.\"
.\" Copyright (c) 1995, 1996 
.\" 	Justin T. Gibbs.  All rights reserved.
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.Dd April 20, 1996
.Dt AHC 4 i386
.Os FreeBSD
.Sh NAME
.Nm ahc
.Nd Adaptec VL/EISA/PCI SCSI host adapter driver
.Sh SYNOPSIS
For one or more VL/EISA cards:
.Cd controller eisa0
.Cd controller ahc0
.Pp
For one or more PCI cards:
.Cd controller pci0
.Cd controller ahc0
.Pp
To enable tagged queueing:
.Cd options AHC_TAGENABLE
.Pp
To enable SCB paging:
.Cd options AHC_SCBPAGING_ENABLE
.Pp
For one or more SCSI busses:
.Cd controller scbus0 at ahc0
.Sh DESCRIPTION
This driver provides access to the
.Tn SCSI
bus connected to an Adaptec 
274x, 284x, 2940, 3940, or controllers based on the
.Tn AIC7770,
.Tn AIC7850,
.Tn AIC7860,
.Tn AIC7870,
or
.Tn AIC7880
host adapter chips.
Features include support for twin and wide busses,
ultra
.Tn SCSI,
two active commands at a time per non-tagged queueing target,
tagged queuing,
and SCB paging which allows up to 255 active commands on all adapters
except those using 
.Tn AIC7770
chips prior to revision E.
Tagged queueing is enabled with the
.Dq Dv AHC_TAGENABLE
configuration option.
SCB paging is enabled with the
.Dq Dv AHC_SCBPAGING_ENABLE
configuration option.
.Pp
Per target configuration performed in the 
.Tn SCSI-Select
menu, accesible at boot
in 
.No non- Ns Tn EISA
models or through an 
.Tn EISA
configuration utility for 
.Tn EISA
models,
is honored by this driver with the stipulation that the 
.Tn BIOS
must be enabled for 
.Tn EISA
adaptors.  This includes synchronous/asynchronous transfers,
maximum synchronous negotiation rate,
disconnection,
and the host adapter's SCSI ID.
.Pp
Note that I/O addresses are determined automatically by the probe routines,
but care should be taken when using a 284x
.Pq Tn VESA No local bus controller
in an
.Tn EISA 
system.  Ensure that the jumpers setting the I/O area for the 284x match the 
.Tn EISA
slot into which the card is inserted to prevent conflicts with other
.Tn EISA
cards.
.Sh BUGS
Some Quantum drives (at least the Empire 2100 and 1080s) will not run on an
.Tn AIC7870
Rev B in synchronous mode at 10MHz.  Controllers with this problem have a
42 MHz clock crystal on them and run slightly above 10MHz, causing the
drive much confusion.  Setting a maximum synchronous negotiation rate of 8MHz
in the 
.Tn SCSI-Select
utility
will allow normal function.
.Sh SEE ALSO
.Xr aha 4 ,
.Xr ahb 4 ,
.Xr cd 4 ,
.Xr scsi 4 ,
.Xr sd 4 ,
.Xr st 4
.Sh AUTHOR
The
.Nm
driver was written by Justin Gibbs.  The
.Tn AIC7xxx
sequencer-code assembler was
written by John Aycock.
.Sh HISTORY
The
.Nm
driver appeared in
.Tn FreeBSD
2.1.