Subject: Re: My problems with merged memory
To: None <>
From: Andreas Heitmann <>
List: amiga-dev
Date: 03/08/1994 09:48:57
>>>>> "Michael" == Michael L Hitch <> writes:

    Michael> On Mar 7, 11:37am, Gregory Kritsch wrote:
    >> > >>>>> "Niklas" == Niklas Hallqvist <>
    >> writes: > > Niklas> I have tested the board both with and
    >> without the DTACK jumper > > I tried to decode it last night,
    >> and have now an idea of what it is > used for.  DaTa
    >> ACKnowledge is driven low as long as the requested data > is
    >> invalid, or if it were the opposite, in order to make slow
    >> memory > (or slow memory-mapped registers) safely deliver
    >> values to the CPU.  > Is this correct?  I don't think the 16bit
    >> mem is to blame in this case

    Michael>   A properly functioning Zorro II board shouldn't drive
    Michael> the DTACK* line, but it might use it for timing.

/DTACK is a M68K signal which is driven low by external asynchronous
memory after a write access to acknowledge that data has been been
taken over. The Problem in the Amiga architecture is that /DTACK is
generated by some internal circuits immediately after /AS occurred, so
that the memory has only one cycle left with stable data lines.

Slower memory has to set XRDY (extern generation of /DTACK) if it is
selected to override the internal generation of /DTACK. With XRDY
active, the processor will do wait states as long as /DTACK is high.

With these precautions, /DTACK can be used for Zorro II boards.

    Michael>   I've got some information from a post on
    Michael> comp.sys.amiga.hardware about the HardCard+8 board and
    Michael> problems on older motherboards.  The article indicated
    Michael> that J15 on the HC+8 board should be jumpered on
    Michael> motherboards older that 4.5.  My board is something like
    Michael> 4.2, and I couldn't get the memory on the HC+8 board to
    Michael> work at all until I put a jumper on J15.  The article I
    Michael> have indicated that the jumper provided a pull-up
    Michael> resistor on the DTACK* line.

The pullup resistor fixes a problem with earlier Amigas. When
connecting Zorro II boards, it can happen that the built-in pullup
resistor is too high, so that /DTACK gets active all the time. As far
as I remember the problem occured only in the A1000.

Hope it helps,

   Andreas Heitmann  (