Pure Amiga - SCSI FAQ SCSI Frequently Asked Questions Information SCSI Examples: Excerpted From The A4000 Hardware Guide Copyright (C) 1995 Warren Block Contact wblock@rapidnet.com. This file may be freely redistributed in either HTML or text format The full Guide can be found at Aminet:hard/misc/a4khard_v3.lha Converted to HTML by Chris Appleton for Pure Amiga by kind permission of the author Introduction It seems that the SCSI bus is one of the most misunderstood aspects of connecting hard drives and other peripherals to the A4000 (or, for that matter, any other Amiga model). This section of the guide is an attempt to provide some simple examples of proper SCSI device connections. Please note that in the following section, and in the Guide as a whole, I have used the common term "controller" when referring to disk adapter boards, although the more accurate description for both SCSI and IDE would be "host adapter." Definitions Since understanding SCSI requires a background in the jargon, a few basic definitions might be helpful: SCSI This is the original standard, now also known as SCSI-1. The maximum theoretical transfer rate is 5 megabytes per second, although most combinations of drives and controllers do much less, usually less than two megabytes per second. Total length of the SCSI bus cannot exceed six meters. SCSI-2 An extension of the SCSI command set. Most CD-ROM drives that are double- speed or faster are SCSI-2. Note that contrary to popular belief, this doesn't go any faster than good old SCSI-1. Fast SCSI-2 Here's where the speed was increased. Fast SCSI-2 has a maximum transfer rate of 10 megabytes per second, synchronous. Again, this is theoretical, and anything more than a third of that should be considered excellent. Wide SCSI And Differential SCSI SCSI transfers data over an 8-bit wide data path. A variation called Wide SCSI uses a 16-bit wide data path, potentially doubling transfer rates. Another variation is differential SCSI, which uses differential signal cables to provide a total bus length of up to 25 meters. Neither of these variations will be described in any detail here, since there don't seem to be any Amiga implementations of controllers for them. Adapters are available to connect Wide or Differential SCSI devices to normal SCSI controllers, though, so it is possible to connect them to the Amiga. Termination SCSI bus systems require an impedance-matching terminator circuit at each end of the bus for reliable operation. Many people find termination to be complex, but the subject can be simplified a great deal by remembering one simple rule: the SCSI bus needs to be terminated at both ends, and only at the ends. The most common mistake in SCSI termination is assuming that the SCSI controller itself doesn't count; in fact, it does count as a device, and the termination rules apply to it just like other devices. Many Amiga controllers have the termination resistors soldered into place, under the assumption that only internal SCSI devices will be attached. If both internal and external devices are to be used, it is necessary to remove these resistors. SIP sockets may be soldered in their place to provide the greatest versatility, or you can just use external terminators. Terminating resistors are usually SIP resistor packs; most are yellow, blue, or black, and there may be one, two, or three of them. External terminators look like a connector with no cable attached, and can be found in Centronics 50-pin, DB25, and high-density 50 configurations. Some devices (notably, many external CD-ROM drives) have a termination switch. All of the termination schemes described so far are known as "passive" terminators. Electronically, they connect each signal pin to +5V through a 220 ohm resistor, and to ground through a 330 ohm resistor. This voltage divider circuit provides impedance matching for the SCSI bus. The alternative to a passive terminator is an "active" terminator, which connects each of the SCSI signal pins through a 110 ohm resistor to a precision +2.85V regulator (an LT1086CT, for example) which is powered by +5V. Active terminators are superior to passive terminators simply because they are active; unlike the fixed resistors in a passive terminator, the active terminator's voltage regulator will track varying voltages and properly terminate the SCSI bus. Active terminators can cure many problems with unreliable SCSI devices; their only disadvantage is that they cost a bit more (Dalco sells them for between thirty and forty dollars). Active termination chips are made by Dallas Semiconductor and Texas Instruments. Any combination of passive and active terminators may be used, although two active terminators would be best. In practice, passive/passive or passive/ active are usually adequate. Termination Power Terminator power (+5V) is supposed to be supplied on pin 26 of the 50-pin IDC header. But SCSI devices are not required to supply this power; many have jumpers to enable or disable it. So it is possible to have a proper termination setup, but no power provided to the terminators. Naturally, this will cause problems. Make sure that at least one device is supplying termination power to the SCSI bus, preferably the controller, since external devices may be turned off, which would deprive the rest of the bus of termination power. Cable Configurations Internal SCSI devices are usually connected with 50-conductor ribbon cable. 50-pin IDC connectors are crimped onto the cable for each device to be attached. "Stub" cables of no more than 3 centimeters off the main cable are allowed by the SCSI standard, but it's better to avoid them altogether by running the cable direct from one device to the next, with no branches off the main bus at all. External SCSI device cables can use several connectors: Centronics 50-pin, DB25, or high-density 50-pin (commonly referred to as SCSI-2, since many Fast SCSI-2 adapters use this type of connector). Adapter cables may have any combination of these three basic types. The SCSI standard states that the total length of the SCSI bus, including internal and external cable, must not exceed six meters. In practice, some devices and cable combinations may limit this severely, particularly cables with DB25 connectors (since Apple created the DB25 "pseudo-SCSI" cable by simply discarding all those "extra" grounds that helped make SCSI capable of running long distances in the first place). Conversely, some SCSI bus implementations can go farther than the standard suggests. SCSI Address Numbers Each SCSI device (including the controller) has an address between 0 and 7 assigned to it by the user. These numbers are usually set as a binary number with three jumpers. Controllers often have no jumpers, either requiring software to change their address, or simply not being able to change it at all. Standard Amiga controllers of either type default to a SCSI address of 7. The rules regulating addresses are pretty simple: each device must have a unique address. (There is no physical "order" in which the addesses must occur; you can use any order or combination of numbers, as long as there is only one device with a given address.) Since the Amiga scans the SCSI bus for bootable devices starting with address 0 and proceeding to address 7, it is advised that you assign address 0 to the boot hard drive, and set "HiID" to "On" for this drive in the Rigid Disk Block (RDB). This will prevent the system for looking for other hard drives with a higher boot priority, making for the quickest booting possible, and preventing the system from trying to boot off of a higher- numbered CD-ROM drive. (Check the Aminet disk/misc directory for RDB utility programs.) LUNs Logical Unit Numbers are a now seldom-used feature of the SCSI standard. LUNs provide a way to access more than one device at a given SCSI address. For example, some Adaptec SCSI-to-MFM adapter boards like the 4000A could control two MFM hard drives. However, the 4000A board used only a single SCSI address; to access each drive, an LUN was used: 0 for the first drive and 1 for the second. With modern SCSI devices, LUNs are relatively rare. Example SCSI Bus Connections These examples show connections to the A2091 controller (see Drives/ 2091 Reference), but other controllers will be similar. In Example 1, the 200M hard drive is used as the boot drive, and the "HiID" flag is set to "On" in this drive's Rigid Disk Block. (The HiID flag may be called by another name, like LastDrive or HighDrive.) For examples 2 and 3, the 540M drive is used as the boot drive, and the HiID flag is set in that drive's RDB. Example 1: 2091 controller, internal 200M SCSI-1 hard drive. Cable connections are 50-conductor ribbon. Example 2: 2091 controller, internal 200M SCSI-1 hard drive, internal 540M Fast SCSI-2 hard drive. Cable connections are 50-conductor ribbon. The SCSI-1 drive has been renumbered as address 1, and the new Fast SCSI-2 drive is now set at address 0 and used as a boot drive, to provide better performance on the system partitions. (Even though it will only be accessed at SCSI-1 rates, it is a newer drive, and will probably have significantly better transfer rates than the older 200M drive.) Example 3: 2091 controller, internal 200M SCSI-1 hard drive, internal 540M Fast SCSI-2 hard drive, external SCSI-2 CD-ROM drive. The cable from the CD-ROM drive to the A2091 is a Centronics 50-pin to DB25 adapter cable, and the internal cables are 50-conductor ribbon. An active terminator is attached to the last available external SCSI connector on the CD-ROM drive. Note that the terminating resistors on the A2091 have been removed so that the SCSI bus is terminated only at the ends (the CD-ROM and the 540M drive). © Warren Block, 1996. If you feel you want more information, try the full A4000 Hardware Guide on Aminet.