This article is a reprint from the Macintosh Enthusiasts Giving Support newsletter, "LocalTalk." It may be reprinted by other User Groups provided MEGS and the author receive credit and a copy of any reprint at 5325 Red Sky Drive, Colorado Springs, CO 80915 (719-637-1016). - Chriss Koch
SCSI (pronounced skuh’-zee) stands for Small Computer Systems Interface. SCSI is a parallel interface used to provide a standard means for connecting computers peripheral devices.
SCSI offers several advantages:
Speed: SCSI transfers data between the computer and peripheral devices much faster than other interfaces.
Flexibility: SCSI is a standard interface used by many different manufacturers.
Expandability: You can connect as many as seven devices to a computer.
Setting up a SCSI network consists of three general tasks:
• connecting the appropriate cables to the computer and the other SCSI devices
• properly terminating the network
• ensuring that each device has a unique identification (priority) number
Many people (including myself) have complained of problems getting SCSI networks to operate. I’ll try to remove some of the mysticism involved in getting a SCSI network to operate correctly by providing guidelines offered to me during my Macintosh Service Training Course.
The first thing we should talk about is SCSI ID numbers. Each device on a SCSI bus (or network) has a SCSI identification number (“ID number” or “priority number”). The SCSI ID number identifies each device regardless of its physical location in the chain. The ID number also determines which device communicates first when more than one device wants to use the bus. Eight numbers (0-7) are used to establish identity and priority on the bus. The higher the number, the higher the priority. The computer is assigned the ID number 7, making it the highest priority device. If two devices have the same ID number, they may send data at the same time, which could damage files. Devices with the same ID number can also cause a failure to boot. When assigning SCSI ID numbers, give higher priority to startup devices and lower priorities to infrequently used equipment. When a Macintosh computer starts up, it looks for the required startup information in the following order:
1. The lower internal floppy drive (on Macintosh II computers the drive on the right).
2. The upper internal floppy drive, if one is installed (on Macintosh II computers the drive on the left).
3. Any external non-SCSI drives.
4. Any startup device selected via the Control Panel’s Startup Device CDEV.
5. SCSI devices beginning with ID 6 and ending with ID 0.
Next lets discuss SCSI cabling. There are three types of SCSI cables.
The System Cable is 18 inches long and is the shortest of the three cables. One end of the cable has a DB-25 connector, which plugs into the SCSI port on the computer. The other end of the System Cable has a 50-pin connector, which plugs into a SCSI device, a Cable Extender, or a Cable Terminator. Only one System Cable should be used in a SCSI network.
The Peripheral Interface Cable is approximately 3 feet long and has identical 50-pin connectors on each end. If you have more than one device, use this cable to connect the devices to each other.
The Cable Extender is the same length as the Peripheral Interface Cable but has different 50-pin connectors at each end. One 50-pin connector is female, and connects to a System Cable 50-pin connector, a Peripheral Interface Cable, or another Cable Extender. The other 50-pin connector is male, and connects to a Cable Terminator or to the next SCSI device. You use the Cable Extender like an extension cord. You can connect several Cable Extenders together, if needed, as long as the total cable length is not more than 20 feet.
Next we should discuss proper SCSI termination. Proper SCSI termination is typically the most frustrating aspect of getting your SCSI network to function properly. It doesn’t have to be. First an explanation of termination is in order.
Electrically terminated resistors are required to ensure signal integrity along a SCSI bus. These resistors control voltage range and keep signals from bouncing off one end of the bus and rippling back, interfering with new signals.
There are two termination techniques. Cable terminators, and built-in terminators. Electrically they are equivalent, however the use of Cable Terminators allows for more flexibility in configuring your SCSI network.
A Cable Terminator is a plastic case containing electrically terminated resistors. You can attach Cable terminators to either the SCSI port of a SCSI device or the end of a SCSI cable.
Some SCSI devices have built-in terminators—electrically terminated resistor packs that are plugged into sockets on the circuit board of a device. All Apple internal SCSI hard disks have built-in terminating resistors. All Apple external SCSI devices do not have built-in terminators. Some third party external SCSI devices are equipped with built-in terminators, others are not. If you are connecting third party external SCSI devices, you need to determine whether or not they have built-in terminators.
Proper SCSI termination is critical! Improper termination can result in a variety of problems. Uncontrolled voltages can damage SCSI circuits inside the computer. Electrical noise in the system can corrupt files. An improperly terminated system may fail to boot or suffer intermittent failures.
When determining the number and placement of terminators in a SCSI chain, first find out whether the computer is equipped with an internal hard disk, and if so, whether the internal hard disk is outfitted with built-in terminating resistors (if it’s not, it should be).
The “two terminator” rule (SCSI networks must be terminated at each end):
• If the computer has an internal hard disk that has built-in terminators, only one other terminator will be placed in the external SCSI chain.
• If the computer is not equipped with an internal hard disk, and the SCSI network consists of more than one cable, two terminators are required, one at each end of the external SCSI chain.
Exception to the “two terminator” rule:
The only time your SCSI network can have only one terminator is when the computer is not equipped with an internal hard disk, and the external SCSI device is connected to the computer using a System Cable. In this case only one terminator is required.
A SCSI network should never have more than two terminators. More than two terminators may cause intermittent problems or inhibit booting.
The number and positioning of terminators depends on how many devices you are connecting, the distance between devices, and if the devices have built-in terminators. Here are some guidelines to follow:
• Make sure the first and last devices in your SCSI chain are terminated.
• Some non-Apple external SCSI devices have built-in terminators. If you have only one such device, place it at the end of your SCSI chain or consider removing the built-in terminators to increase flexibility.
• Make certain that you have only two terminators in the chain. If you have more than one external device that has built-in terminators, you may be required to remove extra terminators in order to conform with the “two terminator” rule.
• If the computer contains an internal hard disk, make sure the hard disk contains built-in terminating resistors.
• If you connect one external SCSI device to a computer without an internal hard disk by using a System Cable and a Cable Extender, place a terminator between the System Cable and the Cable Extender and another to the end of the Cable Extender.
Finally I’d like to touch on the “First and Last” rule. When you want to use any of the devices on a SCSI network, the devices that have terminators attached (the first and last device) must be turned on in order to pass information through the cable system. If you are not using any devices in the middle of the chain, you can leave them switched off. Therefor you may consider placing seldom used devices, such as scanners, in the middle of the chain.
If you follow the guidelines offered here, you should have a minimum amount of trouble setting up and using your SCSI devices. Some will notice that I didn’t cover the differences where a Macintosh IIfx is concerned. I decided it would be best not to complicate the situation any worse, and besides not too many MEGS users have a IIfx! Following is a summary of the areas to be most concerned with:
1) Check all SCSI device ID numbers before connecting a SCSI device. Give each device a unique SCSI ID number before connecting them to your SCSI bus.
2) Assign SCSI ID numbers according to frequency of use. Higher numbers go to more frequently used devices.
3) A SCSI bus will not function properly unless the two devices with terminators are powered on.
4) The exception to the “two terminator” rule is when you connect an unterminated SCSI device to a computer with an 18 inch System Cable. In this instance you need only one terminator, between the cable and the device.
