Defective SCSI on Performa 5260 logic boards - common issue?

[LCGuy]

So as those who read in Conquests will know, yesterday I picked up a free Performa 5260. Anyway, it seems that the SCSI bus on its logic board is blown. Everything else, however, seems to work.

Meanwhile, back at home, I also have another Performa 5260 board that I was running in my LC630 shell for about 4 years, till its internal SCSI blew out (while i was testing an external SCSI HDD, and blamed the HDD for that). Same story...IDE works, as does every other part of the machine, apart from the SCSI bus. Neither internal nor external SCSI works.

I know that the Performa 5260 logic board uses a revised design, which has an onboard ROM (rather than the combination ROM/L2 Cache SIMM the other Performas used)...it just makes me wonder...is there a common issue with these boards that is causing them to lose their SCSI buses? I just think its strange that I have two boards of the same model that have the exact same problem...it'll be interesting to see if anyone else here has encountered this.

 

[Gil]

Not sure if its a common issue, but I had a 5400 board with the SCSI bus blown.

 

[Franklinstein]

I've got a 5420 whose SCSI subsystem blew when I was trying to resurrect a flaky internal CDROM drive, swapping it when the machine was powered up. That's a no-no; these machines aren't designed to handle it. Probably something to do with the active terminator that these things (54/5500s and similar) have on the logic board.

The active terminator is supposed to function automatically, depending on what's connected to the bus and how the devices are configured. The Apple Developer Notes make mention of the presence and function of the thing, though I'm not sure which chip the terminator is, or how to test it. Of course, the Apple Service Manuals only say "replace logic board" when a problem like this occurs.

There could also be a fuse squirreled away somewhere that I couldn't see. Many machines (particularly PowerBooks) had protection for the SCSI bus in case somebody mistakenly plugged a PC parallel device into it or something, but with the cheapness of many of these consumer machines, they may have decided to save the .001 cents required and omitted it.

edit

Found my stash of developer notes, so more info:

The 5260 uses passive termination, actually, but it's still automatic. The SCSI subsystem is part of the F108 chip, originally introduced in the LC 630 and its siblings. From the Developer Notes for the 5260:

The internal end of the SCSI bus is terminated by a 220/330 passive terminator. Theterminator is located on the main logic board near the portion of the internal chassis

connector that contains the signals for the internal CD-ROM drive.

Other machines that use passive termination include those based on the Curio chip, such as the 75/85/9500 series.

Models utilizing the O'Hare chip (54/5500 and similar, PowerBook 2400/3400 thru WallStreet) have active termination. From the 5400 dev note:

The internal end of the SCSI bus is terminated by an active terminator. The terminator islocated on the main logic board near the portion of the internal chassis connector that

contains the signals for the internal CD-ROM drive. On enclosures with only one

internal SCSI device located close to the logic board, the active termination is

automatically enabled. On enclosures with multiple SCSI devices, the active termination

is disabled, and a positive terminator is located at the end of the internal bus.

/edit

 

[LCGuy]

Hmm, thats a thought...I never ever considered that these machines had onboard termination. I'm thinking...the terminator would be a resistor or a set of resistors...surely replacing some of these as a trial wouldn't be too expensive, even if they are the surface mount resistors. I'm looking at my 5260 board now, and on the bottom, there's a bunch of 220's together in one area at the following locatiosn, right near the edge connector: R250, R249, R254, R253, R258, R257

Then there's another couple a bit further away at R236, R237, R246, R245.

Oddly though i can't see any 230's anywhere.

I also had a look for fuses...i can't find any though...maybe they decided not to fit a fuse, as you said. Then again...it might be so small i simply just didn't take any notice.