Macintosh II, BlueSCSI, and termination

[DevyDevly]

Hi All,

Does the Macintosh II require a SCSI terminator on the external SCSI port if you're running a BlueSCSI v2 on the internal SCSI port?
I've not been able to find any clear guidance.

Thanks,
Devy.

 

[robin-fo]

No, as long as your internal BlueSCSI is terminated.

 

[DevyDevly]

Thanks Robin-fo.
My BlueSCSI v2 is definitely jumpered for termination so I must have an issue with my SCSI ports/controller or BlueSCSI, as booting from the internal port only works with an external BlueSCSI (without card) plugged into the DB25 port. I'll investigate further.

 

[ScutBoy]

With SCSI on old Macs, there are two modes.

1. What the standard says should work.

2. What actually works on your machine.

The difference in "why" between the two is sometimes unknowable.

 

[DevyDevly]

Heh… not an answer I was expecting - Cabbage Patch dolls were in vogue around the same time I suppose.
Thanks for the heads up

 

[jmacz]

FWIW, on one of my SE/30s, I had a similar issue where internal only worked when an external device was plugged in. After much debugging, it came down to an overly sensitive req line (due to the scsi controller) and I had to add a resistor to mimic what an active scsi terminator (like the iifx black terminator) does. Not sure if you have the same issue but just an fyi. I had mentioned this in this post below:

Post in thread 'SE/30 won't boot from internal SCSI'
https://tinkerdifferent.com/threads/se-30-wont-boot-from-internal-scsi.4244/post-37625

 

[DevyDevly]

Thanks @jmacz - I've been reading your threads and this sounds like a great lead - the Mac II uses the 53C80 (but not 53C80E) so chances are high of something similar.
I'll also check out the strength of some of my traces (VIAs) around the SCSI controller - the machine was quite corroded when I got it and has already had some trace repair.

 

[DevyDevly]

Problem solved!
But the problem was all of my own making... there's no way I'm ever going to graduate out of "New Tinkerer"

Turns out I didn't properly solder (or test) one of the 110ohm resistors on the BlueSCSI I put together - once I correctly soldered the resistor in place the BlueSCSI worked flawlessly on the internal SCSI connection without anything required to be connected on the external connector. That resistor must have been key to termination (one of the 18 on the board - so makes sense).

Thank for everyone's help and sorry for the false alarm - I learned a little bit more on this SCSI journey so that's a good thing - and I still hope to level up to Tinkerer one day

 

[ScutBoy]

Hey - you found it - that's the important thing!