “virtual fuses” can act as slow-blow or extremely fast-acting fuses. At 350 ms (still much faster than you), the “slow” option is usually enough to prevent any damage without false-positive triggering, but sometimes, the ultra-fast option may be useful. We’ll discuss that later
  • Integrated -5V Converter: the ATX2AT Smart Converter uses a 79M05 to generate the missing -5V rail. It can be used with your nice AWE32 Upgrade Card or any other card that require -5V.
  • -12V/-5V Protection: Both negative voltage is usually never used by the motherboards. Limited amounts of power are available for extension cards, but they may fail. Two 200 mA PTC (resettable fuses) are soldered to prevent damage in case of shorts on negative rails.
  • Current monitoring: real-time current monitoring is displayed on a small 0.96″ OLED display. It also shows the actual current limits set and warn you if an overcurrent is detected (also telling on which rail it occurred). The overall precision is ~2%. The PCB includes a 0.1% voltage reference.
  • Failure-proof: sh*t happens! If, for some very unlikely reasons, the ATMega328 MCU or the current sensor fails, the ATX2AT Smart Converter also includes two very fast acting fuses (10A on +5V and 5A on +12V) as a last-resort protection. They’re mounted on a fuse holder and easy to replace if needed.
  • Current filtering: All 4 rails are filtered with high quality, long life & low ripple electrolytic capacitors.
  • Versatile: Outputs are located on a common 14-pins Molex MINI-FIT connector (same family than the 24-pin ATX connector). You can easily build your own cable to power any other retro-hardware.

Of course, you also have a push button to cycle the power and a status LED. Even the PWR_GOOD signal is current-limited to prevent damage to the ATX2AT Converter in case of failure. Also note that the converter is powered with the 5VSB (Standby) signal from the ATX PSU, so it works even if the PSU is not started.

Slow blow VS Fast Blow

You may think that the faster a fuse acts, the better the protection. Well, not really. On an unpowered board, you’ll find a lot of discharged bypass capacitors. As soon as power is first applied, these capacitors will charge all simultaneously, and acts, for a very short time, as shorts, causing a temporary spike in current consumption. If your fuse is too fast, it will blow because of the very quick overload. The same is true for motors (or a CPU fan), which generated a large peak when they start. To prevent false-triggering, the ATX2AT Converter in SLOW BLOW mode will allow a grace period while the power can be reach +125% of the set limit (but not more than 9A in any case). Let’s see that :

“SLOW BLOW” MODE – 350 ms Reaction Time

This capture show the ATX2AT Smart Converter, configured in the default SLOW BLOW mode with a 1.5A max current limit, connected to an electronic load. The load is then switched from 0.2A to 2.5A. The “grace period” is fixed at 350 ms, then the power is switched off by the micro-controller. This value is set arbitrary and may be changed with a firmware update.

But sometimes, you want a REALLY fast acting protection. For example, on a fragile low-power motherboard. You can set the FAST BLOW mode, which activate almost immediately :

FAST BLOW MODE – 5 ms Reaction Time

In FAST BLOW Mode, the average reaction time is ~5 ms. The OLED display refresh rate is lowered to one time every 5 seconds to let the MCU react as fast as possible. Keep in mind that, after the power down signal is sent, the PSU should react. On my Seasonic Focus+ 550W, it needs ~10 ms, so the whole process between the detected overcurrent condition and the actual power being switched off is ~15 ms. That’s extremely quick and unlikely to cause any damage, even on very sensitive components.

If you wonder if you should use the FAST BLOW Mode, answer is simple: you shouldn’t. And here is why. Both the previous captures were done with lab equipment and an electronic load. In real-world condition, the current peak on failure is usually way bigger, with a much more aggressive ramp time. I tried a suicide run with a working board and a 486 DX2-66 wrongly plugged (180° reverse) in the Socket 3. Here is the screenshot in SLOW BLOW Mode :

SLOW BLOW MODE – “Real World” Failure

With a strong failure, the +125% absolute limit of the SLOW BLOW Mode triggers almost instantly (in about 500 µs). Good news : both the motherboard and the CPU survived. Tried 10 times in a row…


Here are the settings for the 4-way DIP switched locate on the ATX2AT Smart Converter.

Switch 1-2-3 are used to set the current limit:

+5V Limit : 4.00A
+12V Limit : 0.50A
+5V Limit : 1.50A
+12V Limit : 0.20A
+5V Limit : 2.25A
+12V Limit : 0.40A
+5V Limit : 3.50A
+12V Limit : 1.00A
+5V Limit : 6.00A
+12V Limit : 0.50A
+5V Limit : 6.50A
+12V Limit : 2.25A
+5V Limit : 7.50A
+12V Limit : 3.00A
+5V Limit : 9.00A
+12V Limit : 4.75A

Switch 4 is used to configure SLOW or FAST BLOW Mode.


The push button is easy to use: press to power on, press again to power off. If an overcurrent condition is detected, you must keep it pushed for 3 seconds to reset the protection and power on again.

The information displayed on the OLED display are pretty self-explanatory:

The standard 0.96″ I2C OLED is available in three flavors: white, blue and yellow/blue. They all work on the ATX2AT Smart Converter.


A crappy YouTube video for a nice tool !


A small crowdfunding campaign is planned for this tool. Just drop me an email (check the “About Me” page)  if you’re interested and I’ll keep you informed. The price will not be higher than $50 (worst case), even if we’re only a few to want one (complete with OLED display and AT cable).