SCSI FAQ
By Dan DeMaggio
Copyright (c) 1993 Apple Users' Group, Sydney
Republished from Applecations, a publication of the Apple Users' Group, Sydney, Australia.
Source: Internet
The FAQ (Frequently Asked Questions) post
Topic: SCSI
SCSI is a protocol (kind of like serial or parallel) that lets you hook up several devices (up to 8) on a SCSI bus (a bus is just a series of wires). You must give each device it's own unique ID number from 0-7. The SCSI card is usually set to 7. There are two types of SCSI cables: the 50 pin Centronics-type (like on parallel printers) or the 25-pin "D" connector. The 50-pin is the SCSI standard, the 25-pin is the Apple standard. On a SCSI chain, there must be a Terminator at each end. A Terminator is just a bunch of resistors. Some drives have internal terminators (3 small yellow-orange packs), and some drives come with an external terminator (a "plug" to put on the back of the drive). Also, somebody on the bus must supply terminator power (one of the SCSI lines). If There are any problems (multiple things with the same ID, too much termination or not enough, or no terminator power), you may be able to use the drive, but your data will get corrupted. Most of the time, the computer will refuse to recognize the drive.
At first, there was the Apple Rev 'C' SCSI card (named after the final ROM version--all previous versions MUST be upgraded to work with current software). There were several clones from the likes of CMS and Chinook. Then Apple came out with it's High Speed DMA SCSI card. This has the ability to do Direct Memory Access to the RAM in your computer, which speeds things up. This created a lot of problems with cards that were not DMA compatible. CV Technologies also has a DMA SCSI card called the RamFast. This card has 256K or 1MB of on-board RAM to make it even faster than Apple's card. It can also supply terminator power if you drive does not supply it. Both of the new cards support things like SCSI tape backup units, removable SCSI drives, SCSI CD-ROM, and of course SCSI hard drives. Both the new cards also require an Enhanced //e.
Subtopic: Tips on setting up a SCSI system:
You can have multiple drives on one SCSI card, just make sure you remove the termination on all the drives but the last one. This is because the newer SCSI cards are terminated (and they count as a SCSI device).
Always check that the cords are plugged in properly. Never connect/disconnect anything when the computer is on. The computer will boot the hard drive with the highest SCSI ID, which should be ID 6.
Try letting the drive 'warm up' for 15 seconds before turning the computer on. The SCSI cards look for drives only at startup, and may ignore any drives that are not ready.
Try turning off DMA. If this helps, you may have a non-DMA compatible card, such as the early versions of the TransWarp, early versions of the GS RAM, or any 8-bit accelerator.
Check that each device has a unique ID. Most drives have a thumbwheel on the back to set the ID. Your SCSI card (yes, it counts too) is probably ID 7. Number your drives from 6 downwards for best compatibility. The IDs have nothing to do with what slot the card is in.
Is there a terminator at each end of the SCSI bus? (the DMA cards are terminated, and some drives are internally terminated.)
Try the software that came with the card. It may give helpful diagnostic messages (I.E. the Apple DMA SCSI utilities-- Does it say "No Apple SCSI card found" or "No SCSI devices found"?)
Do you get the message "Unable to Load ProDOS"? If so, it's booting your drive but you have no system software on it. Try hitting Control-Reset, then PR#5 (or PR#6) to boot a floppy. Then install the system software (i.e. ProDOS or GS/OS).
In extreme cases, try reformatting the drive, repartitioning, and re-installing the System software.
If the drive access light blinks in a regular pattern before the computer is turned on, it is telling you that it has a hardware malfunction. It needs to be serviced.
Did you try re-installing the System software? Many times, the data on a drive will get corrupted if you run the drive with improper terminators or conflicting SCSI ID's. Sometimes you will not notice the corrupted data until after you fix the problem. If re-installing the System software helps, it was probably a software problem, not a hardware problem.
The Apple HS DMA SCSI card requires an Enhanced //e. It will not work on the older //e without an Enhancement Kit.
To really put a drive through it's paces, copy a LOT of stuff from one partition to another (copy the entire partition if you can). If there is a problem with DMA or SCSI ID's, it will probably show up as a strange GS/OS error. (GS only)
Make sure you do not have the Apple SCSI drivers installed if you have a RamFast. It may cause random problems (they leave an interrupt handler dangling if they can't find their card.) (GS only)
Make sure you are booting the right slot. If the card is in slot 7, you can set the startup slot to Scan or 7. (GS only)
If you boot up and only 1 partition shows up, you need to install the SCSI drivers. (GS only)
If you boot up and it says "Drive XXX is already on the desktop" over and over: Probably a SCSI ID problem. (GS only)
This FAQ is available on-line at apple2.archive.umich.edu and via AFS
(in /afs/umich.edu/group/itd/archive/apple2). Permission granted to
copy portions into non-profit User Group newsletters.
Dan DeMaggio (dmag@caen.engin.umich.edu).

THIS CONTENT COPYRIGHT © 2007, APPLE MACINTOSH USERS' GROUP, SYDNEY
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