SCSI Controller Hanging...

Jules Richardson jules.richardson99 at
Tue Jan 23 12:15:03 CST 2018

On 01/23/2018 10:33 AM, Jack Harper via cctalk wrote:
> Greetings to the List from the Snowy Rocky Mountains.
> Beautiful clear sunny day here at +9F :)

It's a toasty 20F here in northern MN :)

> I have never before dealt with SCSI as a programmer - does this sound like
> something is configured incorrectly?
> There is not much to configure.

I'm not sure about incorrect config, but I'm guessing it could be a 
software issue (rather than hardware) - i.e. board reset is also causing a 
reset of the SCSI bus in some odd way which is then offlining/confusing any 
attached devices. Does it just happen with a reset of one specific CPU 
board, or is it any of them?

> I point out that I am not certain that I have the termination resistors
> correct.

Termination's easy enough - in theory :-)  A single set of termination at 
each end of the bus, never in the middle - and double-termination at either 
end will cause weird things to happen.

If it's a 16-bit SCSI bus then you need termination either end of both 
high/low 'halves' (sometimes people will park an 8-bit device via a cable 
adapter at the end of a 16-bit bus, and stick an 8-bit terminator on the 
device, which leaves the high side of the 16 bit bus unterminated and 
causes glitches)

*Something* on the bus needs to provide +5V on the SCSI TERMPWR line; the 
terminators need this to work. I think typically the bus adapter (i.e. 
controller) will do this, but I seem to recall that the adapter in my Apple 
IIGS has no support for it and expects one of the attached devices to do it 
(some will via a jumper setting, but some won't)

I think that some 16-bit devices will automagically work on an 8-bit SCSI 
bus, but I don't know if there's any guarantee of that.

Other than that, if you've got external devices then try and use the best 
quality cables that you can find - there used to be some really cheap junk 
out there which could cause all sorts of oddball problems.



More information about the cctalk mailing list