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ACSI port not working on Atari Mega ST 2


Calab

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Today, my ACSI port stopped working... It was working previously, but today it got flakier and flakier until it just won't see any devices on the ACSI port. I've tried both an external UltraSatan drive as well as a MegaFile 60 hard drive.

 

I do have a diagnostic cartridge and the Hard Disk test and DMA test are failing. I suspect that the C025913-38 (U27) has failed. I'm reading conflicting information that other chips (C100110-001 for example) will or will not work. Can someone tell me what DMA chips are compatible with an Atari Mega ST 2 machine?

 

One item to note is that the floppy drive appears to be working fine. Wouldn't it also have issues if the DMA chip had failed?


Also, if anyone has any other testing/troubleshooting that I could use to narrow this down, it would be appreciated!

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There are some differences in the communication signals to floppy and HDD, it could be one of those signal lines,

to be sure you would need a scope to check signal levels on each pin.

 

Might also be worth checking U21, pin 10 (IRQ) on 19 pin plug sends a signal to U21 pins 11 & 12.

Although the FDD controller uses the same chip, it uses a different gate.

 

Just in case you don't have the circuit diagram:-

Mega_ST2-4_Schematic_2-19-1987.pdf

Edited by TGB1718
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2 hours ago, TGB1718 said:

Might also be worth checking U21, pin 10 (IRQ) on 19 pin plug sends a signal to U21 pins 11 & 12.

Although the FDD controller uses the same chip, it uses a different gate.

 

Just in case you don't have the circuit diagram:...

Thanks for the diagram. This one is clearer than the one I have.

 

I will check out U21... and I was thinking U26 as well. A 74LS02 and 7406 should be easier to come by than the DMA chip!

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Tried the system again this morning. Still not working.

I swapped out the 7406 at U26, since I had one on hand, and the system worked great for about 5 minutes... then the issue returned. It's not seeing anything on the ACSI connector again.

 

Next will be swapping the 74LS02 at U21, but I have to order one. I will get caps to do the PSU as well at the same time.

I'm also considering the power supply as well. It's original. Getting 4.9v and 11.6v readings on my meter. I just need to find one at a reasonable price that ships to Canada.

 

 

Edited by Calab
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Hrm... Neither Digikey.ca nor Mouser.ca have the 74LS02 in stock. That's not helpful at all.  :(

I think I'm going to swap my PSU for a Meanwell unit instead of messing with the capacitors. Should be more reliable this way.

 

[edit]Meanwell PSU ordered from Digikey... now we wait[/edit]

Edited by Calab
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