Subject: Re: My problems with merged memory
To: None <firstname.lastname@example.org>
From: Andreas Heitmann <email@example.com>
Date: 03/08/1994 09:48:57
>>>>> "Michael" == Michael L Hitch <firstname.lastname@example.org> writes:
Michael> On Mar 7, 11:37am, Gregory Kritsch wrote:
>> > >>>>> "Niklas" == Niklas Hallqvist <email@example.com>
>> 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 (firstname.lastname@example.org)