┌────────────────────────────────────────────────────────────────────┐
│ 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 │▒
│ Domain type connectors, the Centronics ┌───┐ │▒
│ type uses 2 parallel rows of 25 pins. │ └─┐ │▒
│ This arrangement allows the use of extra GND │1 26│<> DB0 │▒
│ grounds for better isolation. │ │2 27│<> DB1 │▒
│ │ │3 28│<> DB2 │▒
│ SCSI HISTORY │ │4 29│<> DB3 │▒
│ │ │5 30│<> DB4 │▒
│ SCSI has it's roots in the mainframe │ │6 31│<> DB5 │▒
│ world, but it's first implementation in │ │7 32│<> DB6 │▒
│ the PC world came soon after the first │ │8 33│<> DB7 │▒
│ PC. Shugart Associates devised an inter- │ │9 34│<> DBP │▒
│ face that they designated the SASI, or │ │10 35│ GND │▒
│ "Shugart Associates Standard Interface" │ │11 36│ GND │▒
│ They proposed that SASI be adopted by ANSI │ │12 37│ GND │▒
│ for small computers, but durring the work │ │13 38│ TERM.PWR. │▒
│ required for ratification, they discovered │ │14 39│ GND │▒
│ the process would take too much effort, and │ │15 40│ GND │▒
│ that the IPI groups were already well into │ │16 41│ > ATN │▒
│ their effort. (which had many features the │ │17 42│ GND │▒
│ same as SASI) A decision was made to take │ │18 43│<> BSY │▒
│ features of both interfaces, and put forth │ │19 44│ > ACK │▒
│ a new specification for a new interface, │ │20 45│<> RST │▒
│ SCSI was born, and ratified in 1986 by │ │21 46│< MSG │▒
│ ANSI. Since then, many have said that the │ │22 47│<> SEL │▒
│ original spec. was not tight enough, and │ │23 48│< C/D │▒
│ that it allowed Manufacturers to make │ │24 49│< REQ │▒
│ drives that met the ANSI spec., but would GND │25 50│< I/O │▒
│ not talk to each other. Recently, the │ ┌─┘ │▒
│ ANSI SCSI committee has proposed newer, └───┘ │▒
│ tighter, more extended specs., for 50 PIN "CENTRONICS" │▒
│ SCSI-2, and now SCSI-3. FOR "PC" TYPE COMPUTERS │▒
└────────────────────────────────────────────────────────────────────┘▒
▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
┌────────────────────────────────────────────────────────────────────┐
│ DIAGRAMS.DOC 8 │▒
├────────────────────┐ CONSTRUCTION PROJECT ┌────────────────────────┤▒
│ SCSI TERMINATION │▒
│ │▒
│ With the advent of increased use of SCSI for peripherals comes │▒
│ the chance that one day you'll need an SCSI terminating resistor. │▒
│ Prepare for a shock, because you might be very suprised at the │▒
│ prices charged, for what you get. Many Manufacturers still have │▒
│ SCSI peripheral hardware priced ┌──────────────────────────────┐ │▒
│ for the Workstation market, not │ 1 ─/\/\/\/─ 26 ─/\/\/\/─┐ │ │▒
│ the PC market. We may see these │ 2 ─/\/\/\/─ 27 ─/\/\/\/─┤ │ │▒
│ prices erode as more PCs adopt │ 3 ─/\/\/\/─ 28 ─/\/\/\/─┤ │ │▒
│ SCSI as their disk interface of │ 4 ─/\/\/\/─ 29 ─/\/\/\/─┤ │ │▒
│ choice, but for now be prepared │ 5 ─/\/\/\/─ 30 ─/\/\/\/─┤ │ │▒
│ to pay a premium for anything to │ 6 ─/\/\/\/─ 31 ─/\/\/\/─┤ │ │▒
│ do with SCSI. │ 7 ─/\/\/\/─ 32 ─/\/\/\/─┤ │ │▒
│ So here you are, with a disk │ 8 ─/\/\/\/─ 33 ─/\/\/\/─┤ │ │▒
│ drive mounted internally, and a │ 9 ─/\/\/\/─ 34 ─/\/\/\/─┤ │ │▒
│ CDRom hanging off the back of the │ 10 35 │ │ │▒
│ PC. Everything looks great, but │ 11 36 │ │ │▒
│ it just doesn't work... Maybe it │ 12 37 │ │ │▒
│ doesn't even recognize the CDRom. │ 13 38 ─────────┤ │ │▒
│ You've checked the connectors, and│ 14 39 │ │ │▒
│ everything looks good... So what's│ 15 40 │ │ │▒
│ the problem? Well, did you check │ 16 ─/\/\/\/─ 41 ─/\/\/\/─┤ │ │▒
│ the terminators? (Say Whaaat??) │ 17 42 │ │ │▒
│ Improper termination of an SCSI │ 18 ─/\/\/\/─ 43 ─/\/\/\/─┤ │ │▒
│ bus can raise havock with the Host│ 19 ─/\/\/\/─ 44 ─/\/\/\/─┤ │ │▒
│ Adapter's interface circuit, and │ 20 ─/\/\/\/─ 45 ─/\/\/\/─┤ │ │▒
│ result in missing peripherals, or │ 21 ─/\/\/\/─ 46 ─/\/\/\/─┤ │ │▒
│ intermittent operation and pos- │ 22 ─/\/\/\/─ 47 ─/\/\/\/─┤ │ │▒
│ sible loss of data. │ 23 ─/\/\/\/─ 48 ─/\/\/\/─┤ │ │▒
│ Well, here's a way to build an │ 24 ─/\/\/\/─ 49 ─/\/\/\/─┤ │ │▒
│ inexpensive terminator that will │ 25 ─/\/\/\/─ 50 ─/\/\/\/─┘ │ │▒
│ connect to the second SCSI con- │ 220Ω 330Ω │ │▒
│ nector on many SCSI peripherals. └──────────────────────────────┘ │▒
│ All you need is a Male 50-pin SCSI Terminator Schematic │▒
│ Centronics type connector, a small │▒
│ length of wire, and 18 resistors of 330Ω and 18 of 220Ω, 1/4 watt. │▒
│ The schematic for connecting the resistors & connector is above, │▒
│ and I'll not go any deeper into construction except to say that if │▒
│ you can't take it from here without explaination, you should buy │▒
│ your terminator instead, as you can do too much damage if you do it│▒
│ wrong. │▒
├────────────────────────────────────────────────────────────────────┤▒
│ │▒
│ │▒
│ │▒
│ │▒
│ (This space left unintentionally blank!) │▒
│ │▒
│ │▒
│ │▒
│ │▒
└────────────────────────────────────────────────────────────────────┘▒
▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
