Re: I/O Expander

From: Andre Fachat (
Date: 1998-01-07 12:35:57

MagerValp wrote:
> >>>>> "Andre" == Andre Fachat <> writes:
> Andre> The 6551 (as well as the 6525, 6526, 6521) doesn't seem to
> Andre> suffer from this as it evaluates the address and select lines
> Andre> when Phi2 is high - the 6522 seems to sample them with the
> Andre> rising edge.
> Ah. Good to know. I'm about to stick a 6551 in my C64.

At least I think that is the case with the 6551, as my C64 used
to have a 6551 at the SID socket (with an extra address line...)
and it worked - besides the horrible rts/cts handshaking which 
is not possible with the 6551. 
Did you have a look at the Sockmaster ACIA6551 fix that 
builds a hardware flow control? A link is on my site.

It is an RS flip-flop that is set when a low-bit arrives (start bit) and
that is reset when register 0 is being read. The output is taken as
RTS. You only have to assure reseting the flipflop after an error
I think (didn't try).

I am now using a 16550A in my C64, in place of the ACIA6551. And here
I really get reliable 9600 baud (probably more when I reduce the 
interrupt latency in my OS... :-( Without the hardware handshake
(which I didn't know about then) I lost too many chars at 1200 baud
already with the 6551 with my selfbuilt OS.


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Andre Fachat, Institute of physics, Technische Universität Chemnitz, FRG

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