SCSI for Dummies Thread - How to properly use SCSI

trag

Well-known member

That's the article I wrote way back when. By today's standards, way too much detail on the soldering and probably some of the (soldering) details are bad advice. But it is more than 25 years old, so...

But there was also discussion in the forum regarding specific cards and firmware versions of the 2940UW. Also, some times there would be a front page item from Mike when I reader reported converting a newer version of the card.
 

volvo242gt

Well-known member
Jeff,

Ah, ok. So, that info may be found in one of the more recent scrapes of the site done by the wayback machine... I didn't check to see if the forum had been archived or not, though.
 

Realitystorm

Well-known member
The Quadra 900 and 950 did indeed have two separate SCSI busses with each one having their own 53C96 controller.
Technical Note HW540: SCSI Port Q&As: The Macintosh Quadra 900 is the first Macintosh computer to provide a separate, internal SCSI bus. This bus is physically isolated from the external SCSI bus and must also be properly terminated. The cable provided with the machine includes all the termination necessary, so ALL internal devices must have SCSI termination removed before connecting to the internal Macintosh Quadra 900 SCSI cable. If extra termination is supplied it may cause intermittent hardware failures as well as physical damage to the device.
 

Realitystorm

Well-known member
I wanted to put together a short guide for how to use SCSI devices on my site as part of my page on comparing modern storage options. Based on what I read in the 1990 Macintosh Reference Guide, and the Macintosh Plus Owner's Guide. It looks like the Macintosh Plus requires a pass through terminator at the start of it's SCSI chain (two or more devices) if the initial device does not contain a terminator. Based on what I've read, this is also a requirement for other Macintosh systems if you have more than one external device connected and you do not have an internal SCSI device installed, or your internal SCSI device is not providing termination (E.g. if you have a LC 630 with an IDE hard drive you internal SCSI CD-ROM drive must be terminated)

Capture.JPGSCSI_chain.JPG
 

ironborn65

Well-known member
Hi @Realitystorm thanks for your contribution, I used your content very often, thank you so much.

About the diagram, can I ask how it can address the cases where many SCSI devices are inside the Mac?
Can it be taken into account?
 

Realitystorm

Well-known member
About the diagram, can I ask how it can address the cases where many SCSI devices are inside the Mac?
Can it be taken into account?
Hi @ironborn65, I've been looking in to what happens when you are dealing with both internal and external devices in a SCSI chain, and I've updated my page to add more information for the main situations I could think of:
  1. No internal SCSI devices and only one external SCSI device,
  2. No internal SCSI devices and two or more SCSI external devices
  3. Only one internal SCSI device is installed (optionally with external devices too)
  4. Multiple internal SCSI devices are installed (optionally with external devices too)
It has been difficult to find answers for the 2nd option, it appears that for some Macintosh systems if you remove all the internal devices you need to double terminate the external SCSI chain, so if you have removed all your internal SCSI devices and are getting errors with your external SCSI devices you may have to add a second termination point at the beginning of your external SCSI chain.

For the termination of the internal part of your SCSI chain, there appears to be 3 options:
  1. jumpers/resistors on the device (LC520 and others)
  2. a terminator attached to the end of the cable for systems supporting multiple internal devices (Quadra 900)
  3. or a terminator attached to the beginning of the cable / on the motherboard if only one internal SCSI device is supported (LC630)
For the 2nd termination for internal SCSI chains, that appears to be automatically handed by the system where the system provides the termination if no External SCSI devices are installed.
 
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