Received: from lclark.edu by terminator.rs.itd.umich.edu (5.67/2.2) with SMTP id AA20509; Tue, 11 May 93 23:23:42 -0400 Received: by lclark.edu (4.1/SMI-4.1) id AA29736; Tue, 11 May 93 20:23:41 PDT Date: Tue, 11 May 1993 20:21:24 -0700 (PDT) From: Beth Corbo Sender: Beth Corbo Reply-To: Beth Corbo Subject: Quadra Utilities To: macgifts@mac.archive.umich.edu Cc: GOLDENJ@nic.CSU.net, nguyen@lenti.med.umn.edu Message-Id: Mime-Version: 1.0 Content-Type: MULTIPART/MIXED; BOUNDARY="603987746-1640726462-737177021:#29293" --603987746-1640726462-737177021:#29293 Content-Type: TEXT/PLAIN; CHARSET=US-ASCII Here are the utilities that I found for dealing with setting the amount of time the Mac waits for the internal hard drive to spin-up and some notes on SCSI from Micronet. Enjoy! Beth Corbo (corbo@lclark.edu) --603987746-1640726462-737177021:#29293 Content-Type: APPLICATION/octet-stream; name="SCSIInfo.sit.hqx" Content-ID: Content-Description: Macintosh SCSI Termination by the Engineering Department of MicroNet Technology, Inc. Dated: 11/1/90 What is Termination? Termination involves the placement of impedance matching circuits on the bus. In the case of SCSI, these circuits are typically placed at each end of the SCSI Bus (cable). These circuits are known as "Thevenin Terminators" and are simple resistor networks in the case of SCSI. These resistor circuits are composed of a 220 ohm resistor connected to Vcc and the signal node, while a 330 ohm resistor is connected to ground and the signal node. One of these resistor pairs is connected to each of the 18 active signal lines. The Vcc (+5 nominal) for the terminators is typically provided by line 26 on the SCSI bus cable. In practice, however, this voltage is sometimes supplied by the SCSI device (hard disk) directly to the terminator circuit if the circuit resides on the device itself. In this case, Vcc on line 26 of the SCSI would not be required. Why Terminate? Terminating a SCSI bus preserves high transition speeds, and when properly placed will clean up the signal along the entire length of the line. They also provide a reasonable degree of noise immunity. The most important reason to terminate an SCSI bus is that termination is required for the bus to work! The ANSI SCSI specification calls for the bus to be terminated in the way described above. How to Properly Terminate a SCSI Bus A SCSI bus is properly terminated by placing bus terminators at each end of the bus. There are a couple of exceptions. If the bus is very short (ie. 18" or less) then generally it is best to terminate just one end. Also, if the cable length (6 meters maximum) between two devices is greater than 10 feet then the cable should be terminated at the 10 foot point in addition to each end. When daisy chaining three or more SCSI devices the bus will often exceed this 10 foot specification and will require three terminators in the chain to work properly. Physically, terminators generally have three forms in the Macintosh world. Electrically, these three types of terminators are equivalent and vary only in where and how they are installed. First, are the ON-DRIVE terminators which are known as resistor packs (or sips, or dips). They reside right on the device (hard disk) itself and are almost always removable. Always note the orientation of these parts before attempting to remove these! They have a polarity and must not be inserted backwards. Second, there are the terminator blocks or plugs. These are short "plug like" devices and are inserted between a hard drive cabinetÕs SCSI connector and the SCSI cable or on the second cabinet connector if one exists. DonÕt confuse these with the ON-DRIVE terminators described above. ON-DRIVE means literally that, on the drive (HDA) not on the cabinet. Third, are the "Mother Board" Terminators. These may look like a SIMM or they may look like a narrow plug. They are used ONLY when there is no ÔinternalÕ hard drive in the Macintosh AND you are connecting an external SCSI device (hard drive) with an SCSI cable longer than 18". They are inserted into the 50 pin SCSI connector inside the Macintosh where the cable for an internal drive would normally connect. These Mother Board Terminators are keyed (a polarity notch) and must never be inserted backwards! Never use the second type of terminator when the drive inside a cabinet has terminators installed on it. Common Termination Problems Poor Quality Cables. Always use cables which are double shielded (foil and braid). Apple and MicroNet brand SCSI cables are built with specifications that include twisted pair and double shielding. Never use simple printer type RS232 cables! Make sure the connector hoods (or shrouds) are each connected to the shield braid! Too Many SCSI Devices Terminated. Only the last SCSI device (disk drive/tape drive/scanner/printer) should be terminated. If the Macintosh has no hard disk drive installed inside it, then a Mother Board Terminator should be installed if the cable to the external device is longer than 18". MicroNet Technology developed what is called the "MBT", which can be used to terminate the Macintosh SE, SE/30, II, IIx, IIcx, IIci when an internal hard disk drive is not present and is available through MicroNet resellers. Total SCSI Cabling Exceeds 6 Meters (19.6'). The total length of all cables used (added together) must not be greater than 6 meters. DonÕt forget to add the internal cabinet wiring which is generally about 1 foot. Never use a plug type terminator designed for installation on a cabinet connector if the SCSI device (hard disk drive or tape drive) inside the cabinet already has terminators installed on the drive! Never Exceed the Maximum Cabling Length of 6 Meters (19.6 feet) Total. Never mix cable brands, types or styles. This is a common source of trouble. Each cable has a different type of construction, impedance and wire placement which can result in bus reflections. In mixed cable configurations some devices may simply not work even if all other guidelines are followed! Macintosh and The SCSI Reset Line and On-Drive and Off-Drive Termination Pin 40 of the 50 pin SCSI connector is the hardware reset line. When the Macintosh is first turned on (cold boot time) it repeatedly asserts this reset line in order to reset all attached SCSI devices. Generally, this poses no problem. Some non-block devices can have trouble recovering from this reset condition and cause other attached devices to have trouble. Most notable are older SCSI scanners and tape drives. The most common symptom of this problem is a Macintosh that wonÕt boot from a hard drive when a scanner or tape drive is attached to the same cable. This problem can also occur when using some of the first generation SCSI hard disks. They will enter a ÔUnit AttentionÕ mode and will not respond to read or write commands until a ÔRequest SenseÕ SCSI command is issued by the Macintosh. Newer drives will simply abort the Unit Attention condition after it receives several read or write commands and eventually will respond to the data requests. Symptoms of this later problem will be a drive which will not show up on the desktop the first time the Macintosh is booted but will show up fine the second time around. It should also be noted that whenever a SCSI device is first powered on it also enters this Unit Attention condition. The symptom for this is again a drive which will not show up on the desktop (mount) the first time the Macintosh is booted. The purpose of the Unit Attention condition is to alert the Host (Macintosh) that an important event has occurred which the host should be aware of before accessing that device. The Host is then supposed to ask the SCSI device what occurred by using the Request Sense command. The Host would then act take an appropriate action before making data requests of the device. A good example of this scenario is the resultant Unit Attention condition which follows the swapping of media on a removable media device, such as an optical cartridge device. It should be obvious why the host should be alerted to this swap before it "writes data to the newly inserted cartridge"! The Unit Attention condition following the swap would prompt the Host (Macintosh) to determine what event has caused the device to respond with a Unit Attention condition. When the Host (Macintosh) follows up with a Request Sense command it then learns of the cartridge swap and takes appropriate action. In this case it grays out or removes the icon on the desktop and mounts the new cartridge. Hardware resets and drive power being turned off and then on again are considered important events. That is why they cause most SCSI devices to enter the Unit Attention condition. Older Macintoshes have trouble with devices which enter the Unit Attention condition because they donÕt follow up and query the device to find out the reason for the condition. Therefore, if the device doesnÕt clear the condition itself, the MacintoshÕs data requests will not be honored and the result can be a drive which wonÕt boot reliably or at all. The fix for these types of problems is simply to cut the reset line somewhere in the cabling (line 40 at the 50 pin end). Many subsystem manufacturers have taken this approach successfully. While this approach remains debatable it can be the only remedy for severe cases. While it wonÕt stop devices from entering Unit Attention conditions at power up time, it will stop the hardware resets issued by Macintoshes at boot time from reaching the device. This can make the difference in booting and not booting! The reason this topic is being presented here in a discussion on termination is that it is all important that the reset line (pin 40) always be terminated! If it is not, the device will be constantly resetting itself and will not work! Normally this is never a problem since line 40 is one of the 18 active signal lines which normally get terminated. But if you are working with a device which has line 40 cut inside its cabinet, AND you remove the ON-DRIVE terminators in favor of an ON CABINET (external) terminator then line 40 wonÕt be terminated since the cabling inside the cabinet has this line cut. Therefore, pin 40 on the drive wonÕt be terminated since this line is open and wonÕt reach the terminator which is now on the outside of the cabinet! The warning here is simply to verify continuity of line 40 on a cabinetÕs internal cabling before determinating the drive inside it. If line 40 has been cut on the cabling inside a drive cabinet YOU CANÕT DETERMINATE THE DRIVE ITSELF unless you change the internal cabling to pass line 40 through to the outside connector of the cabinet! Enough of these type of cabinets exist in the world that it is a real problem when reconfiguring bus termination. Another approach to handling a case like this is to put this drive (or device) on the end of the SCSI which should be terminated anyway and donÕt use an external terminator on the bus at all. If, however, you have more than one of these devices with line 40 cut inside its cabinet then you must consider changing their internal cable if you wish to determinate the drive (device) inside it and use external ÔplugÕ type terminators. Macintosh Configurations and Termination Issues Point #1) DonÕt Be a Worry Wart There is a lot of bad advice floating around. The purist will tell you that if you have more than two SCSI devices terminated on the bus you will blow up your Macintosh. This simply isnÕt true. If you have a third terminated device on the bus or even (heaven forbid) a fourth you are not going to pop the SCSI chip in your Macintosh. In some cases with older SCSI drives it may be necessary to leave the ON-DRIVE terminators in place in order for them to power up and to work at all. Another purist might tell you that both ends of a SCSI bus MUST be terminated in order for anything to work. This again is junk talking. Take for example the fact that Apple doesnÕt terminate the mother board of certain Macintosh computers! If there is no internal drive in these Macintosh computers it simply never gets terminated like it should be (hence the recent availability of mother board terminators from MicroNet Technology, Inc., and Apple themselves). And since these Macintosh computers are always one end of the SCSI bus one might ask the purist why any SCSI devices work at all if there is no internal drive! The fact is that if the cable is fairly short (18" to 6') the Macintosh SCSI bus will generally function fine even if the Macintosh isnÕt terminated. However, the faster Macintosh computers, like the IIci and the IIfx may need internal termination inorder for them to work properly. The pursuit of the Perfect SCSI bus can be elusive. As soon as you add SCSI devices from different vendors and use the different cables supplied by each, you have a potential problem. Mixing cable types, lengths, and styles is an invitation to trouble. But in the real world, there isnÕt much choice! This is largely why switching cables around and trying cables of different lengths has solved many problems. A part of configuring SCSI peripherals will always be determined empirically. Questions and Answers Q) Should I ever remove the ON-DRIVE terminators from a drive which is installed inside a Macintosh? A) NO. Q) Should I ever use a Mother Board Terminator if there is a drive installed inside the Macintosh? A) NO. You canÕt ! Each plugs into the same 50 pin connector on the mother board and you know what they say about two pieces of matter occupying the same space! Q) What is a ÔBlack terminatorÕ and when should I use it? A) The black terminator is a product from Apple and is for use only with the Macintosh IIfx. It is an external plug type terminator and would only be used Ôwhere and whenÕ a normal external plug type terminator would be used! This black terminator differs (electrically) only slightly from a normal plug type terminator and not physically. ItÕs purpose in life is to compensate for some changes in the new SCSI chip used in the Macintosh IIfx. If later models of the Macintosh use this new SCSI chip then it is likely to be recommended for those machines as well. Prepared by the Engineering Department of MicroNet Technology, Inc. 20 Mason Irvine, CA 92718 TEL: (714) 837-6033 FAX: (714) 837-1164 Copyright 1990 by MicroNet Technology, Inc.