Turbo XT board refusing to enable Turbo mode

[nestor]

I have a strange problem with a Turbo XT clone motherboard. Originally the motherboard runs at 8 MHz, and shorting two pins switches to 4.77 MHz (there are another two pins to connect a Turbo LED indicator).

This afternoon I installed the motherboard in a case with a VGA card and a floppy disk controller, and run some software. Some minutes after, the board started to lock randomly, so I removed it from the case and started to track the problem, removing all cards and all ram and started to put it again. The motherboard booted, but after POST the turbo led was turned off and it switched to 4.77 MHz. It continues to do it, no matter if the turbo jumper is shorted or not. During memory RAM test I can switch turbo on and off and the speed changes (ram count speed is affected and also does the Turbo LED), but after POST beep the speed is locked at 4.77 MHz and the turbo LED doesn't lit again. A Ctrl-Alt-Del sequence make the turbo switch mechanism to work again just before the BEEP, then it is disabled again.

I have tried to:
- Replace the 8088-2 with a V20
- Replace the power supply
- Install only some banks of RAM
- Remove all cards
- Replace the original BIOS with another different (AMI 8088 BIOS)

and the behaviour remains the same. What can be causing this problem?


Here is a picture of the board:


Click to enlarge

 

[Chuck(G)]

Your board looks like one of the generic "Turbo XT" boards from Taiwan, using the ERSO/DTK design. I think I have one like yours--this is mine

If that's the case, have you tried the ERSO/DTK BIOS that was made for this board?

Switching between normal and turbo mode is Ctrl+Alt+(keypad something, I don't recall exactly).

Oh, and did you always have those extra EPROMs? I assume they're BASIC, but you might want to test the system with them removed if they're new.

 

[nestor]

Mine is not identical to the one on th99 you linked, it has less jumpers and different placed. The JP3 jumper in mine selects 4164 only / 41256 + 4164 mode there is no JP4. JP6 and JP7 for instance are not present, and they control the Turbo software switching feature.

The extra EPROMs are BASIC, I removed them and I found no changes. But after that I found what makes this behaviour: the DMA2. If there is not floppy controller or the one installed is in any other DMA than 2, the board switches to 4.77 clock. I don't find it logical, but it seems to be in this way... also I couldn't find any key combination (Ctrl+Alt+ key) to switch turbo, so I think this board doesn't support software switching.

 

[Chuck(G)]

I can understand the "switch to 4.77MHz" in the case of floppy access--many of the Taiwanese BIOSes drop the speed of the CPU to 4.77MHz during floppy access. If an error results, I can well imagine that the original speed is not restored.

On the BIOS that I attached, the turbo mode is controlled by Ctrl-Alt-Keypad -; in reality, most of these designs use a software switch via port 61h. On my board it's bit 2 on that port; 1 = turbo; 0 = non turbo. Obviously, since your BIOS changes the speed during floppy access, it's software programmable.

What few people understand is that the Taiwanese clones were mostly variations on the ERSO design. And no, I don't mean the European Rope-Skipping Organization; I mean the Electronic Research and Services Organization, sponsored by the government of Taiwan. In an effort to boost high-tech innovation, they designed and made freely available much software and many reference designs.

See, for example, this document.

It was the Taiwanese clone-makers that were the game-changer for IBM, not Compaq.

 

[strollin]

[deleted]