386 / 486 Mhz LED displays - how do they work?

Discussion about old PC hardware.

386 / 486 Mhz LED displays - how do they work?

Postby Silent Loon » 2008-11-29 @ 08:11

Does anyone know how those old mhz led displays work that were very common in 386 / 486 AT cases?
I read somewhere that it is all faked, and that those panels actually are not connected to the board. The display is factory pre-configured, so it allways shows factory defaults only changing when you push or release the turbo button. So if you change the board or /and the cpu i.e. from a 486DX33 to a 486DX4-100 it still displays 33mhz.
Is this true? I recently bought such an old machine. It runs fine, but the led mhz display doesn't work at all. There is no connection from the lcd panel to the mobo (only from the power button, I think) How to connect it right?
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Re: 386 / 486 Mhz LED displays - how do they work?

Postby Amigaz » 2008-11-29 @ 08:40

Yep, the were just pre-set with jumpers and often just goty s signal from one of the turboled pins to switch display mode.
Some didn't connect to the mobo at all like on my Vobis Highscreen Colani tower...it's controlled directly from the turbo button
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Re: 386 / 486 Mhz LED displays - how do they work?

Postby dh4rm4 » 2008-11-29 @ 09:22

In other words, bling was just as useless then as it is now. :)
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Re: 386 / 486 Mhz LED displays - how do they work?

Postby Harekiet » 2008-11-29 @ 10:00

Pah it was great, you could sell your 100 mhz as a 120mhz one just by changing the display. It's 120 it says so!
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Re: 386 / 486 Mhz LED displays - how do they work?

Postby dh4rm4 » 2008-11-29 @ 10:31

Nothing like a little lipstick on a pig eh? hehe
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Re: 386 / 486 Mhz LED displays - how do they work?

Postby Malik » 2008-11-29 @ 11:20

Silent Loon wrote:Does anyone know how those old mhz led displays work that were very common in 386 / 486 AT cases?
I read somewhere that it is all faked, and that those panels actually are not connected to the board. The display is factory pre-configured, so it allways shows factory defaults only changing when you push or release the turbo button. So if you change the board or /and the cpu i.e. from a 486DX33 to a 486DX4-100 it still displays 33mhz.
Is this true? I recently bought such an old machine. It runs fine, but the led mhz display doesn't work at all. There is no connection from the lcd panel to the mobo (only from the power button, I think) How to connect it right?


Initially the main reason the LEDs were put up was to show what speed the system is currently running. Those days, most systems come with a turbo button which when you "un-turbo" it, it runs in a "slower" mode.

My first intel 8088 XT PC ran at a super speed of 10MHz!!. When turbo is off, it ran at a slower but yet spectacular 6MHz!

Nope, afaik, the LED is not connected to the motherboard. It's just to show the MAX speed capable of running and an approximate non-turbo slower speed. The slower speed is only an "approximate" since different motherboards utilize different methods to "slow" the system down.

To make it work, you have to connect an extra power line, sometimes labelled in some casings, to the LED board. This power line is usually a 2-pin jumper connector, usually with a red and a black wire. Otherwise, some good AT casings will have the jumper connector labelled as LED. Otherwise you can buy a 2-pin jumper connector with a molex connector at the other end to connect to the power supply's connector.

The connectors for the power supply to the LED board is usually situated at the centre.

There'll be 2 double rows of jumper pins, at the top and bottom of the board. This is where you place the jumpers to display the numbers.

As for the numbers itself, you have to play around with jumpers directly on the LED board to get the numbers you want.

Usually we set the numbers to be at the default speed. Like my previous AT-based Pentium 1 was 133MHz. The numbers at turbo-off mode is set at 66MHz. (an easy approximate to set it to half the actual speed).

Of course, some opportunistic 3rd party dealers / resellers sold the system with the LEDs displaying higher false values to non-tech conscious customers. Even if it IS slow, they'll never know.
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Re: 386 / 486 Mhz LED displays - how do they work?

Postby Moogle! » 2008-11-29 @ 16:24

Turbo doesn't seem to halve the CPU speed on my HOT433. Normally it reads 134MHz, but with turbo off, it reads 133, but it is still slower. I think it disables the L2 cache, but I haven't checked it.

I have actually seen an LED display that seemed to actually have some kind of microprocessor on the back, programmed to say 333, and occasionally it would bouce up to 334, or down to 332, though it still only seemed to be hooked up at the turbo switch. The system itself was a PII Xeon.
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Re: 386 / 486 Mhz LED displays - how do they work?

Postby GL1zdA » 2008-11-29 @ 22:03

Moogle! wrote:I have actually seen an LED display that seemed to actually have some kind of microprocessor on the back, programmed to say 333, and occasionally it would bouce up to 334, or down to 332, though it still only seemed to be hooked up at the turbo switch. The system itself was a PII Xeon.


Wasn't it some server monitoring LCD? They would probably display accurate data instead of predefined values.
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Re: 386 / 486 Mhz LED displays - how do they work?

Postby Moogle! » 2008-11-30 @ 04:53

Nope. It was just an LED display, as far as I know. Didn't have any mysterious wires going to it. Just power, ground, and switch.
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