ADB Fuse Query

[beachycove]

Not sure where to put this but I will try here: I have come across a troublesome LC575.

The machine will start up by pressing the power on button on the keyboard, and will boot normally to the desktop. However, that's not a lot of good, because it will not recognize the keyboard or mouse (cursor will not move, etc.) — apart from the power on button. The mouse and keyboard are known working.

Is this consistent with a blown ADB fuse or is there some other explanation? (N.B.: There are two "ADB" looking fuses on the logic board (silver, about 1/4" long).

 

[ClassicHasClass]

It's possible, though I've never opened the ADB fuses myself with any of my Macs.

 

[uniserver]

silly question, have you tried a different keyboard?

 

[olePigeon]

I think he mentioned that the keyboard and mouse are known to be good. If you have a soldering iron, the ADB port is one of the easiest things to replace. I replaced a broken ADB port on an Apple Extended Keyboard.

 

[beachycove]

Both fuses are good, showing 0 Ohms or thereabouts.

I think i will try replacing the ADB port itself.

 

[uniserver]

you can try with 2 hands put some pressure on the ADB keyboard plug, dunno up,down,left,right at the same time tap on keys or wiggle the mouse... or wiggle the port while wiggling the mouse. see if the cursor moves at all...

nice thing the ADP is plug and play.

i had a LCII i was playing around with, i was pinching the capacitor cans to try and make the sound not squeal, i ended up pinching the ADB can because mouse and keyboard stopped working, i was lucky i pinched the can the opposite way and it the mouse and keyboard came back, so at least i do know with the LCII there is a cap that could keep the keyboard from working.

 

[ClassicHasClass]

Both fuses are good, showing 0 Ohms or thereabouts.
I think i will try replacing the ADB port itself.

I think the port probably fails more than the fuses do. I've hotswapped ADB keyboards and mice for years, even though this is "bad" and technically unsafe, and never opened the ADB fuses on any Mac.

 

[Cory5412]

nice thing the ADP is plug and play.

By "plug and play" -- do you mean "hot swappable"? (they are different things.)

Technically, nothing horrible will happen if you accidentally unplug an ADB device and plug it back in while the machine is running, all ideal circumstances considered, but it is not hot-pluggable and Apple's user manuals are always quick to caution that you should turn your machine off if you plan on re-configuring the ADB devices.

It's not wildly common (aka "it won't happen every single time") but hot-plugging ADB devices can damage the ADB controller on the Mac's motherboard, as well as chips on the ADB devices.

 

[Bunsen]

If you have a multitester, which, having a soldering iron, I am going to go ahead and presume you do, try testing for continuity between ADB pins and the matching solder points on the logic board. This will tell you whether the port itself even needs replacing. Also, inspect the solder joints themselves, and/or touch them up.

 

[defor]

On the technical side of things, ADB is NOT hotswap nor was it ever designed to be.

The ADB table is normally only initialized with data from devices at startup by the ADB manager. A reinit can be forced (there are utilities for this), but it is not very clean and can cause issues on a running system.

The ADB table contains a list of every device connected to the system ADB bus and unique ids. by hotswapping, it is possible (usually, but not exclusively, with devices that have custom drivers beyond the basic apple keyboard/mouse) to create issues with data being sent to the wrong spot in the device table if the devices don't re-enumerate in the same order.

Also, in general, there is no hotswap circuit protection required in adb devices.

Once upon a time I had a friend who insisted that his ISA cards were hotswap, they just needed a restart after you put a new card in a slot or they wouldn't show up. Same general logic. ADB is a rather simplistic bus, but it has no provision for being truly hotswap.

 

[Dennis Nedry]

Definitely try booting from a floppy or CD just to verify for sure that it's not the OS doing it. If that's not easily available, take the back cover off, unplug the hard drive, turn on the Mac, and try moving the mouse when you see the question mark / disk icon. Also try unplugging the keyboard and attaching the mouse directly after you turn it on. As mentioned, it's not too bad to hot-plug those cables, especially if you have a good reason to do it.

These are just easy, harmless things to try that could maybe provide useful clues before you get out the soldering iron.