旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 � DIAGRAMS.DOC Supplement to TheRef(tm) Drive & Controller Listing 낢 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑� � In "publishing" TheRef(tm), I've often been asked the difference 낢 � between the types of drive controllers and recording methods. I'm 낢 � not going to get into that in this document, as it would require a 낢 � good sized doc. of it's own. What I have supplied are diagrams of 낢 � the different connectors associated with the technology today. 낢 � frf 낢 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴캑 CABLES 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑� � 낢 � Controller Drive 2(or none) Drive 1 낢 � 낢 � 1�袴� 컴컴컴컴�1�袴� 컴컴stripe컴컴�1�袴� Pins 10-16 낢 � FLOPPY cable �::븟拙拙拙拙拙봤뉴拙拙拙拙拙拙拙拙齊척� are twisted 낢 � with twist �::븟拙拙拙拙拙봤뉴拙拙拙拙拙拙�XX拙봤뉴 before the 낢 � (control & �::븟拙拙拙拙拙봤뉴拙拙拙拙拙拙拙拙齊척� connector. 낢 � data, 34 pin) �::븟拙拙拙拙拙봤뉴拙拙拙拙拙拙拙拙齊척� (7 wires) 낢 � 훤暠 훤暠 훤暠 낢 � 1�袴� 컴컴컴컴�1�袴� 컴컴stripe컴컴�1�袴� Pins 25-29 낢 � ST412 & ESDI �::븟拙拙拙拙拙봤뉴拙拙拙拙拙拙拙拙齊척� are twisted 낢 � Hard Drive �::븟拙拙拙拙拙봤뉴拙拙拙拙拙拙拙拙齊척� before the 낢 � cable w/twist �::븟拙拙拙拙拙봤뉴拙拙拙拙拙拙�XX拙봤뉴 connector. 낢 � (control) �::븟拙拙拙拙拙봤뉴拙拙拙拙拙拙拙拙齊척� (5 wires) 낢 � 훤暠 훤暠 훤暠 낢 � 1�袴� 컴컴컴컴컴훥tripe컴컴컴컴컴컴�1�袴� (no twists) 낢 � ST412 & ESDI �::븟拙拙拙拙拙拙拙拙拙拙拙拙拙拙拙齊척� Each drive 낢 � Hard Drive �::븟拙拙拙拙拙拙拙拙拙拙拙拙拙拙拙齊척� has it's 낢 � (data, 20 pin)�::븟拙拙拙拙拙拙拙拙拙拙拙拙拙拙拙齊척� own data 낢 � 훤暠 훤暠 cable 낢 � 낢 � IMPORTANT NOTE: Pin #1 on any drive cable SHOULD be indicated by a 낢 � a colored stripe. If you should find the stripe 낢 � by connector pin 34 (or 20), inspect the whole 낢 � cable VERY throughly! 낢 � 낢 � DRIVE SELECT For both Floppy and Hard drives, when the 34 pin 낢 � JUMPERS: cable has a twist, the device number should be set 낢 � to the second position. Drives numbered 0-3, set to 낢 � 1, those numbered 1-4, set to 2. When cables with- 낢 � out a twist are used, Floppy "A", and(or) Hard drive 낢 � "C" should be set to 1, and the second Floppy and 낢 � (or) Hard drive should be set to 2. 낢 � 낢 � TERMINATORS: When using more than one drive on a cable (ie; 2FDs 낢 � or 2HDs), the terminating resistor pack should be 낢 � left on the drive furthest from the controller, and 낢 � removed from the drive closest to the controller. 낢 � 낢 � NOTE: On SCSI drives, the Host Adapter also has resistors. 낢 � These are needed to terminate both ends of the bus. 낢 � Since the SCSI bus can have up to 7 devices attached 낢 � to it, only the Host Adapter and the device farthest 낢 � from it will retain the resistors. All devices in- 낢 � between should have theirs removed. 낢 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸� 굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 � DIAGRAMS.DOC 2 낢 쳐컴컴컴컴컴컴컴컴컴컴컴컴캑 CONNECTIONS 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴눙 � 낢 � FLOPPY DRIVES 旼컴컴� 낢 � HI/LO DENSITY >�2 1� GND 낢 � The connector on a floppy drive N/C �4 _ 3� | 낢 � consists of 34 conductors. Both N/C �6 5� | 낢 � control and data use this same INDEX <�8 7� | 낢 � cable. Most cables have a twist MOTOR ENAB. A >�10 9� | 낢 � that interchanges pins 10 through DRIVE SEL. B >�12 11� | 낢 � 16 at the end of the cable (on DRIVE SEL. A >�14 13� | 낢 � drive 1). Most floppy connect- MOTOR ENAB. B >�16 15� | 낢 � ors have a "key" between pins DIRECTION SEL. >�18 17� | 낢 � 4 & 6, and 3 & 5, to prevent the HEAD STEP >�20 19� | 낢 � cable from being reversed. At WRITE DATA >�22 21� | 낢 � the other end, the dual row con- WRITE GATE >�24 23� | 낢 � nector that attaches to the con- TRACK 00 <�26 25� | 낢 � troller card will usually have a WRITE PROTECT <�28 27� | 낢 � set of ridges that coincide with READ DATA <�30 29� | 낢 � cutouts in the controller card's HEAD SELECT >�32 31� | 낢 � connector. Note that old style DISK CHANGE <�34 33� GND 낢 � floppy-only controllers used a 읕컴컴� 낢 � card-edge connector just like that > Input ( At the 낢 � of the drive. < Output Drive Conn.) 낢 � 낢 � ST506/412 HARD DRIVE (MFM & RLL) 낢 � 낢 � This standard drive system uses 旼컴컴� 낢 � two cables; a 34 conductor control HEAD SEL. 8 �2 1� GND 낢 � cable, and a 20 conductor data HEAD SEL. 4 �4 _ 3� | 낢 � cable. The control cable contains WRITE GATE �6 5� | 낢 � a twist of the conductors going to SEEK COMPLETE �8 7� | 낢 � the farthest drive, which is drive TRACK 0 �10 9� | 낢 � "C" on most systems. This twist WRITE FAULT �12 11� | 낢 � consists of conductors 25 through HEAD SEL. 1 �14 13� | 낢 � 29. As with the floppy cable, the RESERVED �16 15� | 낢 � ST506/412 cables normally have a HEAD SEL. 2 �18 17� | 낢 � key to prevent reversal, and the INDEX �20 19� | 낢 � controller end has a pin-type con- READY �22 21� | 낢 � nector, while the drive end has a STEP �24 23� | 낢 � card-edge type connector. DRIVE SEL. 1 �26 25� | 낢 � DRIVE SEL. 2 �28 27� | 낢 � 旼컴컴� DRIVE SEL. 3 �30 29� | 낢 � DRIVE SEL'D �1 2� GND DRIVE SEL. 4 �32 31� | 낢 � RESERVED �3 _ 4� | DIRECTION IN �34 33� GND 낢 � | �5 6� | 읕컴컴� 낢 � | �7 8� GND 낢 � RESERVED �9 10� RESERVED Though control signals 낢 � GND �11 12� GND go through a single 34 낢 � * WRITE DATA+ �13 14� * WRITE DATA- conductor cable, data 낢 � GND �15 16� GND flows through seperate 낢 � * READ DATA+ �17 18� * READ DATA- 20 conductor cables 낢 � GND �19 20� GND for each drive (C,D). 낢 � *(MFM or RLL) 읕컴컴� 낢 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸� 굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 � DIAGRAMS.DOC 3 낢 쳐컴컴컴컴컴컴컴컴컴컴컴컴캑 CONNECTIONS 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴눙 � 낢 � ESDI HARD DRIVES 旼컴컴� 낢 � HEAD SEL. 3 �2 1� GND 낢 � Though ESDI and ST506/412 drives HEAD SEL. 2 �4 _ 3� | 낢 � share similar looking cables, WRITE GATE �6 5� | 낢 � even to the point of having a CONFIG/STAT DATA �8 7� | 낢 � twist, the actual data and con- TRANSFER ACK. �10 9� | 낢 � trol signals are very different. ATTENTION �12 11� | 낢 � One should never mix components HEAD SEL. 0 �14 13� | 낢 � from these two drive types. SECT/ADD.MK. FOUND �16 15� | 낢 � While the ST506/412 interface HEAD SEL. 1 �18 17� | 낢 � utilizes a standard pulse code INDEX �20 19� | 낢 � to transmit data between the READY �22 21� | 낢 � drive and controller, ESDI uses TRANS.REQUEST �24 23� | 낢 � a pulse code that does not require DRIVE SEL. 1 �26 25� | 낢 � the level to return to zero between DRIVE SEL. 2 �28 27� | 낢 � pulses. This format is refered to DRIVE SEL. 3 �30 29� | 낢 � as NRZ, or Non Return to Zero. By READ GATE �32 31� | 낢 � utilizing NRZ, the clock that data COMMAND DATA �34 33� GND 낢 � is transfered by can be increased, 읕컴컴� 낢 � thereby increasing the troughput to 낢 � and from the ESDI disk. 낢 � 旼컴컴� 낢 � DRIVE SEL'D �1 2� SECT/ADD.MK. FOUND 낢 � SEEK COMPLETE �3 _ 4� ADDRESS MARK ENABLE 낢 � RESV'D FOR STEP MODE �5 6� GND 낢 � WRITE CLOCK+ �7 8� WRITE CLOCK- 낢 � CARTRIDGE CHANGED �9 10� READ REF. CLOCK+ 낢 � READ REF. CLOCK- �11 12� GND 낢 � NRZ WRITE DATA+ �13 14� NRZ WRITE DATA- 낢 � GND �15 16� GND 낢 � NRZ READ DATA+ �17 18� NRZ READ DATA- 낢 � GND �19 20� GND 낢 � 읕컴컴� 낢 � 낢 � 컴컴컴컴컴컴컴커 And in this corner... Recording 旼컴컴컴컴컴컴컴� 낢 � 낢 � Times were, you had a simple choice for type of disk drive... 낢 � Any kind, as long as it was ST506/412. Those were the heydays of 낢 � MFM drives. But many manufacturers weren't content with the 17 낢 � sectors/track that MFM provided. They devised a newer encoding 낢 � scheme to pack data tighter, and called it RLL, or Run Length 낢 � Limited, as opposed to MFM, or Modified Frequency Modulation. It 낢 � involves using groups of 16 bits rather than each individual bit, 낢 � thus achieving a sort of "compression" of the information as it is 낢 � encoded. Since the same information takes up less space as RLL 낢 � encoded data, more info can be writen to the disk. The most com- 낢 � mon RLL technique, known as 2,7 RLL, can pack roughly 50% more on 낢 � a disk than MFM. Of course, there is always a trade-off, and the 낢 � timing and media required for RLL is it. RLL requires a higher 낢 � grade of media because of it's dense bit-packing, and timing is 낢 � more critical, since the data is flowing at 50% higher rate than 낢 � an MFM drive. Also, the mechanics of the drive must have tighter 낢 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸� 굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 � DIAGRAMS.DOC 4 낢 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑� � tolerences because head positioning becomes more critical. These 낢 � requirements kept RLL drives at a premium. It has only been the 낢 � last two years, that RLL drives have outsold MFM, and have all but 낢 � wiped them from the marketplace. This turnabout has come from the 낢 � need to increase disk capacity more and more. Both ESDI, and SCSI 낢 � type drives utilize RLL.(1*) encoding to achieve high capacity and 낢 � transfer rates (from the disk). And the newest interface, IDE, or 낢 � Integrated Drive Electronics, is also based on this technology. 낢 � 컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 낢 � 旼컴컴� 낢 � SCSI HARD DRIVES DB0 <>�2 1� GND 5 낢 � DB1 <>�4 3� | 0 낢 � The normal internal cable for SCSI DB2 <>�6 5� | 낢 � is a 50 conductor ribbon, with all DB3 <>�8 7� | P 낢 � odd numbered conductors grounded. DB4 <>�10 9� | I 낢 � Two conductors, numbers 25 & 26, are DB5 <>�12 11� | N 낢 � often left not-connected, as they DB6 <>�14 13� | 낢 � deal with Terminator power, and can DB7 <>�16 15� | D 낢 � be easily shorted by cable reversals. DBP <>�18 17� | U 낢 � There are no twists in this cable, GND �20 19� | A 낢 � and it's length may be a maximum of GND �22 21� | L 낢 � 6 meters. But one is advised to use GND �24 23� | 낢 � minimum lengths to improve timing. TERM PWR �26 25� | R 낢 � Up to seven drives, or devices may be GND �28 27� | O 낢 � attached to an SCSI cable. Each is GND �30 29� | W 낢 � daisy-chained on the cable, or, when ATN < �32 31� | 낢 � a device has two connectors, another GND �34 33� | C 낢 � cable may be "spliced" into the chain BSY <>�36 35� | O 낢 � starting at the second connector, and ACK < �38 37� | N 낢 � continued on. Care must be taken to RST <>�40 39� | N 낢 � insure that cables and connectors are MSG >�42 41� | E 낢 � not reversed, as this would short pin SEL <>�44 43� | C 낢 � 26 (TERMPWR) to ground, and likely C/D >�46 45� | T 낢 � damage the drive or controller. Also, REQ >�48 47� | O 낢 � as explained earlier, the terminating I/O >�50 49� GND R 낢 � resistors should remain only on the 읕컴컴� 낢 � controller (Host Adapter) and the LAST 旼커 DB-25F CONN. 낢 � drive on the cable, regardless of it's GND �1 읕커 낢 � address. DB1 <>�2 14�<> DB0 낢 � Most SCSI Host Adapters also have DB3 <>�3 15�<> DB2 낢 � a connector for external drives in the DB5 <>�4 16�<> DB4 낢 � form of a Centronics(tm) type 50 pin, DB7 <>�5 17�<> DB6 낢 � or an "alternate", DB-25F connector. GND �6 18�<> PARITY 낢 � Only the internal 50-pin, and the SEL <>�7 19� GND 낢 � "alternate" external connector are GND �8 20� > ATN 낢 � shown here. (see also: MORE SCSI) TMPWR �9 21�< MSG 낢 � Also, these diagrams refer to the RST <>�10 22� > ACK 낢 � single-ended SCSI connections, since C/D �11 23�<> BSY 낢 � this is the most common arrangement I/O >�12 24�< REQ 낢 � for PCs today. The Differential SCSI GND �13 25� GND 낢 � requires balanced lines, and is used � 旼켸 낢 � mostly on high-end workstations. 읕켸 FUTURE DOMAIN낢 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸� 굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 � DIAGRAMS.DOC 5 낢 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴캑 CABLES 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑� � SCSI (cont.) 낢 � (T) 旼(DC)� (T) 낢 � On an SCSI cable, the 1�袴뼛stripe�1�袴뼛�1�袴뼛�1�袴箇敲컴1�袴� 낢 � terminating resistors �::븟拙拙拙拙�::븟拙�::븟拙�::�::븟拙�::� 낢 � (T) remain at the END �::븟拙拙拙拙�::븟拙�::븟拙�::�::븟拙�::� 낢 � devices on the cable, �::븟拙拙拙拙�::븟拙�::븟拙�::�::븟拙�::� 낢 � even when 2 cables are �::븟拙拙拙拙�::븟拙�::븟拙�::�::븟拙�::� 낢 � "Daisy-Chained" (DC). �::븟拙拙拙拙�::븟拙�::븟拙�::�::븟拙�::� 낢 � Also, the external �::븟拙拙拙拙�::븟拙�::븟拙�::�::븟拙�::� 낢 � connector may be used, 훤暠 훤暠 훤暠 훤菰袴� 훤暠 낢 � requiring the removal (HA) Drives 1-7 (in any order) 낢 � of the Host Adapter's 낢 � internal Term. resistors. 낢 쳐컴컴컴컴컴컴컴컴컴컴컴컴캑 CONNECTORS 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴캑� � 낢 � IDE (AT) HARD DRIVES (<> AT THE DRIVE CONN) 낢 � 旼컴컴� 낢 � IDE, or Integrated Drive Electronics RST >�1 2� GND 낢 � is the most recent drive interface to SD7 <>�3 4�<> SD8 낢 � gain popularity. Often, the control SD6 <>�5 6�<> SD9 낢 � circuitry is built into the mother- SD5 <>�7 8�<> SD10 낢 � board, eliminating the requirement for SD4 <>�9 10�<> SD11 낢 � a seperate Host Adapter. There are 2 SD3 <>�11 12�<> SD12 낢 � types of IDE interfaces...those for the SD2 <>�13 14�<> SD13 낢 � 8-bit XT bus, and those for the 16-bit SD1 <>�15 16�<> SD14 낢 � AT bus (detailed here). The cable for SD0 <>�17 18�<> SD15 낢 � IDE contains 40 conductors and has no GND �19 20쿙/C (KEY) 낢 � twists. Like an SCSI cable, the IDE RES.N/C�21 22� GND 낢 � cable uses a Dual-row Pin connector for IOW >�23 24� GND 낢 � both ends. A single cable may be used IOR >�25 26� GND 낢 � to connect two drives, or two cables RES.N/C�27 28쿙/C RES. 낢 � may be Daisy-Chained. Most IDE Host RES.N/C�29 30� GND 낢 � Adapters will support two hard drives. IRQ14 <�31 32�> I/O CS16 낢 � The first drive should be jumpered as SA1 <>�33 34�<> PDIAG 낢 � the Master drive, and the second as the SA0 <>�35 36�<> SA2 낢 � Slave drive. Plug-in IDE Host Adapters CS0 >�37 38�< CS1 낢 � are often called Paddle-Boards, and ACTIVE <�39 40� GND 낢 � may contain a floppy controller, and 읕컴컴� 낢 � serial and parallel ports. 낢 � 낢 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴캑 CABLES 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑� � Note: 낢 � 1�袴뼛컴훥tripe컴컴�1�袴뼛컴컴컴컴컴1�袴� 낢 � The IDE Host Adapter �::븟拙拙拙拙拙拙拙齊::븟拙拙拙拙拙齊::� 낢 � connector may be on �::븟拙拙拙拙拙拙拙齊::븟拙拙拙拙拙齊::� 낢 � a plug-in Paddle-Board �::븟拙拙拙拙拙拙拙齊::븟拙拙拙拙拙齊::� 낢 � or may be integrated �::븟拙拙拙拙拙拙拙齊::븟拙拙拙拙拙齊::� 낢 � on the Motherboard. �::븟拙拙拙拙拙拙拙齊::븟拙拙拙拙拙齊::� 낢 � 훤暠 훤暠 훤暠 낢 � Host Adapter Drives 1-2 (any order) 낢 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑� � 1* There ARE some SCSI drives that utilize MFM, but very few. 낢 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸� 굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 � DIAGRAMS.DOC 6 낢 쳐컴컴컴컴컴컴컴컴컴컴컴� More on Recording 旼컴컴컴컴컴컴컴컴컴컴컴캑� � 낢 � WRITE PRECOMPENSATION 낢 � 낢 � OK, so we've all seen it listed, and maybe even had to set it 낢 � in the CMOS. So what IS it? And what does it do? 낢 � PreComp. is the way in which the electronics compensates for 낢 � eventual "drift" of the magnetic domains written on the disk. A 낢 � simple explaination is that it allows the head to space bits that 낢 � would attract each other, further apart, while it puts those that 낢 � repel each other, closer together. It does this by analyzing the 낢 � data stream, and adjusting the timing for each bit, to allow it to 낢 � be recorded earlier or later, if needed. 낢 � Not all disks require you to set their PreComp value. Those 낢 � that do are asking for a cylinder to start PreComp. at. Since the 낢 � packing of the bits on a disk increases as you get closer to the 낢 � center of the disk (higher cylinders), the requirement for PreComp.낢 � increases too. The PreComp. value specified by the Manufacturer 낢 � for a disk is his way of insuring your long term data stability. 낢 � 낢 � 컴< THE EFFECT OF PRECOMPENSATION OVER TIME >컴 낢 � 낢 � When recorded (w/o PreComp) When recorded (with PreComp) 낢 � 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 낢 � � +- -+ +- +- -+ -+ � � +- -+ +- -+ -+ -+� 낢 � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸 낢 � 낢 � After time (w/o PreComp) After time (with PreComp) 낢 � 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 낢 � �+- -+ +- +- -+ -+ � � +- -+ +- -+ -+ -+ � 낢 � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸 낢 � 낢 � From the figures above, we can see how a slight amount of Pre- 낢 � Compensation can insure long term stability. The disk that didn't 낢 � employ PreComp was eventually unreadable. Of course, this would 낢 � take time to happen, but no one can give cold hard specs on how 낢 � much drift will occure. (Of course, this example is a gross sim- 낢 � plification of the process, but, hey, who's counting?) 낢 � 낢 쳐컴컴컴컴컴컴컴컴컴컴컴� For Notes & Such 쳐컴컴컴컴컴컴컴컴컴컴컴컴눙 � 낢 � 낢 � 낢 � 낢 � 낢 � 낢 � 낢 � 낢 � 낢 � 낢 � 낢 � 낢 � 낢 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸� 굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 � DIAGRAMS.DOC 7 낢 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑� � APPLE SCSI 낢 � 낢 � Unlike in the PC world, the Apple APPLE DB-25 SCSI 낢 � standardized on one drive interface, 旼컴커 낢 � SCSI. Also, Apple standardized on REQ >�1 읏 낢 � a 25 pin connector for external con- MSG >�2 14� GND 낢 � nections. However, Apple decided not I/O >�3 15�< C/D 낢 � to implement the complete ANSI spec., RST <>�4 16� GND 낢 � so one must be careful that peripherals ACK < �5 17� > ATN 낢 � used are certified to work with Apple's BSY <>�6 18� GND 낢 � SCSI bus. GND �7 19�<> SEL 낢 � Apple also developed it's own pin- DB0 <>�8 20�<> PARITY 낢 � configuration. The Apple and Future GND �9 21�<> DB1 낢 � Domain 25-pin SCSI connectors are as DB3 <>�10 22�<> DB2 낢 � close to "Standards" as there are in DB5 <>�11 23�<> DB4 낢 � the world of PCs. But the real ANSI DB6 <>�12 24� GND 낢 � Standard called for a 50 pin connector DB7 <>�13 25� TMPWR 낢 � commonly referred to as a "Centronics" � 粕 낢 � type (made popular by the Centronics 읕컴켸 낢 � printer company). Instead of the 25 낢 � staggered pins of the Apple & Future 낢 � Domain type connectors, the Centronics 旼컴� 낢 � type uses 2 parallel rows of 25 pins. � 읕� 낢 � This arrangement allows the use of extra GND �1 26�<> DB0 낢 � grounds for better isolation. � �2 27�<> DB1 낢 � � �3 28�<> DB2 낢 � SCSI HISTORY � �4 29�<> DB3 낢 � � �5 30�<> DB4 낢 � SCSI has it's roots in the mainframe � �6 31�<> DB5 낢 � world, but it's first implementation in � �7 32�<> DB6 낢 � the PC world came soon after the first � �8 33�<> DB7 낢 � PC. Shugart Associates devised an inter- � �9 34�<> DBP 낢 � face that they designated the SASI, or � �10 35� GND 낢 � "Shugart Associates Standard Interface" � �11 36� GND 낢 � They proposed that SASI be adopted by ANSI � �12 37� GND 낢 � for small computers, but durring the work � �13 38� TERM.PWR. 낢 � required for ratification, they discovered � �14 39� GND 낢 � the process would take too much effort, and � �15 40� GND 낢 � that the IPI groups were already well into � �16 41� > ATN 낢 � their effort. (which had many features the � �17 42� GND 낢 � same as SASI) A decision was made to take � �18 43�<> BSY 낢 � features of both interfaces, and put forth � �19 44� > ACK 낢 � a new specification for a new interface, � �20 45�<> RST 낢 � SCSI was born, and ratified in 1986 by � �21 46�< MSG 낢 � ANSI. Since then, many have said that the � �22 47�<> SEL 낢 � original spec. was not tight enough, and � �23 48�< C/D 낢 � that it allowed Manufacturers to make � �24 49�< REQ 낢 � drives that met the ANSI spec., but would GND �25 50�< I/O 낢 � not talk to each other. Recently, the � 旼� 낢 � ANSI SCSI committee has proposed newer, 읕컴� 낢 � tighter, more extended specs., for 50 PIN "CENTRONICS" 낢 � SCSI-2, and now SCSI-3. FOR "PC" TYPE COMPUTERS 낢 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸� 굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 � DIAGRAMS.DOC 8 낢 쳐컴컴컴컴컴컴컴컴컴커 CONSTRUCTION PROJECT 旼컴컴컴컴컴컴컴컴컴컴컴캑� � SCSI TERMINATION 낢 � 낢 � With the advent of increased use of SCSI for peripherals comes 낢 � the chance that one day you'll need an SCSI terminating resistor. 낢 � Prepare for a shock, because you might be very suprised at the 낢 � prices charged, for what you get. Many Manufacturers still have 낢 � SCSI peripheral hardware priced 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 낢 � for the Workstation market, not � 1 �/\/\/\/� 26 �/\/\/\/커 � 낢 � the PC market. We may see these � 2 �/\/\/\/� 27 �/\/\/\/캑 � 낢 � prices erode as more PCs adopt � 3 �/\/\/\/� 28 �/\/\/\/캑 � 낢 � SCSI as their disk interface of � 4 �/\/\/\/� 29 �/\/\/\/캑 � 낢 � choice, but for now be prepared � 5 �/\/\/\/� 30 �/\/\/\/캑 � 낢 � to pay a premium for anything to � 6 �/\/\/\/� 31 �/\/\/\/캑 � 낢 � do with SCSI. � 7 �/\/\/\/� 32 �/\/\/\/캑 � 낢 � So here you are, with a disk � 8 �/\/\/\/� 33 �/\/\/\/캑 � 낢 � drive mounted internally, and a � 9 �/\/\/\/� 34 �/\/\/\/캑 � 낢 � CDRom hanging off the back of the � 10 35 � � 낢 � PC. Everything looks great, but � 11 36 � � 낢 � it just doesn't work... Maybe it � 12 37 � � 낢 � doesn't even recognize the CDRom. � 13 38 컴컴컴컴캑 � 낢 � You've checked the connectors, and� 14 39 � � 낢 � everything looks good... So what's� 15 40 � � 낢 � the problem? Well, did you check � 16 �/\/\/\/� 41 �/\/\/\/캑 � 낢 � the terminators? (Say Whaaat??) � 17 42 � � 낢 � Improper termination of an SCSI � 18 �/\/\/\/� 43 �/\/\/\/캑 � 낢 � bus can raise havock with the Host� 19 �/\/\/\/� 44 �/\/\/\/캑 � 낢 � Adapter's interface circuit, and � 20 �/\/\/\/� 45 �/\/\/\/캑 � 낢 � result in missing peripherals, or � 21 �/\/\/\/� 46 �/\/\/\/캑 � 낢 � intermittent operation and pos- � 22 �/\/\/\/� 47 �/\/\/\/캑 � 낢 � sible loss of data. � 23 �/\/\/\/� 48 �/\/\/\/캑 � 낢 � Well, here's a way to build an � 24 �/\/\/\/� 49 �/\/\/\/캑 � 낢 � inexpensive terminator that will � 25 �/\/\/\/� 50 �/\/\/\/켸 � 낢 � connect to the second SCSI con- � 220� 330� � 낢 � nector on many SCSI peripherals. 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸 낢 � All you need is a Male 50-pin SCSI Terminator Schematic 낢 � Centronics type connector, a small 낢 � length of wire, and 18 resistors of 330� and 18 of 220�, 1/4 watt. 낢 � The schematic for connecting the resistors & connector is above, 낢 � and I'll not go any deeper into construction except to say that if 낢 � you can't take it from here without explaination, you should buy 낢 � your terminator instead, as you can do too much damage if you do it낢 � wrong. 낢 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑� � 낢 � 낢 � 낢 � 낢 � (This space left unintentionally blank!) 낢 � 낢 � 낢 � 낢 � 낢 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸� 굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇굇