Diagnosing SCSI problems is best done in a logical fashion. It is next to impossible to figure out
exactly what is causing a problem if you have everything connected… so the best way to diagnose a
problem is to simplify your test setup. This is done by disconnecting everything and starting with only
one device connected.
Working with one device at a time is much easier than trying to figure out what is going on when you
have multiple devices connected with multiple cables and terminators.
What we will do first is to take the host computer and connect a cable and a known working drive.
The Active Diagnostic Terminator should be connected to the drive. (See Diagram 5.)
Test All Cables
Cables are the easiest part of the system to test. After you have a system running, with a minimum
configuration, remove the working cable and replace it with another cable you want to test. Use the
Terminator to help test the cables. Turn the computer on briefly and verify that the TRM
(Termination Power) LED illuminates. Turn the computer off then turn on the drive sub-system.
Again, check the TRM (Termination Power) LED and make sure it illuminates. Boot up the system
and copy some files to the sub-system, a small application is good to copy since you can start it up
and see if the copy works like the original. Verify when you start to copy to the drive sub-system
that the SEL (Select Line) LED illuminates. Also the REQ (Request Line) and the ACK
(Acknowledge Line) LED should illuminate dimly. They should be flashing so fast that they look
dim. If the copy worked correctly and the LED indicators worked correctly then this cable is good.
Continue with the next cable.
After testing all the cables, visually inspect them to make sure the contacts are all straight and clean.
Make sure there is no obstruction keeping any of the contacts from making a good connection.
Check around the strain relief area for any cracks or breaks, (See Diagram 6.)
If you find any kind of oxidation on the cable contacts use a pencil eraser to clean them, but do not
rub too hard or you will remove some of the gold plating on the contacts. Check all the connectors
on any target devices. Again, if you find the connectors oxidized clean them gently with a small
pencil eraser making sure to remove any residue left from the eraser.
Check for cracks and breaks around the strain relief. To promote longer life try not to bend the
cable too much. The strain relief can only do so much to protect your cable.
Test All Target Devices
Now that the cables have been tested you can next test all the SCSI Target Devices. The first thing to
do before testing the hardware is to make sure that your operating system is not creating problems.
Quite often the problems associated with SCSI are software conflicts. Since we are only testing
hardware right now we must make it as simple as possible. So it’s now time to remove all INITs
(Macintosh), rename CONFIG.SYS and AUTOEXEC.BAT (Dos), and start with a fresh KERNEL
(UNIX). The goal here is to test with only the basics.
After you have removed, changed, recopied, all the unnecessary items in your operating system,
boot the system a couple of times without any external SCSI devices connected. Make sure that you
have removed all device drivers and items that might cause conflicts. Now load in only the necessary
device drivers that will allow you to test the target device that you will be connecting. You will again
be testing one device at a time. Use one of the known good cables that you previously tested, and
connect the suspect target device. (See Diagram 7.)
Connect the Diagnostic Terminator and power up the computer only. Check the LED on the
terminator and make sure that the TRM (Termination Power) LED lights up. If it does not, the
computer is not supplying termination power. Check with the manufacturer of the computer or host
adapter for service advice. (Some host adapters have removable fuses that can be replaced by the
end user. Take a look at the card to see if there is a removable fuse. Replace it and repeat the
Now with the computer turned off, turn on the suspect target device. The TRM (Termination
Power) LED should come on. If it does not, the drive fuse has also probably gone out. Call the
manufacturer for details on how to fix the problem. If the LED goes on we can now proceed to the
With both the host and target supplying termination power we can now turn them both on and try to
operate the suspect device. Data transfers are the best way to tell if a device is working or not. If
the device is a hard drive, tape drive, removable drive, floptical drive, or any other device that stores
information, the simplest thing to do is copy over a few megabytes of information.
It is best to test data-transfers with a freshly formatted device that has been tested for bad blocks
and is free of viruses. It is also useful to test a target device for a prolonged amount of time, thus
testing for thermal problems that will only arise after hours of testing. If the device does not have any
problems after such testing you can feel reasonably assured that the drive is operating properly. So,
if possible, reformatting the device is a good safeguard against viruses and bad blocks. Both can
Using the Diagnostic Terminator to help check how things are working can be done very easily.
During the data-transfer state, you should be able to monitor the ACK (Acknowledge Line) LED,
the REQ (Request Line) LED and the SEL (Select Line) LED. Both the ACK and REQ should be
glowing dimly (actually flashing on and off very fast), and the SEL line should be on brightly. If this
is not the case, then you have a problem with the host or target. Refer to our section on page 6 for
Continue until all the SCSI targets have been individually tested. At this point you have tested all the
SCSI components on your system. Now connect one device at a time and test the system as you
add each device. If the system still fails as you add the last device then SCSI Cable quality should