486DX266: Rebuilding my childhood PC

16Mar/170

486DX266: Rebuilding my childhood PC

I've been itching to do this for a while. There've been some nice 486 motherboards coming up on eBay, but I haven't been in the position to get them until now. A Hippo VL+ board appeared and, although it had suffered battery damaged (and been repaired!), it was a bargain for the price. I've always wanted to tinker with the VESA Local Bus and now was my chance. I did have a VLB board back in the day, but hardly knew what it meant.

Base Components

This was to be a completely build-it-yourself project. Each item was individually selected from whatever came on offer via eBay. First up was the motherboard with 4mb of RAM and that all-important exactly-what-I-wanted 486 DX2/66 processor.

You can see where the previous owner has repaired a leaking CMOS battery. The wiring has been well-done and is all hot-glue-gunned down. The coin cell slot is in its place, but I seem to still be getting CMOS battery low issues. It could be that the coin cell battery is only 3v when the CMOS needs 3.5v+.

Next up was the VLB controller card. These full-length half-height cards provide two IDE channels, two floppy drives (via one cable), 2 com ports, a game port and a printer port! The HDD activity LED is also attached to this card.

Finally, the all-important video card. I happened across a cheap S3 86C805i VLB card. It has expandable RAM of which I'll try and fill later on.

There are only three slots where these cards fit into. You'll also find that they are named as to their purpose! There are two 'master' slots and a 'slave'. For those playing at home, the 'master' slots allow cards that support bus mastering; where the card will receive direct information rather than data via the CPU, and have it's own processing power on-board. The slave slot is for cards that don't support bus mastering but want the additional bandwidth of the VLB.

A quick trial assembly then took place. Be really careful when doing this on a flat surface! The end plates have tabs that are meant to slot into the case when the board is mounted... when not, they will simply push into your desk/table and pressing further is strongly discouraged!

Housing

I picked up a AUD$1.00 ATX case from eBay. Yes. ATX. It'll never work, you say? I have previously already put a 386 in an ATX case, but that was whilst keeping its old AT power supply and having the power cord hanging out the side. Not very neat. This time around we're going to do things differently.

Meet the case, creepy photo, right? It's full ATX... soft buttons and all. It even has an ATX power supply.

How do we possibly use an ATX supply with an AT motherboard? With an adapter plug! This beauty was found on eBay for the handsome sum of AUD$5.00.

So, simple mounting of the board ensued. Turns out the back plane is riveted into the case; annoying as previous cases I've dealt with usually allow this panel to be removed for easier motherboard mounting. Either way, the motherboard was mounted in and the adapter was hooked to the power supply.

The other end was then slapped onto the board itself. Remember with AT plugs, keep the black wires together!

Great ... so ... now what? The front panel can be rudimentarily hooked up. Reset, Power LED, Turbo LED? and that's it.

The case was stood up, all cables checked and arranged neatly so as to not cause any shorting issues and then a monitor was connected. The power supply was then jump started. This is simple: simple join the 'dangling' green and black wires together.

It worked!

ATX case + power supply without an ATX motherboard

Here's the trick you've been waiting for. The power button on the front of the ATX is momentary. This means that when you press it, power is conducted for only the timespan until you let go. So, if we hooked this up directly to the green/black wires from the power supply, we'd need to hold/tape the button down until we were done with the computer... not very handy.

Because of this, we're going to use a relay to latch the required power supply wires. The basic idea is that we'll put the momentary button on the green and black wires from the power supply and then latch those down when power starts to flow. To do this, we'll use a drive power plug which will supply us with 12v/5v when we've pressed the power button. This power feed will then latch a relay that'll latch the power button. When we let go, the power will continue as the relay will emulate an extended press!

So, apologies for the crude diagram, but that's the basic idea above. The relay is a 12v single-pole-double-throw and therefore has power, common, normally-connected and normally-open terminals. Power is used to actually throw the relay and is fed from the 12v drive power. Normally-opened isn't used as we only care about the actuation of the relay. The normally-closed side is therefore put in parallel with the power switch. When the switch is pressed, the relay fires and reinforces the power button.

The best part? It worked perfectly! The machine came on and I played A-Train. The problem? Hitting Reset did as expected... it reset the machine, but I had no way of turning it off without yanking power cables! I originally hoped that reset would trip the drive power and reset the relay; alas it turns out that the 12v stays constant. We therefore have to amend the circuit to allow powering down the machine from the reset button.

Here, we've taken the reset switch off the motherboard and used it for an off switch. This means that the machine is power-for-on, reset-for-off. It makes sense... doing an actual reset will just mean hitting reset-and-then-power instead of just reset.

In the above diagram, we've used a second identical relay to interrupt the power supply of the first relay. To do this, we've fed one of the coil power feeds of the first relay through the normally-closed line of the second relay. This means that, when the second relay is not active, the current can flow through it to the first relay. Exactly as we want, as we don't want to do anything until the reset button is pressed.

When the reset button is pressed, it'll actuate the second relay and break the current to the first relay. This will, in turn, break the latch on the power switch and the ATX supply will shut off. Pressing the power switch again will re-latch the power and we'll be up and running again.

Actually building it

I used some crappy veroboard and a spare drive power plug that I had laying around. Hot glue then helped to insulate everything. Note that in my initial sketch, I was trying to be overly-tricky and had the 5v power fed through the other side of a DPDT really. This is redundant and really not needed!