
       Ŀ
        DIAGRAMS.DOC   Supplement to TheRef(tm) Drive & Controller Listing 
       Ĵ
          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 
       Ĵ CABLES Ĵ
                                                                           
                   Controller     Drive 2(or none)       Drive 1           
                                                                           
                     1ͻ 1ͻ stripe1ͻ Pins 10-16  
        FLOPPY cable  ::ôô are twisted 
        with twist    ::ôXXô before the  
        (control &    ::ôô connector.  
        data, 34 pin) ::ôô  (7 wires)  
                      ͼ           ͼ                 ͼ             
                     1ͻ 1ͻ stripe1ͻ Pins 25-29  
        ST412 & ESDI  ::ôô are twisted 
        Hard Drive    ::ôô before the  
        cable w/twist ::ôXXô connector.  
        (control)     ::ôô  (5 wires)  
                      ͼ           ͼ                 ͼ             
                     1ͻ stripe1ͻ (no twists) 
        ST412 & ESDI  ::ô Each drive  
        Hard Drive    ::ô has it's    
        (data, 20 pin)::ô own data    
                      ͼ                                ͼ 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.                  
       ٱ
        
       
       Ŀ
        DIAGRAMS.DOC                                                     2 
       Ĵ CONNECTIONS Ĵ
                                                                           
        FLOPPY DRIVES                                      Ŀ         
                                            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                    Ŀ         
        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  |      
                          Ŀ             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)                                               
       ٱ
        
                                                                             
       Ŀ
        DIAGRAMS.DOC                                                     3 
       Ĵ CONNECTIONS Ĵ
                                                                           
        ESDI HARD DRIVES                                      Ŀ      
                                                  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.                                            
                                               Ŀ                     
                                   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                 
                                                                    
                                                                           
        Ŀ 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  
       ٱ
        
                                                                              
       Ŀ 
        DIAGRAMS.DOC                                                     4 
       Ĵ
        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.    
        Ŀ  
                                                         Ŀ           
        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           Ŀ DB-25F CONN. 
        drive on the cable, regardless of it's     GND   1 Ŀ           
        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
       ٱ
        
                                                                             
       Ŀ
        DIAGRAMS.DOC                                                     5 
       Ĵ CABLES Ĵ
        SCSI (cont.)                                                       
                                 (T)                       (DC)    (T)  
        On an SCSI cable, the  1ͻstripe1ͻ1ͻ1ͻ1ͻ  
        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,  ͼ         ͼ   ͼ   ͼ   ͼ  
        requiring the removal    (HA)          Drives 1-7 (in any order)   
        of the Host Adapter's                                              
        internal Term. resistors.                                          
       Ĵ CONNECTORS Ĵ
                                                                           
        IDE (AT) HARD DRIVES                       (<> AT THE DRIVE CONN)  
                                                         Ŀ           
        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 20N/C (KEY)  
        twists.  Like an SCSI cable, the IDE      RES.N/C21 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/C27 28N/C RES.   
        may be Daisy-Chained.  Most IDE Host      RES.N/C29 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.                                         
                                                                           
       Ĵ CABLES Ĵ
        Note:                                                              
                               1ͻstripe1ͻ1ͻ   
        The IDE Host Adapter    ::::::   
        connector may be on     ::::::   
        a plug-in Paddle-Board  ::::::   
        or may be integrated    ::::::   
        on the Motherboard.     ::::::   
                                ͼ                ͼ            ͼ   
                                Host Adapter       Drives 1-2 (any order)  
       Ĵ
         1*  There ARE some SCSI drives that utilize MFM, but very few.    
       ٱ
        
           
       Ŀ
        DIAGRAMS.DOC                                                     6 
       Ŀ More on Recording Ĵ
                                                                           
        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)   
        Ŀ  Ŀ 
         +-   -+   +-   +-   -+   -+       +-  -+  +-   -+    -+    -+ 
           
                                                                           
            After time (w/o PreComp)          After time (with PreComp)    
        Ŀ  Ŀ 
        +-    -+    +- +-     -+ -+       +-   -+   +-   -+   -+   -+  
           
                                                                           
           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?)             
                                                                           
       Ĵ For Notes & Such Ĵ
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
       ٱ
        
        
       Ŀ
        DIAGRAMS.DOC                                                     7 
       Ĵ
        APPLE SCSI                                                         
                                                                           
          Unlike in the PC world, the Apple          APPLE DB-25 SCSI      
        standardized on one drive interface,             Ŀ            
        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           Ŀ             
        type uses 2 parallel rows of 25 pins.               Ŀ           
        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  
       ٱ
        

       Ŀ
        DIAGRAMS.DOC                                                     8 
       Ŀ CONSTRUCTION PROJECT Ĵ
        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   Ŀ 
        for the Workstation market, not      1 /\/\/\/ 26 /\/\/\/Ŀ   
        the PC market.  We may see these     2 /\/\/\/ 27 /\/\/\/Ĵ   
        prices erode as more PCs adopt       3 /\/\/\/ 28 /\/\/\/Ĵ   
        SCSI as their disk interface of      4 /\/\/\/ 29 /\/\/\/Ĵ   
        choice, but for now be prepared      5 /\/\/\/ 30 /\/\/\/Ĵ   
        to pay a premium for anything to     6 /\/\/\/ 31 /\/\/\/Ĵ   
        do with SCSI.                        7 /\/\/\/ 32 /\/\/\/Ĵ   
          So here you are, with a disk       8 /\/\/\/ 33 /\/\/\/Ĵ   
        drive mounted internally, and a      9 /\/\/\/ 34 /\/\/\/Ĵ   
        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 Ĵ   
        You've checked the connectors, and  14           39             
        everything looks good... So what's  15           40             
        the problem?  Well, did you check   16 /\/\/\/ 41 /\/\/\/Ĵ   
        the terminators?   (Say Whaaat??)   17           42             
        Improper termination of an SCSI     18 /\/\/\/ 43 /\/\/\/Ĵ   
        bus can raise havock with the Host  19 /\/\/\/ 44 /\/\/\/Ĵ   
        Adapter's interface circuit, and    20 /\/\/\/ 45 /\/\/\/Ĵ   
        result in missing peripherals, or   21 /\/\/\/ 46 /\/\/\/Ĵ   
        intermittent operation and pos-     22 /\/\/\/ 47 /\/\/\/Ĵ   
        sible loss of data.                 23 /\/\/\/ 48 /\/\/\/Ĵ   
          Well, here's a way to build an    24 /\/\/\/ 49 /\/\/\/Ĵ   
        inexpensive terminator that will    25 /\/\/\/ 50 /\/\/\/   
        connect to the second SCSI con-          220         330       
        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 330 and 18 of 220, 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.                                                             
       Ĵ
                                                                           
                                                                           
                                                                           
                                                                           
                      (This space left unintentionally blank!)             
                                                                           
                                                                           
                                                                           
                                                                           
       ٱ
        
