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     MCCONFIG(F)	      XENIX System V		   MCCONFIG(F)

     Name
	  mcconfig - Irwin tape driver parameters.

     Description
	  /etc/default/mcconfig contains information on Irwin tape
	  driver parameters.  mcconfig entries are in the following
	  format:

	       variable=parameterlist

	  variable is a case insensitive character string that names a
	  configuration parameter.  parameterlist is a string of one
	  or more parameter values, in formats that vary depending on
	  the variable used.

	  The following variables are defined:

	  IROPT	   driver options
	  IRDBG	   debugging aids
	  SYSFDC   system floppy controller parameters
	  ALTFDC   alternate controller parameters
	  4100	   Irwin 4100 PC bus controller parameters
	  4100B	   second 4100 PC bus controller parameters
	  IRDRV	   drive searching sequence (old method 2.00)
	  IRSRCH   drive searching sequence (new method 2.02)
	  4251	   4251 address

	  When configuring parameters, space and tab characters can
	  not be used.	For example,

	  irdrv=3     is correct, while
	  irdrv = 3   is incorrect and will be ignored.

	  Parameters are passed to the tape driver by the daemon
	  program /etc/mcdaemon. Configuration parameters are given on
	  separate lines.  The pound sign character (#) may be used
	  open a comment.  Comments are terminated by a newline.  For
	  example the mcconfig file might contain:

	       # this is a comment in the mcconfig file
	       iropt=F
	       4251=31f

	  Changes made to the mcconfig file do not take effect until
	  the system is rebooted.

	IROPT: Configuration Option String
	  The tape driver configuration variable IROPT may be used
	  override certain default or automatically determined
	  configuration parameters.  Multiple values can be specified,
	  for example:

     Page 1					      (printed 2/7/91)

     MCCONFIG(F)	      XENIX System V		   MCCONFIG(F)

	       iropt=Bdf

	  The values for IROPT are as follows:

	  B/b: 64K DMA Boundary Present/Absent

	  B	 This computer's hardware architecture has a 64K DMA
		 memory boundary.  Tape data transfer buffers may not
		 cross a 64K physical boundary.	 This is the case for
		 most PC and AT compatible machines.

	  b	 This computer's hardware architecture does not have a
		 64K DMA physical memory boundary.  Tape data transfer
		 buffers may be allocated any where in memory.	This
		 is true for PS/2s with the Micro Channel
		 Architecture.

	  When neither (B) nor (b) is set, configuration is based upon
	  the result of Micro Channel presence determination (see the
	  M/m option).	In a Micro Channel machine, (b) is assumed,
	  otherwise (B) is used.

	  D/d: Use Demand/Single Byte DMA with Controllers Having a
	  FIFO

	  D    When running in PC or AT class machine and using a
	       controller which has a first-in-first-out (FIFO)
	       buffer, use demand mode DMA transfers.  Both the Intel
	       82072 and 82077 floppy controller chips (the later is
	       used in the 4100PC) have a 16 byte FIFO.

	  d    When running in a PC or AT class machine use the
	       standard single byte DMA transfer mode regardless of
	       the floppy controller type.

	  When neither (D) nor (d) is set, automatic configuration
	  determines whether a floppy controller chip with a FIFO is
	  present on a per controller basis.  When a controller having
	  a FIFO is found (e.g., Intel 82072/82077 parts return a
	  positive response to the CONFIGURE command), DMA transfers
	  with respect to that controller are setup using the demand
	  mode.	 Using demand mode decreases the portion of the bus
	  bandwidth consumed by tape read/write transfers and improves
	  system performance during tape access.

	  F/f: Floating/Pulled-Up Drive Search

	  F    When searching for drives on the system controller, use
	       a special ``floating track 0'' drive search.  The
	       ``floating'' drive search assumes the track 0 floppy
	       interface line floats (can be high or low) when no

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     MCCONFIG(F)	      XENIX System V		   MCCONFIG(F)

	       drive is attached. This algorithm works in all machines
	       but can't locate a drive which is executing a load-
	       point operation.	 The floating search is required on
	       certain Adaptec controllers.

	  f    When searching for drives on the system controller, use
	       the standard ``pulled-up track 0'' drive search.	 The
	       standard algorithm assumes the floppy interface's track
	       0 line is pulled up (is high) when no tape drive is
	       attached.  When the standard search is employed on a
	       controller which ``floats'' the track 0 line, a drive
	       may be erroneously detected at a line where none is
	       present. To deal with this condition either the IRDRV
	       configuration variable may be set to specify the drive
	       line (preferred) or the ``floating track 0'' drive
	       search (F) may be specified.

	  When neither (F) nor (f) is set, automatic configuration of
	  this option is performed by examining the model information
	  returned from the BIOS ``Get Machine Configuration'' service
	  (int 15, AH = C0).  The following model uses the
	  ``floating'' drive search (F):

		      Model   Type   Sub-type  PS/2 Model
		      ___________________________________
		      F8      0D     24 MHz Model 70
	  All other models use the ``pulled-up track 0'' search (f).

	  H/h: Do/Don't Test for 4100 PC Bus Controller Signature

	  H    Test for Irwin 4100 PC Bus controller (default).

	  h    No 4100 PC controller present.

	  In the PC or AT (not Micro Channel) hardware environment
	  (see the M/m option), when testing for the presence of a
	  4100 PC controller, the driver reads a byte from a signature
	  port on the controller and compares this against the value
	  45 hexadecimal.  The I/O port address of the signature port
	  is found by adding six to the board's base port address (see
	  the controller configuration section).  For a 4100 PC Bus
	  controller with switches set to ``as shipped from the
	  factory'' positions, the signature port address is 0370
	  (hexadecimal) + 6.  If the byte compares the 4100 PC is
	  present.  Otherwise it isn't.	 This option is intended to be
	  used when peeking at the factory set (0376 hexadecimal)
	  signature port causes the disruption of some other adapter
	  which is present at this address.  Note that the driver can
	  be instructed to find the controller at a different address
	  by setting the 4100 parameter.

	  I/i: Do/Don't Wait-for-Index

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     MCCONFIG(F)	      XENIX System V		   MCCONFIG(F)

	  I    Wait-for-index before data transfer of each tape block.

	  i    No need to wait-for-index before data transfer.

	  When neither (I) nor (i) is set, wait-for-index is enabled
	  by default only when an Olivetti Micro Channel machine is
	  present, otherwise wait-for-index is disabled.

	  If the following symptoms are experienced, after installing
	  the MC driver in certain Micro Channel machines, the wait-
	  for-index algorithm may need to be enabled:

	   o   On the first backup this message is seen:

	  mc tape write error: Defect list has unrecoverable error

      o	  If tape format gives the error:

	       Formatting failed: Block 0 medium error :
	       phase: CERTIFICATION,  track:  0,  cylinder:  0

      o	  Extremely poor performance is experienced while listing the
	  content of or restoring a previously written tape.

	  A condition exists in some Micro Channel computers which
	  causes errors reading the first sector of each tape block.
	  Included are the IBM models 50, 60, and 80, and the Olivetti
	  P-500.

	  These machines employ 72065 (except for the Olivetti which
	  has a 765) floppy controllers and data separators with
	  certain characteristics.  The 72065 differs from other
	  controllers in that it does not inhibit VCO SYNC when an
	  INDEX signal is received.  Characteristically the data
	  separator circuit will: 1) have a phase lock loop (PLL)
	  which totally loses synchronization when confronted with a
	  50/50 duty cycle read data signal and 2) be slow to re-
	  synchronize while in the ``data following mode.''  Most
	  Irwin drives generate a read data signal with the 50/50 duty
	  cycle when transiting servo headers.

	  When these factors are combined, the following sequence of
	  events occurs during a tape read operation:  A servo header
	  crosses the head.  The drive sends a 50/50 duty cycle 250
	  KHz signal on the read data line.  The PLL loses sync (that
	  is, the loop control voltage goes to a rail).	 The end of
	  the servo header crosses the head and the drive gives an
	  INDEX pulse.	No corresponding VCO SYNC inhibit is generated
	  by the 72065 (this would normally put the PLL back on
	  track).  Sector 1 crosses the head but the PLL is still too
	  far off to read the sector.  The 72065 generates a record-

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     MCCONFIG(F)	      XENIX System V		   MCCONFIG(F)

	  not-found error.

	  Some Irwin drives are fitted with a data compensator board.
	  This board has a circuit which alters the 50/50 duty cycle
	  to a value which allows most of these controllers to
	  maintain PLL synchronization.	 One exception is certain
	  Model 80s.

	  For Micro Channel systems which don't have the compensator
	  (and certain Model 80s which do), this problem can be
	  circumvented by software.  The technique relies on a feature
	  of the 72065 (and other controllers in the 765 class):  A
	  VCO SYNC inhibit is generated just after the last byte of a
	  READ command is sent to the controller.  Inhibiting the VCO
	  SYNC pin (which is normally telling the PLL to lock on
	  incoming read data) causes the VCO's input to be switched to
	  a reference.	This results in quickly returning the PLL to a
	  state in which it will be nearly synchronized with the
	  ``real'' read data.  VCO SYNC inhibition results from
	  programming the floppy controller using a ``wait-for-index''
	  algorithm.

	  The wait-for-index algorithm sends all but the last byte of
	  the data transfer command to the 72065.  It then waits for a
	  logical high to low transition of the floppy INDEX signal.
	  The wait is accomplished by polling a special I/O port (at
	  address 03F0h) provided by the Micro Channel floppy
	  controller.  The wait is used to delay the writing of the
	  last byte of the 72065 transfer command until after the
	  INDEX transition.  As a result, the 72065 generates an
	  inhibit pulse on VCO SYNC after INDEX, but with sufficient
	  lead time to allow the PLL to achieve synchronization.
	  Thus, sector one's ID can be correctly read.

	  As no index interrupt is available, wait-for-index polls to
	  accomplish its task.	The sought INDEX event is time
	  critical. A high priority daemon is awakened to poll for the
	  index transition.  Using the wait-for-index algorithm has
	  the following drawback:  All other system task time
	  processing is stopped until index polling is complete.  This
	  means the user will see sluggish system performance at
	  certain times.  Typically a 3 or 4 second dead period at
	  tape track switch time.  This may prove unacceptable in
	  certain installations.

	  M/m: Micro-Channel-Architecture/PC-Bus

	  M    This computer has a Micro Channel Architecture bus.

	  m    This machine doesn't have a Micro Channel Architecture.

	  When neither (M) nor (m) is set, automatic configuration

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     MCCONFIG(F)	      XENIX System V		   MCCONFIG(F)

	  determines if Micro Channel Architecture hardware is
	  present. The M/m option is used for automatic configuration
	  of the B/b, I/i, and P/p options.

	  If the string ``EISA'' is found at physical memory location
	  0xfffd9, (BIOS ROM location F000:FFD9) this is not a Micro
	  Channel Architecture.	 Otherwise if all 8 bits of the I/O
	  port at address 0x0080 (DMA page register 0 in an AT
	  compatibles) can be modified this is an AT 286/386
	  compatible.  Otherwise this is a Micro Channel Architecture.

	  O/o: System Controller Does/Doesn't Support 1-Meg Transfers

	  O    The system controller supports one Megabit data
	       transfers.

	  o    One Megabit transfers are not supported by the system
	       controller.

	  When neither (O) nor (o) is set, automatic configuration
	  determines whether the system controller supports 1-Megabit
	  transfer rates.  This is important when a 2120 is attached
	  to the system controller.  If the controller does not
	  support 1- Megabit transfers, 500-Kilobit transfers are used
	  for 80 and 120 Megabyte tapes.  The driver detects the
	  presence of the following 1-Megabit controllers:  Intel
	  82072 and 82077.  80 and 120 Megabyte drives do not work if
	  the driver thinks the hardware is capable of 1-Megabit
	  transfers and it is not.  In the reverse situation, transfer
	  performance is degraded.

	  P/p: 4251 Is/Isn't Present

	  P    A 4251 board is present in the system and has its
	       jumpers configured to address the 4251 digital output
	       register (DOR) at 0372h.	 When present the tape driver
	       echoes commands sent to the system floppy controller's
	       DOR (at I/O port address 03F2h) to the 4251 DOR.	 This
	       address can be configured using the 4251 parameter.

	  p    No 4251 board present.

	  When neither (P) nor (p) is set, and when running in a PC-
	  bus (non-Micro Channel) machine (see the M/m option),
	  automatic configuration determines the presence of a 4251
	  board by reading I/O port 0372h and comparing the input byte
	  to the signature of the 4251.	 The 4251 signature byte is
	  42h.	This address can be configured using the 4251
	  parameter.

	  Q/q: Compaq Portable III Piggy Back Tape Unit Is/Isn't
	  Present

     Page 6					      (printed 2/7/91)

     MCCONFIG(F)	      XENIX System V		   MCCONFIG(F)

	  Q    A Compaq Portable III piggy back tape unit is present.

	  q    No Compaq Portable III piggy back tape unit is present.

	  When neither (Q) nor (q) is set, the algorithm used to test
	  for presence of an alternate (Compaq Portable III piggy
	  back) controller does the following:	First the model byte
	  is checked to see if the machine is other than an 8086 class
	  machine (that is, the model byte must be less than FE).  If
	  this test passes, the BIOS address F000:FFEA is checked for
	  the string 'COMPAQ'.	When a match is found, the I/O port at
	  0374 (that is, the alternate floppy controller chip status
	  port) is read and the three low order bits are tested.  If
	  all three bits are zero, the alternate controller is
	  present.

	  When an alternate floppy controller is present, the
	  following port addresses are used by default:

		       765    765
	  Base	DOR    Stat   Data   Clock
	  _______________________________________________________________
	  03F0	03F2   03F4   03F5   03F7    Primary FLOPPY controller
	  0370	0372   0374   0375   0377    Alternate TAPE controller

	  See the Controller Parameter Configuration section for
	  information on reconfiguration of the default base address.

	  X/x: One Megabit Transfers Are/Aren't Allowed

	  X    Allow 1 Megabit transfers when conditions permit.

	  x    Never allow 1 Megabit transfers.

	  By default, 1 Megabit transfers (X) are allowed.  If 1
	  Megabit transfers overload the system bus, the (x) option
	  should be configured.

	IRDBG: Debugging Options
	  Several debugging flags are available:

	  s    Drive search debug

	  When (s) is set, the result of the tape drive search
	  (presence test) is shown.  The following shows an example:

	  4100MC:3=CTLRNOTFND :2=CTLRNOTFND :1=CTLRNOTFND :0=CTLRNOTFND
	  4100MCB:3=CTLRNOTFND :2=CTLRNOTFND :1=CTLRNOTFND :0=CTLRNOTFND
	  4100:3=DRVNOTFND :2=tapedrive :1=DRVNOTFND :0=DRVNOTFND
	  4100B:3=CTLRNOTFND :2=CTLRNOTFND :1=CTLRNOTFND :0=CTLRNOTFND
	  ALTFDC:3=CTLRNOTFND :2=CTLRNOTFND :1=CTLRNOTFND :0=CTLRNOTFND
	  SYDFDC:3=DRVNOTFND :2=tapedrive :1=nottested

     Page 7					      (printed 2/7/91)

     MCCONFIG(F)	      XENIX System V		   MCCONFIG(F)

	  The order of drive presence testing is shown left to right
	  and top to bottom.  On a given line, the left most field has
	  a symbol which represents a controller.  Numeric fields
	  preceded by a colon (:) give the unit select in the range 0
	  through 3.  Fields preceded by an equal sign (=) have a
	  symbol which represents result of tape drive presence
	  testing for the controller and unit.	These fields normally
	  have a upper case symbol which represents a driver error
	  code.	 Two special strings are used: ``tapedrive'' if a
	  drive was found, or ``nottested'' if drive presence was not
	  tested.

	  i    Initialization value debug

	  When (i) is set, certain initialization values are
	  displayed.  The following is an example:

	       hz=60 12_us_scaler=12 scaler_loops=27510 model=0x1FC
	       is64kdma=1 demanddma_ok=1
	       isuchannel=0 port_4251=3F0
	       timers=[ 0 1 2 1 2 7 19 37 181 235 ]

	  r    Interrupt debug

	  When (r) is set, a character is displayed for each interrupt
	  processed by the driver's finite state machine.  In
	  addition, reset cycles are shown.  The following lists the
	  characters and their meanings:

	  Character  Meaning
	  _____________________________________________________________________
	  N		       Floppy controller (NEC) interrupt
	  T		       Timer Interrupt
	  R		       Reset sent to floppy controller (start of reset)
	  r		       Reset complete

	  x    Data transfer debug

	  When (x) is set the status of a transfer request is
	  displayed at interrupt time.	The display is similar to that
	  shown below:

	       Cylinder
	       |
	       |     DMA
	       |     Overruns
	       |     |			   Alternating
	  Track|     |	  Positional	   Asterisk
	  |    |     |	  Retries	   |
	  |    |     |	  |		   |
	  |    |     |	  |    Interrupt   |

     Page 8					      (printed 2/7/91)

     MCCONFIG(F)	      XENIX System V		   MCCONFIG(F)

	  |    |     |	  |    Status	   |	    Sector Map
	  +--+ +---+ +--+ +--+ +---------+ | +--------------------+
	  |  | |   | |	| |  | |	 | | |			  |
	  T= 2 C= 42 O=12 R= 0 CRC	   *[-c-- --Cs ---- M-O- --]

	  Track (T=decimal number) has the transfer request's track
	  number.

	  Cylinder (C=decimal number) has the transfer request's
	  cylinder number (tape block for the given track).

	  DMA Overruns (O=decimal number) has a count of DMA overruns
	  (excluding, if indicated by in the Interrupt Status, the
	  current DMAOVERRUN).

	  Positional Retry (R=decimal number) has the current
	  positional retry number for the request.  Note that a
	  ``free'' retry is allowed under the following conditions: 1)
	  A track switch was performed.	 2) The tape is moving
	  logically forward, this transfer request's target head,
	  cylinder, and sector addresses match current values, but
	  there is some positional uncertainty because this transfer
	  request was not not started on the completion thread of the
	  previous request (That is, the period of time the tape has
	  been moving between requests is not known).  3) A DMA
	  overrun has occurred during the previous pass for a given
	  read/write/verify request.

	  Interrupt Status has the current reason for the interrupt
	  displayed symbolically.

	  Alternating Asterisk (*).  This one character field is
	  alternately set with an asterisk (*) and a space (' ')
	  character so that screen updates may be distinguished.

	  Sector Map displays a visual indication of the status of
	  each sector when an error occurs.  For example:

	       ([-c-- --Cs ---- M-O- --])

	  Each printing character in the sector map represents the
	  status of a sector.  Before the start of a transfer, each
	  entry is set to (s).	On successful transfer of a sector,
	  the corresponding entry is set to a hyphen (-).  The
	  following is a list of characters which appear in the sector
	  map and their meanings:

	  Character  Interrupt Error

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     MCCONFIG(F)	      XENIX System V		   MCCONFIG(F)

				       Num     Symbol	       Description
	  _________________________________________________________________________
	  -			       0       IE_NOERR	       No error
	  C			       12      IE_CRC	       Data CRC error
	  c			       13      IE_IDCRC	       ID CRC error
	  s			       14      IE_RECNOTFND    Record not found
	  M			       16      IE_DATAMARK     No data address mark
	  O			       17      IE_DMAOVERRUN   DMA overrun
	  ?			       other   unexpected      Unexpected value

	IRDRV, IRSRCH: Drive Search Control
	  IRDRV	 drive searching sequence (old method)
	  IRSRCH  drive searching sequence (new method)

	  The tape driver uses a default drive searching sequence to
	  test for the presence of tape drives.	 The default sequence
	  may be replaced with a user configured sequence using either
	  the IRDRV or IRSRCH variables.  This is useful in situations
	  where tape drives are erroneously detected by the default
	  sequence, or where multiple tape drives are supported and a
	  different mapping of logical to physical drives is desired.
	  For example:

	       IRSRCH=SYSFDC:3,4100:2

	  This searches for a tape drive at unit select 3 on the
	  system floppy controller, and unit select 2 on an Irwin 4100
	  PC bus controller.

	  The equivalent IRDRV specification is:

	       IRDRV=04,43

	  or alternately:

	       IRDRV=4,43

	  IRDRV specifications use a 2-digit number to specify a
	  controller and unit select.  The high-order digit gives the
	  controller, and the low-order the unit select.  If the
	  high-order digit is missing, 0 (for the system floppy
	  controller) is assumed.  Note that the unit select used by
	  IRDRV is in the range 1-4 while the unit select used by
	  IRSRCH is in the range 0-3.

	  The following is a list of controllers supported by IRSRCH
	  and IRDRV:

	  IRSRCH    IRDRV

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     MCCONFIG(F)	      XENIX System V		   MCCONFIG(F)

	  Name	    High
			    Digit
	  ___________________________________________
	  SYSFDC    0	    System floppy
	  ALTFDC    1	    Alternate floppy
	  4100MC    2	    Irwin 4100 Micro Channel
	  4100MCB   3	    Second 4100 Micro Channel
	  4100	    4	    Irwin 4100 PC Bus
	  4100B	    5	    Second 4100 PC Bus

	  The syntax of an IRSRCH drive search sequence specification
	  is:

	  IRSRCH=searchlist

	  searchlist =	      searchspec
			      searchspec,searchlist

	  searchspec =	      controller:unitlist

	  controller =	      SYSFDC (System floppy controller)
			      ALTFDC (Alternate controller)
			      4100MC (Irwin 4100 Micro Channel tape controller)
			      4100MCB (Second 4100 Micro Channel controller)
			      4100 (Irwin 4100 PC Bus tape controller)
			      4100B (Second 4100 PC Bus controller)

	  unitlist =	      unit
			      unit:unitlist

	  unit =	      0
			      1
			      2
			      3

	  The syntax of an IRDRV drive search sequence specification
	  is:

	  IRDRV=searchlist

	  searchlist =	      searchspec
			      searchspec,searchlist

	  searchspec =	      controllerdigitunitdigit

	  controllerdigit =   0 (System floppy controller, may be omitted)
			      1 (Alternate controller)
			      2 (Irwin 4100 Micro Channel tape controller)

     Page 11					      (printed 2/7/91)

     MCCONFIG(F)	      XENIX System V		   MCCONFIG(F)

			      3 (Second 4100 Micro Channel controller)
			      4 (Irwin 4100 PC Bus tape controller)
			      5 (Second 4100 PC Bus controller)

	  unitdigit =	      1
			      2
			      3
			      4

	SYSFDC, ALTFDC, 4100, 4100B: Controller Parameter
	  Configuration
	  Certain variables may be set to specify tape controller
	  specific parameters.	For example:

	       4100=P:370,I:6,D:2,T:2,T:0

	  says an Irwin 4100 PC bus controller is installed and
	  configured with a base I/O Port address (P) 0370
	  hexadecimal, using IRQ (I) 6, DMA channel (D) 2, and has two
	  tape units (T), one wired for physical unit select number 2,
	  and the other 0.

	  The general form for controller parameter specifications is:

	  controller=paramlist

	  paramlist =	 parameter
			 parameter,paramlist

	  parameter =	 name:value

	  controller =	 SYSFDC (System floppy controller)
			 ALTFDC (Alternate controller)
			 4100 (Irwin 4100 PC Bus controller)
			 4100B (Second 4100 PC Bus controller)

	  name =	 P (Base I/O Port address)
			 I (Interrupt Request line (IRQ))
			 D (DMA channel)
			 T (Tape unit number [0-3])

	  value =	 [0123456789abcefABCDEF]+ (Hexadecimal number)

	4100 PC Configuration Switch Settings
	  The following tables contain the 4100 PC bus switch
	  settings.  (4100 Micro Channel settings are modified with
	  the PS/2 reference (setup) diskette.

	  Base	 |

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     MCCONFIG(F)	      XENIX System V		   MCCONFIG(F)

	  Address|  SW1	  SW2	SW3   SW4
	  _______|_______________________
	     300 |  ON	  ON	ON    ON
	     310 |  off	  ON	ON    ON
	     320 |  ON	  off	ON    ON
	     330 |  off	  off	ON    ON
	     340 |  ON	  ON	off   ON
	     350 |  off	  ON	off   ON
	     360 |  ON	  off	off   ON
	   * 370 |  off	  off	off   ON
	     380 |  ON	  ON	ON    off
	     390 |  off	  ON	ON    off
	     3a0 |  ON	  off	ON    off
	     3b0 |  off	  off	ON    off
	     3c0 |  ON	  ON	off   off
	     3d0 |  off	  ON	off   off
	     3e0 |  ON	  off	off   off
	     3f0 |  off	  off	off   off

	  DMA	 |
	  Channel|  SW5	  SW6	SW7   SW8
	  _______|_______________________
	  1	 |  ON	  off	ON    off
	  2*	 |  off	  ON	off   ON

	  IRQ|	SW9   SW10
	  ___|____________
	  3  |	ON    off
	  6* |	off   ON

	  *  factory setting

	4251: Floppy Extender Address Configuration
	  The Irwin 4251 adapter board augments the system floppy
	  controller. It extends the total number of drives which may
	  be attached from 2 to 4, and allows for the attachment of an
	  external drive.  The 4251 uses a single drive select I/O
	  port. By design, the 4251 I/O port partially mimics the
	  functionality of the system floppy controller's drive select
	  port.	 The system controller's drive select port is called
	  the Digital Output Register (DOR).  When written with
	  certain values, both the system controller's DOR and the
	  4251 drive select port activate a drive select line at the
	  floppy interface.  In the standard ``as shipped from the
	  factory'' configuration, the 4251 port is addressed at 03F2
	  hexadecimal.	The same address is used by system floppy
	  controller's DOR.  Thus, in the standard configuration, the
	  4251 monitors (that is, listens to and uses) bytes written
	  to the system's DOR to select a drive.  The 4251 uses unit
	  selects 2 and 3.  Unit selects are used by the software and
	  should not be confused with the DRIVE SELECT jumpers on the
	  tape drive which are almost always set to DRIVE SELECT 2.

     Page 13					      (printed 2/7/91)

     MCCONFIG(F)	      XENIX System V		   MCCONFIG(F)

	  In certain hardware environments, the standard 4251
	  configuration either doesn't detect the presence of or fails
	  to write tapes in a tape drive.

	  When a 4251 is configured for the standard address and is
	  connected to:

	   o   a DTC controller, data is never written to tape.	 The
	       reason: DTC controllers disable the floppy interface
	       WRITE GATE signal when unit selects 2 or 3 (the third
	       and fourth) selects are activated.  This means the tape
	       drive's write circuitry is never enabled.

	   o   an Adaptec suffix 'B' controller (e.g., ACB-2xxxB or
	       1542B SCSI controllers), driver software never detects
	       the presence of a tape drive.  The reason: Adaptec
	       suffix 'B' controllers drive the TRACK 0 line active
	       for unit selects 2 or 3.	 The TRACK 0 line is the line
	       used by the drive to return the results of status
	       requests and motion commands issued by the driver
	       software.

	  The conditions listed in the above three paragraphs can be
	  overcome.  Typically reconfiguring the 4251 to use the
	  recommended alternate address by installing the A7 jumper
	  allows the tape drive to function correctly.	When this is
	  done, the 4251 I/O address moves from 3F2 to 372
	  hexadecimal.

	  When configuring the address of the Irwin 4251, the board
	  address jumpers are changed from the ``as shipped'' A0, A2,
	  A3 position. Normally the change involves reinstalling a
	  jumper stored on one pin of the A7 pin pair to connect the
	  ``A7'' pin pair.  This selects the address 372.  However,
	  when a secondary floppy controller (such as the Irwin 4100)
	  or other adapter is present the 372 address may be in
	  conflict.  In general, a secondary floppy controller uses
	  addresses in the range 370 through 377, which includes the
	  alternate ``372'' address of the 4251.  To resolve this
	  conflict, the 4251 can be re-addressed.  In addition, the
	  tape driver software must be informed of the new address.

	  The following information is given to aid in understanding
	  of the relationship of the 4251 and tape driver software,
	  the meaning of the 4251 jumpers A0 through A9, and an
	  example of a non-standard configuration.

	  At initialization, the tape driver software tests for the
	  presence of a 4251 at an alternate address.  By default, the
	  alternate address is 372 hexadecimal.	 (To select the 372
	  address on the 4251 install jumpers across the A0, A2, A3,
	  and A7 pin pairs.)  The test reads a byte from the alternate

     Page 14					      (printed 2/7/91)

     MCCONFIG(F)	      XENIX System V		   MCCONFIG(F)

	  address and compares the byte with the signature.  When the
	  4251 select port is read, a signature byte (42 hexadecimal)
	  is returned.	If the signature compares, the driver sends
	  select bytes to both the system's DOR and the 4251 port.
	  The default alternate address may overridden by using the
	  variable named ``4251.''  For example,

	       4251=31f

	  tells the driver to test and use, if present, the port at
	  31F hexadecimal.

	  The 4251 port uses a single 10-bit I/O port address.	The
	  address is set using the jumper pin pairs labeled A0 through
	  A9.  Each jumper pin pair corresponds directly with an I/O
	  port address bit. When a jumper pin pair is connected, the
	  corresponding address bit is set to a logical 0.  When the
	  pin pair is disconnected, the address bit is set to a
	  logical 1.

	  For example, to address the 4251 at 31F (an address which is
	  unlikely to conflict with standard adapters), connect jumper
	  pin pairs A5, A6, and A7.

     Files
	  /etc/default/mcconfig
	  /etc/mcdaemon

     See Also
	  tape(C), tape(HW)

     Page 15					      (printed 2/7/91)

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