IIci Unterminated When No SCSI Devices At All Are Attached

[mars]

I'm having some weird SCSI reliability issues with my IIci, but the very first thing I want to confirm is:

Is it expected behavior for the SCSI bus in a IIci to apparently be unterminated (as claimed by SCSIProbe 4.3) when zero devices are attached (meaning no internal HDD and nothing external)? I suppose it wouldn't really matter if it's unterminated in such a situation, since you don't have any devices using the bus anyway, but shouldn't there still be some kind of termination built into the SCSI controller in effect still? And wouldn't SCSIProbe detect that? An annoying side effect of this situation is that if I do connect a device internally that doesn't provide termination, SCSIProbe can't detect the device and continues to claim the bus is unterminated, until I put a terminator on the external SCSI port (by way of attaching an external CD drive that I can plug a centronics terminator into).

 

[aladds]

The chain is technically unterminated if you don't have anything plugged into it but that doesn't really matter, as you say.

There isn't termination built in, but the controller is there, in the middle between the external and internal chains, and provides the required pull up / down if the chain only goes "one way" due to its signalling anyway. I suppose there is the small "cable" from the SCSI controller to the port on the back of the machine, but I would assume that that is sufficiently short that it doesn't have a noticeable effect at SCSI speeds.

Bitsavers have an old controller data sheet here: http://www.bitsavers.org/components/ncr_symbios/scsi/53CF94/NCR53CF9x-2.pdf
Chapter 6 has some good details about timings and capacitance of the various SCSI signals.

An internal device at the end of the chain should have termination. You need to terminate both ends of the chain, unless one end is the Mac itself. If the drive in the Mac can't be terminated you can get a terminator which goes on the end of the internal SCSI cable (which will need one more connector than you have drives)

 

[mars]

Is SCSIProbe then incorrectly reporting the bus as terminated, when only the external end is terminated (in the setup where I attach an external CD drive with a terminator but the internal drive is still unterminated)? Maybe it can't distinguish between one or both ends being terminated.