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┌────────────────────────────────────────────────────────────────────┐ │ DIAGRAMS.DOC Supplement to TheRef(tm) Drive & Controller Listing │▒ ├────────────────────────────────────────────────────────────────────┤▒ │ In "publishing" TheRef(tm), I've often been asked the difference │▒ │ between the types of drive controllers and recording methods. I'm │▒ │ not going to get into that in this document, as it would require a │▒ │ good sized doc. of it's own. What I have supplied are diagrams of │▒ │ the different connectors associated with the technology today. │▒ │ frf │▒ ├────────────────────────────┤ CABLES ├──────────────────────────────┤▒ │ │▒ │ Controller Drive 2(or none) Drive 1 │▒ │ │▒ │ 1╔══╗ ─────────1╔══╗ ────stripe─────1╔══╗ Pins 10-16 │▒ │ FLOPPY cable ║::║≡≡≡≡≡≡≡≡≡≡≡║├┤║≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡║├┤║ are twisted │▒ │ with twist ║::║≡≡≡≡≡≡≡≡≡≡≡║├┤║≡≡≡≡≡≡≡≡≡≡≡≡≡XX≡≡║├┤║ before the │▒ │ (control & ║::║≡≡≡≡≡≡≡≡≡≡≡║├┤║≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡║├┤║ connector. │▒ │ data, 34 pin) ║::║≡≡≡≡≡≡≡≡≡≡≡║├┤║≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡║├┤║ (7 wires) │▒ │ ╚══╝ ╚══╝ ╚══╝ │▒ │ 1╔══╗ ─────────1╔══╗ ────stripe─────1╔══╗ Pins 25-29 │▒ │ ST412 & ESDI ║::║≡≡≡≡≡≡≡≡≡≡≡║├┤║≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡║├┤║ are twisted │▒ │ Hard Drive ║::║≡≡≡≡≡≡≡≡≡≡≡║├┤║≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡║├┤║ before the │▒ │ cable w/twist ║::║≡≡≡≡≡≡≡≡≡≡≡║├┤║≡≡≡≡≡≡≡≡≡≡≡≡≡XX≡≡║├┤║ connector. │▒ │ (control) ║::║≡≡≡≡≡≡≡≡≡≡≡║├┤║≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡║├┤║ (5 wires) │▒ │ ╚══╝ ╚══╝ ╚══╝ │▒ │ 1╔══╗ ───────────stripe─────────────1╔══╗ (no twists) │▒ │ ST412 & ESDI ║::║≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡║├┤║ Each drive │▒ │ Hard Drive ║::║≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡║├┤║ has it's │▒ │ (data, 20 pin)║::║≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡║├┤║ own data │▒ │ ╚══╝ ╚══╝ cable │▒ │ │▒ │ IMPORTANT NOTE: Pin #1 on any drive cable SHOULD be indicated by a │▒ │ a colored stripe. If you should find the stripe │▒ │ by connector pin 34 (or 20), inspect the whole │▒ │ cable VERY throughly! │▒ │ │▒ │ DRIVE SELECT For both Floppy and Hard drives, when the 34 pin │▒ │ JUMPERS: cable has a twist, the device number should be set │▒ │ to the second position. Drives numbered 0-3, set to │▒ │ 1, those numbered 1-4, set to 2. When cables with- │▒ │ out a twist are used, Floppy "A", and(or) Hard drive │▒ │ "C" should be set to 1, and the second Floppy and │▒ │ (or) Hard drive should be set to 2. │▒ │ │▒ │ TERMINATORS: When using more than one drive on a cable (ie; 2FDs │▒ │ or 2HDs), the terminating resistor pack should be │▒ │ left on the drive furthest from the controller, and │▒ │ removed from the drive closest to the controller. │▒ │ │▒ │ NOTE: On SCSI drives, the Host Adapter also has resistors. │▒ │ These are needed to terminate both ends of the bus. │▒ │ Since the SCSI bus can have up to 7 devices attached │▒ │ to it, only the Host Adapter and the device farthest │▒ │ from it will retain the resistors. All devices in- │▒ │ between should have theirs removed. │▒ └────────────────────────────────────────────────────────────────────┘▒ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ ┌────────────────────────────────────────────────────────────────────┐ │ DIAGRAMS.DOC 2 │▒ ├──────────────────────────┤ CONNECTIONS ├───────────────────────────┤▒ │ │▒ │ FLOPPY DRIVES ┌─────┐ │▒ │ HI/LO DENSITY >│2 1│ GND │▒ │ The connector on a floppy drive N/C │4 _ 3│ | │▒ │ consists of 34 conductors. Both N/C │6 5│ | │▒ │ control and data use this same INDEX <│8 7│ | │▒ │ cable. Most cables have a twist MOTOR ENAB. A >│10 9│ | │▒ │ that interchanges pins 10 through DRIVE SEL. B >│12 11│ | │▒ │ 16 at the end of the cable (on DRIVE SEL. A >│14 13│ | │▒ │ drive 1). Most floppy connect- MOTOR ENAB. B >│16 15│ | │▒ │ ors have a "key" between pins DIRECTION SEL. >│18 17│ | │▒ │ 4 & 6, and 3 & 5, to prevent the HEAD STEP >│20 19│ | │▒ │ cable from being reversed. At WRITE DATA >│22 21│ | │▒ │ the other end, the dual row con- WRITE GATE >│24 23│ | │▒ │ nector that attaches to the con- TRACK 00 <│26 25│ | │▒ │ troller card will usually have a WRITE PROTECT <│28 27│ | │▒ │ set of ridges that coincide with READ DATA <│30 29│ | │▒ │ cutouts in the controller card's HEAD SELECT >│32 31│ | │▒ │ connector. Note that old style DISK CHANGE <│34 33│ GND │▒ │ floppy-only controllers used a └─────┘ │▒ │ card-edge connector just like that > Input ( At the │▒ │ of the drive. < Output Drive Conn.) │▒ │ │▒ │ ST506/412 HARD DRIVE (MFM & RLL) │▒ │ │▒ │ This standard drive system uses ┌─────┐ │▒ │ two cables; a 34 conductor control HEAD SEL. 8 │2 1│ GND │▒ │ cable, and a 20 conductor data HEAD SEL. 4 │4 _ 3│ | │▒ │ cable. The control cable contains WRITE GATE │6 5│ | │▒ │ a twist of the conductors going to SEEK COMPLETE │8 7│ | │▒ │ the farthest drive, which is drive TRACK 0 │10 9│ | │▒ │ "C" on most systems. This twist WRITE FAULT │12 11│ | │▒ │ consists of conductors 25 through HEAD SEL. 1 │14 13│ | │▒ │ 29. As with the floppy cable, the RESERVED │16 15│ | │▒ │ ST506/412 cables normally have a HEAD SEL. 2 │18 17│ | │▒ │ key to prevent reversal, and the INDEX │20 19│ | │▒ │ controller end has a pin-type con- READY │22 21│ | │▒ │ nector, while the drive end has a STEP │24 23│ | │▒ │ card-edge type connector. DRIVE SEL. 1 │26 25│ | │▒ │ DRIVE SEL. 2 │28 27│ | │▒ │ ┌─────┐ DRIVE SEL. 3 │30 29│ | │▒ │ DRIVE SEL'D │1 2│ GND DRIVE SEL. 4 │32 31│ | │▒ │ RESERVED │3 _ 4│ | DIRECTION IN │34 33│ GND │▒ │ | │5 6│ | └─────┘ │▒ │ | │7 8│ GND │▒ │ RESERVED │9 10│ RESERVED Though control signals │▒ │ GND │11 12│ GND go through a single 34 │▒ │ * WRITE DATA+ │13 14│ * WRITE DATA- conductor cable, data │▒ │ GND │15 16│ GND flows through seperate │▒ │ * READ DATA+ │17 18│ * READ DATA- 20 conductor cables │▒ │ GND │19 20│ GND for each drive (C,D). │▒ │ *(MFM or RLL) └─────┘ │▒ └────────────────────────────────────────────────────────────────────┘▒ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ ┌────────────────────────────────────────────────────────────────────┐ │ DIAGRAMS.DOC 3 │▒ ├──────────────────────────┤ CONNECTIONS ├───────────────────────────┤▒ │ │▒ │ ESDI HARD DRIVES ┌─────┐ │▒ │ HEAD SEL. 3 │2 1│ GND │▒ │ Though ESDI and ST506/412 drives HEAD SEL. 2 │4 _ 3│ | │▒ │ share similar looking cables, WRITE GATE │6 5│ | │▒ │ even to the point of having a CONFIG/STAT DATA │8 7│ | │▒ │ twist, the actual data and con- TRANSFER ACK. │10 9│ | │▒ │ trol signals are very different. ATTENTION │12 11│ | │▒ │ One should never mix components HEAD SEL. 0 │14 13│ | │▒ │ from these two drive types. SECT/ADD.MK. FOUND │16 15│ | │▒ │ While the ST506/412 interface HEAD SEL. 1 │18 17│ | │▒ │ utilizes a standard pulse code INDEX │20 19│ | │▒ │ to transmit data between the READY │22 21│ | │▒ │ drive and controller, ESDI uses TRANS.REQUEST │24 23│ | │▒ │ a pulse code that does not require DRIVE SEL. 1 │26 25│ | │▒ │ the level to return to zero between DRIVE SEL. 2 │28 27│ | │▒ │ pulses. This format is refered to DRIVE SEL. 3 │30 29│ | │▒ │ as NRZ, or Non Return to Zero. By READ GATE │32 31│ | │▒ │ utilizing NRZ, the clock that data COMMAND DATA │34 33│ GND │▒ │ is transfered by can be increased, └─────┘ │▒ │ thereby increasing the troughput to │▒ │ and from the ESDI disk. │▒ │ ┌─────┐ │▒ │ DRIVE SEL'D │1 2│ SECT/ADD.MK. FOUND │▒ │ SEEK COMPLETE │3 4│ ADDRESS MARK ENABLE │▒ │ RESV'D FOR STEP MODE │5 6│ GND │▒ │ WRITE CLOCK+ │7 8│ WRITE CLOCK- │▒ │ CARTRIDGE CHANGED │9 10│ READ REF. CLOCK+ │▒ │ READ REF. CLOCK- │11 12│ GND │▒ │ NRZ WRITE DATA+ │13 14│ NRZ WRITE DATA- │▒ │ GND │15 16│ GND │▒ │ NRZ READ DATA+ │17 18│ NRZ READ DATA- │▒ │ GND │19 20│ GND │▒ │ └─────┘ │▒ │ │▒ │ ───────────────┐ And in this corner... Recording ┌──────────────── │▒ │ │▒ │ Times were, you had a simple choice for type of disk drive... │▒ │ Any kind, as long as it was ST506/412. Those were the heydays of │▒ │ MFM drives. But many manufacturers weren't content with the 17 │▒ │ sectors/track that MFM provided. They devised a newer encoding │▒ │ scheme to pack data tighter, and called it RLL, or Run Length │▒ │ Limited, as opposed to MFM, or Modified Frequency Modulation. It │▒ │ involves using groups of 16 bits rather than each individual bit, │▒ │ thus achieving a sort of "compression" of the information as it is │▒ │ encoded. Since the same information takes up less space as RLL │▒ │ encoded data, more info can be writen to the disk. The most com- │▒ │ mon RLL technique, known as 2,7 RLL, can pack roughly 50% more on │▒ │ a disk than MFM. Of course, there is always a trade-off, and the │▒ │ timing and media required for RLL is it. RLL requires a higher │▒ │ grade of media because of it's dense bit-packing, and timing is │▒ │ more critical, since the data is flowing at 50% higher rate than │▒ │ an MFM drive. Also, the mechanics of the drive must have tighter │▒ └────────────────────────────────────────────────────────────────────┘▒ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ ┌────────────────────────────────────────────────────────────────────┐ │ DIAGRAMS.DOC 4 │▒ ├────────────────────────────────────────────────────────────────────┤▒ │ tolerences because head positioning becomes more critical. These │▒ │ requirements kept RLL drives at a premium. It has only been the │▒ │ last two years, that RLL drives have outsold MFM, and have all but │▒ │ wiped them from the marketplace. This turnabout has come from the │▒ │ need to increase disk capacity more and more. Both ESDI, and SCSI │▒ │ type drives utilize RLL.(1*) encoding to achieve high capacity and │▒ │ transfer rates (from the disk). And the newest interface, IDE, or │▒ │ Integrated Drive Electronics, is also based on this technology. │▒ │ ───────────────────────────────┐ ┌──────────────────────────────── │▒ │ ┌─────┐ │▒ │ SCSI HARD DRIVES DB0 <>│2 1│ GND 5 │▒ │ DB1 <>│4 3│ | 0 │▒ │ The normal internal cable for SCSI DB2 <>│6 5│ | │▒ │ is a 50 conductor ribbon, with all DB3 <>│8 7│ | P │▒ │ odd numbered conductors grounded. DB4 <>│10 9│ | I │▒ │ Two conductors, numbers 25 & 26, are DB5 <>│12 11│ | N │▒ │ often left not-connected, as they DB6 <>│14 13│ | │▒ │ deal with Terminator power, and can DB7 <>│16 15│ | D │▒ │ be easily shorted by cable reversals. DBP <>│18 17│ | U │▒ │ There are no twists in this cable, GND │20 19│ | A │▒ │ and it's length may be a maximum of GND │22 21│ | L │▒ │ 6 meters. But one is advised to use GND │24 23│ | │▒ │ minimum lengths to improve timing. TERM PWR │26 25│ | R │▒ │ Up to seven drives, or devices may be GND │28 27│ | O │▒ │ attached to an SCSI cable. Each is GND │30 29│ | W │▒ │ daisy-chained on the cable, or, when ATN >│32 31│ | │▒ │ a device has two connectors, another GND │34 33│ | C │▒ │ cable may be "spliced" into the chain BSY <>│36 35│ | O │▒ │ starting at the second connector, and ACK >│38 37│ | N │▒ │ continued on. Care must be taken to RST <>│40 39│ | N │▒ │ insure that cables and connectors are MSG < │42 41│ | E │▒ │ not reversed, as this would short pin SEL <>│44 43│ | C │▒ │ 26 (TERMPWR) to ground, and likely C/D < │46 45│ | T │▒ │ damage the drive or controller. Also, REQ < │48 47│ | O │▒ │ as explained earlier, the terminating I/O < │50 49│ GND R │▒ │ resistors should remain only on the └─────┘ │▒ │ controller (Host Adapter) and the LAST ┌──┐ DB-25F CONN. │▒ │ drive on the cable, regardless of it's GND │1 └──┐ │▒ │ address. DB1 <>│2 14│<> DB0 │▒ │ Most SCSI Host Adapters also have DB3 <>│3 15│<> DB2 │▒ │ a connector for external drives in the DB5 <>│4 16│<> DB4 │▒ │ form of a Centronics(tm) type 50 pin, DB7 <>│5 17│<> DB6 │▒ │ or an "alternate", DB-25F connector. GND │6 18│<> PARITY │▒ │ Only the internal 50-pin, and the SEL <>│7 19│ GND │▒ │ "alternate" external connector are GND │8 20│< ATN │▒ │ shown here. (see also: MORE SCSI) TMPWR │9 21│ > MSG │▒ │ Also, these diagrams refer to the RST <>│10 22│< ACK │▒ │ single-ended SCSI connections, since C/D < │11 23│<> BSY │▒ │ this is the most common arrangement I/O < │12 24│ > REQ │▒ │ for PCs today. The Differential SCSI GND │13 25│ GND │▒ │ requires balanced lines, and is used │ ┌──┘ │▒ │ mostly on high-end workstations. └──┘ FUTURE DOMAIN│▒ └────────────────────────────────────────────────────────────────────┘▒ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ ┌────────────────────────────────────────────────────────────────────┐ │ DIAGRAMS.DOC 5 │▒ ├────────────────────────────┤ CABLES ├──────────────────────────────┤▒ │ SCSI (cont.) │▒ │ (T) ┌─(DC)┐ (T) │▒ │ On an SCSI cable, the 1╔══╗─stripe─1╔══╗──1╔══╗──1╔══╦══╗──1╔══╗ │▒ │ terminating resistors ║::║≡≡≡≡≡≡≡≡≡║::║≡≡≡║::║≡≡≡║::║::║≡≡≡║::║ │▒ │ (T) remain at the END ║::║≡≡≡≡≡≡≡≡≡║::║≡≡≡║::║≡≡≡║::║::║≡≡≡║::║ │▒ │ devices on the cable, ║::║≡≡≡≡≡≡≡≡≡║::║≡≡≡║::║≡≡≡║::║::║≡≡≡║::║ │▒ │ even when 2 cables are ║::║≡≡≡≡≡≡≡≡≡║::║≡≡≡║::║≡≡≡║::║::║≡≡≡║::║ │▒ │ "Daisy-Chained" (DC). ║::║≡≡≡≡≡≡≡≡≡║::║≡≡≡║::║≡≡≡║::║::║≡≡≡║::║ │▒ │ Also, the external ║::║≡≡≡≡≡≡≡≡≡║::║≡≡≡║::║≡≡≡║::║::║≡≡≡║::║ │▒ │ connector may be used, ╚══╝ ╚══╝ ╚══╝ ╚══╩══╝ ╚══╝ │▒ │ requiring the removal (HA) Drives 1-7 (in any order) │▒ │ of the Host Adapter's │▒ │ internal Term. resistors. │▒ ├──────────────────────────┤ CONNECTORS ├────────────────────────────┤▒ │ │▒ │ IDE (AT) HARD DRIVES (<> AT THE DRIVE CONN) │▒ │ ┌─────┐ │▒ │ IDE, or Integrated Drive Electronics RST >│1 2│ GND │▒ │ is the most recent drive interface to SD7 <>│3 4│<> SD8 │▒ │ gain popularity. Often, the control SD6 <>│5 6│<> SD9 │▒ │ circuitry is built into the mother- SD5 <>│7 8│<> SD10 │▒ │ board, eliminating the requirement for SD4 <>│9 10│<> SD11 │▒ │ a seperate Host Adapter. There are 2 SD3 <>│11 12│<> SD12 │▒ │ types of IDE interfaces...those for the SD2 <>│13 14│<> SD13 │▒ │ 8-bit XT bus, and those for the 16-bit SD1 <>│15 16│<> SD14 │▒ │ AT bus (detailed here). The cable for SD0 <>│17 18│<> SD15 │▒ │ IDE contains 40 conductors and has no GND │19 20│N/C (KEY) │▒ │ twists. Like an SCSI cable, the IDE RES.N/C│21 22│ GND │▒ │ cable uses a Dual-row Pin connector for IOW >│23 24│ GND │▒ │ both ends. A single cable may be used IOR >│25 26│ GND │▒ │ to connect two drives, or two cables RES.N/C│27 28│N/C RES. │▒ │ may be Daisy-Chained. Most IDE Host RES.N/C│29 30│ GND │▒ │ Adapters will support two hard drives. IRQ14 <│31 32│> I/O CS16 │▒ │ The first drive should be jumpered as SA1 <>│33 34│<> PDIAG │▒ │ the Master drive, and the second as the SA0 <>│35 36│<> SA2 │▒ │ Slave drive. Plug-in IDE Host Adapters CS0 >│37 38│< CS1 │▒ │ are often called Paddle-Boards, and ACTIVE <│39 40│ GND │▒ │ may contain a floppy controller, and └─────┘ │▒ │ serial and parallel ports. │▒ │ │▒ ├────────────────────────────┤ CABLES ├──────────────────────────────┤▒ │ Note: │▒ │ 1╔══╗────stripe─────1╔══╗───────────1╔══╗ │▒ │ The IDE Host Adapter ║::║≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡║::║≡≡≡≡≡≡≡≡≡≡≡≡║::║ │▒ │ connector may be on ║::║≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡║::║≡≡≡≡≡≡≡≡≡≡≡≡║::║ │▒ │ a plug-in Paddle-Board ║::║≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡║::║≡≡≡≡≡≡≡≡≡≡≡≡║::║ │▒ │ or may be integrated ║::║≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡║::║≡≡≡≡≡≡≡≡≡≡≡≡║::║ │▒ │ on the Motherboard. ║::║≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡║::║≡≡≡≡≡≡≡≡≡≡≡≡║::║ │▒ │ ╚══╝ ╚══╝ ╚══╝ │▒ │ Host Adapter Drives 1-2 (any order) │▒ ├────────────────────────────────────────────────────────────────────┤▒ │ 1* There ARE some SCSI drives that utilize MFM, but very few. │▒ └────────────────────────────────────────────────────────────────────┘▒ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ ┌────────────────────────────────────────────────────────────────────┐ │ DIAGRAMS.DOC 6 │▒ ├───────────────────────┐ More on Recording ┌────────────────────────┤▒ │ │▒ │ WRITE PRECOMPENSATION │▒ │ │▒ │ OK, so we've all seen it listed, and maybe even had to set it │▒ │ in the CMOS. So what IS it? And what does it do? │▒ │ PreComp. is the way in which the electronics compensates for │▒ │ eventual "drift" of the magnetic domains written on the disk. A │▒ │ simple explaination is that it allows the head to space bits that │▒ │ would attract each other, further apart, while it puts those that │▒ │ repel each other, closer together. It does this by analyzing the │▒ │ data stream, and adjusting the timing for each bit, to allow it to │▒ │ be recorded earlier or later, if needed. │▒ │ Not all disks require you to set their PreComp value. Those │▒ │ that do are asking for a cylinder to start PreComp. at. Since the │▒ │ packing of the bits on a disk increases as you get closer to the │▒ │ center of the disk (higher cylinders), the requirement for PreComp.│▒ │ increases too. The PreComp. value specified by the Manufacturer │▒ │ for a disk is his way of insuring your long term data stability. │▒ │ │▒ │ ──< THE EFFECT OF PRECOMPENSATION OVER TIME >── │▒ │ │▒ │ When recorded (w/o PreComp) When recorded (with PreComp) │▒ │ ┌──────────────────────────────┐ ┌──────────────────────────────┐ │▒ │ │ +- -+ +- +- -+ -+ │ │ +- -+ +- -+ -+ -+│ │▒ │ └──────────────────────────────┘ └──────────────────────────────┘ │▒ │ │▒ │ After time (w/o PreComp) After time (with PreComp) │▒ │ ┌──────────────────────────────┐ ┌──────────────────────────────┐ │▒ │ │+- -+ +- +- -+ -+ │ │ +- -+ +- -+ -+ -+ │ │▒ │ └──────────────────────────────┘ └──────────────────────────────┘ │▒ │ │▒ │ From the figures above, we can see how a slight amount of Pre- │▒ │ Compensation can insure long term stability. The disk that didn't │▒ │ employ PreComp was eventually unreadable. Of course, this would │▒ │ take time to happen, but no one can give cold hard specs on how │▒ │ much drift will occure. (Of course, this example is a gross sim- │▒ │ plification of the process, but, hey, who's counting?) │▒ │ │▒ ├───────────────────────┤ For Notes & Such ├─────────────────────────┤▒ │ │▒ │ │▒ │ │▒ │ │▒ │ │▒ │ │▒ │ │▒ │ │▒ │ │▒ │ │▒ │ │▒ │ │▒ │ │▒ └────────────────────────────────────────────────────────────────────┘▒ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ ┌────────────────────────────────────────────────────────────────────┐ │ DIAGRAMS.DOC 7 │▒ ├────────────────────────────────────────────────────────────────────┤▒ │ APPLE SCSI │▒ │ │▒ │ Unlike in the PC world, the Apple APPLE DB-25 SCSI │▒ │ standardized on one drive interface, ┌────┐ │▒ │ SCSI. Also, Apple standardized on REQ < │1 └┐ │▒ │ a 25 pin connector for external con- MSG < │2 14│ GND │▒ │ nections. However, Apple decided not I/O < │3 15│ > C/D │▒ │ to implement the complete ANSI spec., RST <>│4 16│ GND │▒ │ so one must be careful that peripherals ACK >│5 17│< ATN │▒ │ used are certified to work with Apple's BSY <>│6 18│ GND │▒ │ SCSI bus. GND │7 19│<> SEL │▒ │ Apple also developed it's own pin- DB0 <>│8 20│<> PARITY │▒ │ configuration. The Apple and Future GND │9 21│<> DB1 │▒ │ Domain 25-pin SCSI connectors are as DB3 <>│10 22│<> DB2 │▒ │ close to "Standards" as there are in DB5 <>│11 23│<> DB4 │▒ │ the world of PCs. But the real ANSI DB6 <>│12 24│ GND │▒ │ Standard called for a 50 pin connector DB7 <>│13 25│ TMPWR │▒ │ commonly referred to as a "Centronics" │ ┌┘ │▒ │ type (made popular by the Centronics └────┘ │▒ │ printer company). Instead of the 25 │▒ │ staggered pins of the Apple & Future 50-PIN "CENTRONICS" │▒ │ Domain type connectors, the Centronics TYPE SCSI │▒ │ type uses 2 parallel rows of 25 pins. ┌─────┐ │▒ │ This arrangement allowed use of extra │1 26│ │▒ │ grounds for better isolation. │2 27│ │▒ │ │3 28│ │▒ │ │4 29│ │▒ │ │5 30│ │▒ │ │6 31│ │▒ │ │7 32│ │▒ │ │8 33│ │▒ │ (WORK IN PROGRESS) │9 34│ │▒ │ │10 35│ │▒ │ │11 36│ │▒ │ │12 37│ │▒ │ │13 38│ │▒ │ │14 39│ │▒ │ │15 40│ │▒ │ │16 41│ │▒ │ │17 42│ │▒ │ │18 43│ │▒ │ │19 44│ │▒ │ │20 45│ │▒ │ │21 46│ │▒ │ │22 47│ │▒ │ │23 48│ │▒ │ │24 49│ │▒ │ │25 50│ │▒ │ └─────┘ │▒ │ │▒ │ │▒ │ THE END? │▒ └────────────────────────────────────────────────────────────────────┘▒ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒