When I set about to interface to the Hard Disk Port on my Atari
1040ST I assembled all the documentation I could find and discovered that,
though I found plenty, I still ended up without quite enough to design
the interface.  I attacked the problem with the Scientific Method and
now have a working one-megabyte-per-second interface.  This article was
written to give others the information that I needed in hopes that they
will not be frightened of using what has turned out to be a very simple
and easy-to-use port.
        Since much of the information in this article is based on 
experiment with only one DMA chip it is possible that some of it will
not apply to other revisions of the chip.  Perhaps someone at Atari will
correct any misconceptions or outright errors.  Also, the interface I
built only does DMA transfers into the ST (read operations); therefore,
while it is easy to infer the appropriate protocol, I haven't tested
DMA transfers in the opposite direction.
        The same DMA channel is used for both the floppy disks and the
Hard Disk Port.  The floppies are controlled by a floppy disk controller
chip which is connected to the DMA chip.  The DMA chip is capable of
servicing data requests at about 1.25 MHz.  If you attempt to transfer
data at a rate greater than that data will be lost.  I have been able
to transfer data at very close to this rate without any problems.  The
DMA chips buffers 16 bytes of data before requesting the memory bus
then holds the bus until all buffered data has been transfered.  Memory
cycles used for DMA are taken from the processor;  since the memory bus
makes 4 MByte/s available, 1 MByte/s of DMA should take 25% of the cycles
normally available to the processor, plus a fudge factor to account for
the time spent in arbitration.
        The Hard Disk Port is a 19-pin D connector - apparently
completely unavailable through normal commercial channels.  I made my
own from a metal-shelled male DB25 connector as follows:
        - remove and discard the metal shell revealing that the pins are
          held in place and insulated with two loose plastic retainers;
        - remove the pins from the retainers (for once you can lose a
	  couple since you will end up with six spares);
        - using a fretsaw or a fine hacksaw remove one end of both
	  retainers leaving 19 holes;
        - replace the pins and glue the retainers together.
You now have the connector you need.  It has no mounting ears but the
retaining force of 19 pins is quite substantial.  If this is not adequate
you could perhaps ingeniously salvage the metal shell somehow.
        The pins on the Hard Disk Port are connected as follows (pin one
is at the upper right of the connector facing the rear of the computer):
        1-8     DMA data bus bits 0-7 (bidirectional)
        9       Hard Disk Controller Chip Select (_HDCS) (output)
        10      Interrupt Request (_IRQ) (input, internal 1k pull-up)
        11      Ground
        12      Reset (bidirectional)
        13      Ground
        14      Acknowledge (_ACK) (output)
        15      Ground
        16      A1 (output)
        17      Ground
        18      Read/_Write (R/_W) (output)
        19      Data Request (_DRQ) (input, internal 1k pull-up)
        The Hard Disk Port is connected directly to the DMA chip WITHOUT
ANY BUFFERING on most pins - be careful,
a small mistake (e.g., static) could blow your DMA chip.  The system reset
line is also directly connected to the port meaning that accidental shorts
reset your computer - very irritating.
        The DMA chip is accessed through memory-mapped registers:
ff8604  R/W  16 bits    When bit 4 of the Mode register is zero a read or
                        write access to this word causes a read or write
                        cycle on the DMA bus.  If bit 3 is set the Hard
                        Disk Chip Select controls the cycle; otherwise,
                        the Floppy Disk Controller is selected.  R/_W is
                        set according to the type of the CPU access and
                        A1 is set according to bit 1 of the Mode register.
                        NOTA BENE: what is called A0 in the DMA chip is
                        called A1 on the Hard Disk Port.  For some reason, 
                        if bit 7 is not set no DMA bus cycle will take
                        When bit 4 of the Mode register is one the internal
                        sector count register is selected.  This register
                        counts down the number of 512-byte blocks that have
                        been transferred.  When the sector count register
                        reaches zero the chip will cease to transfer data.
ff8606  R    16 bits    Status word:
                        bit 0: 1 if error.
                        bit 1: 1 if sector count is zero.
                        bit 2: 1 if _DRQ is active.
                        If this word is polled during a DMA operation the
                        transfer will be disrupted.  End of transfer must
                        be signalled with the _IRQ signal.  The status is
                        cleared by toggling bit 8 in the Mode register.
ff8606  W    16 bits    Mode register:
                        bit 0:  not used.
                        bit 1:  A0; state of Hard Disk Port line A1 (sic)
                                during a DMA bus cycle.
                        bit 2:  A1; used for Floppy Controller only.
                        bit 3:  HDC/_FDC chip select; if this bit is 1
                                the _HDCS chip select will be generated;
                                otherwise the Floppy Controller select.
                        bit 4:  If one, select the the internal sector
                                count register for access at ff8604.
                        bit 5:  Reserved; set to zero.
                        bit 6:  Disable DMA when set; this is not used by
                        bit 7:  FDC/_HDC transfer select; if set DRQ from
                                the Floppy Controller is acknowledged;
                                otherwise, DRQ from the Hard Disk Port.
                                This bit must also be set to get a DMA bus
                                cycle for some reason.
                        bit 8:  Write/_Read; if set, data will be
				transferred out of memory; otherwise,
				into. Toggling this bit also clears the
				DMA chip status.
ff8609  R/W  8 bits     DMA Address Counter High byte.
ff860b  R/W  8 bits     DMA Address Counter Mid byte.
ff860d  R/W  8 bits     DMA Address Counter Low byte.
                        The DMA Address Counter must be loaded in Low, Mid,
                        High order.
        There are two eight-word FIFOs in the chip; one for buffering
read operations and one for writes.  The FIFOs are not flushed auto-
matically so you can only transfer data in multiples of 16 bytes.
Actually, use 512-byte units like the sector count register does.
        The DMA chip has no interrupt capability; This is ickky, but
cheap.  End-of-transfer  interrupts must  be  generated  by
the  external controller and are  masked and vectored by the 68901  MFP
chip on the General Purpose  I/O Port, bit 5, interrupt level  7. The
MFP chip interrupts  are controlled  with the XBIOS  calls 'mfpinit',
'jdisint',  and 'jenabint'.  'jenabint(7)' and  'jdisint(7)' will
enable and disable the DMA interrupt. 'mfpint(7,dmahandler)' will
initialize  the vector  to 'dmahandler',  wiping out  any earlier one.
If  you want  instead  to  simply  test  the state  of  the interrupt
request line,  without taking  any interrupts  you can test the MFP
GPIP data register. Read a byte from fffa01 and mask with $20; if the
result is zero there is no interrupt.
        Because the floppy controller shares the DMA chip you have to
be very careful about two things: 1) do not leave DMA interrupts enabled
accidentally or floppy operations will call your DMA interrupt handler;
2) turn off the floppy check routine while using DMA by jamming a 1 into
FLOCK=$43e to prevent this periodic operation from screwing up your
transfers by altering the DMA chip registers.
        These timing contraints are my interpretation of sketchy,
preliminary-looking Atari documents dated "27 September 1985".  If
someone at Atari can give better figures, please inform me.
        There are two separate types of transfer on the DMA bus: 
processor cycles, which are initiated by the processor, and DMA cycles,
which are initiated by the external controller.  The processor 
initiates a DMA transfer by waking up the controller with a few command
bytes then gives it control of the DMA bus.  The controller then uses
the DMA bus as long as it needs it and returns control by interrupting
the processor. 
        Processor cycles are controlled by the _HDCS, R/_W, and A1
lines in the usual fashion.  On a write cycle, the DMA chip gives you
60 ns of setup on A1, data, and R/_W before asserting _HDCS for about
250 ns, then holds for no more than 20 ns.  On a read cycle, you get
the same setup for A1 and R/_W, and you must give the DMA chip 100 ns
setup oe data before _HDCS is retracted.  Hold time should be less than
80 ns.
        Data cycles are initiated by the external controller by asserting
_DRQ.  _ACK will be asserted by the DMA chip no more than 240 ns later.
_DRQ may then be retracted.  _ACK is asserted for about 250 ns.  In a
read operation (data from controller to memory) data should be setup
60(?) ns before _ACK is retracted and held for 50(?) ns.  In a write
operation (data from memory to controller) data is valid 60 ns before
_ACK is asserted until 60 ns after _ACK is retracted. _DRQ for the next
cycle may not be asserted earlier than 240 ns after _ACK has been
        This interface transfers a byte into the the DMA chip whenever
a rising edge is seen on the data strobe input.  This interface cannot
coexist with a hard disk because it will respond to commands intended
for the hard disk.  Bear with me, we'll tackle that later.
        Rather than attempt to render several schematics in ASCII I will
describe the circuits and let you draw the schematics.  If the 
instructions don't make any sense you probably should not attempt to
make the interface.  It's all very simple if you know exactly what
you are doing; if you don't - you're dead.
        If all you want to do is read and write bytes it is very simple:
You simply use the Chip Select, R/_W, and A1 lines to select two write
registers and two read registers.  If you need more than that all you
have to do is use A1=0 to select an address register which selects the
register you get on a cycle with A1=1.  One thing that you have to
consider is that the ST writes to the Hard Disk Port several times
during its booting procedure, presumably to determine if there are any
hard disks connected.  Apparently, if no interrupt is received in
response to these writes the ROM assumes that no controller is connected.  
        In my interface there is a mode register which is accessed when
A1=0.  It has two control bits and two bits connected to a decoder which
determines which of three other registers are selected when A1=1.
        If an interrupt is needed (probably why you want to use this
port) you need a flipflop to store the interrupt state and some way
to reset it.  I used a 74LS74 for the flipflop with a diode on the output
to get the open-collector action.  This diode is not needed if there is
no other device on the Hard Disk Port. (But don't scrimp on a diode...)
I reset the interrupt with any access to the mode register but other
methods would work as well.  Make sure that the circuit will always
leave reset with the interrupt cleared.  I arranged this by having a
circuit enable bit in the Mode register, which is cleared by system
reset.  If you screw up on this your floppy will not work because the
same interrupt is used for both floppy and hard disk controllers.  I
learned a little about the file system the hard way while debugging
this interface - I recommend getting the interrupt reset system right
the first time.
        Once you can write to your mode register adding DMA transfers
(at least for read operations) is very simple.  All you need is 
another flipflop to do the Data Request synchronization and a 3-state
octal latch (74LS374) to hold the data.  Connect
the output to the DRQ line with a diode as with the interrupt flipflop.
When data is available clock it into the data register and clock a zero
into the flipflop.  This generates a data request for the DMA chip.
When it gets around to it the DMA chip will assert _ACK, which means it
is ready to accept the data.  Since you have the _ACK signal connected
to the three-state enable pin on the data register the data is driven
onto the bus.  You also use the _ACK signal to set the flipflop.
When the DMA chip no longer needs the data it will retract _ACK and
you can begin another transfer.  Spurious data requests are not as
nasty as spurious interrupts because the DMA chip does not listen to
hard disk data requests if it is listening to the floppy disk controller.
Naturally, if there is a real hard disk out there, there had better not
be any spurious data requests.
        I do not own a hard disk; therefore, the suggestions in this
section are totally untested.
        The Atari-defined protocol allows seven different controllers on
the DMA bus.  In order to make an interface which will not interfere with
other controllers you must understand the protocol.  Each command is
initiated with a processor write cycle which sends out a Command Byte
(A1=0) on the DMA bus.  The most-significant three bits contain the
controller number and the remaining bits contain an operation code. The
controller whose number matches the controller number in the Command Byte
responds to the operation code.  Some operations require parameters,
which are sent as Data Bytes (A1=1) following the Command Byte. The
controller generates an interrupt after coping with each byte. After
the last Data Byte is sent the controller takes over the bus for the DMA
transfer. When the transfer is finished the controller generates an
        In order to keep out of the way of other controllers on the bus it
should only be necessary to select your interface with a Command Byte with
the proper format.  After that use data bytes as you wish, since all other
controllers will not be selected.
        Using this information anyone with any experience in interfacing
can produce a working DMA interface.  Best of luck and drop me a line if
you find this useful.
                Doug Collinge
                School of Music, University of Victoria,
                PO Box 1700, Victoria, B.C., Canada,  V8W 2Y2  
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(C) Marko, Suomen Atari-sivut / ArkiSTo 2003