[Coco] 6820 and 6821 pin compatible?

[jdaggett at gate.net]

On 5 Nov 2009 at 20:53, Mike Pepe wrote:

> Should work just fine. As I recall the major difference between the two was the 6820 had dynamic registers while the 6821 was static. But I'm sure if I'm wrong someone will chime in.
> 


Mike

You maybe right, though I seem to remember something about the port sink and source 
currents were not very strong and the part blew port pins. Though I could be wrong as this 
recollection dates back now almost 30 yrs. I think the MC6820 was discontinued in very early 
80's. 

I do know that the MC6822 differed in that the ports were open drain and would work with 
voltages upwards to 15VDC. 

james



> > -----Original Message-----
> > From: coco-bounces at maltedmedia.com [mailto:coco-
> > bounces at maltedmedia.com] On Behalf Of Joel Ewy
> > Sent: Thursday, November 05, 2009 7:34 PM
> > To: CoCoList for Color Computer Enthusiasts
> > Subject: [Coco] 6820 and 6821 pin compatible?
> > 
> > I'm restoring an SWTPC 6809 computer.  I intend to run OS-9 on it.  I
> > also want to try out FLEX and UNIFLEX (the latter of which I'm sure
> > this
> > computer once ran).
> > 
> > I've got a timer board, that I believe (judging from the OS-9
> > configuration of the SWTPC emulator) provides the system clock
> > interrupts.  It has a socket that, according to the documentation, once
> > housed a 6820, but now sits empty.  I can probably dig up the
> > documentation, but I bet somebody on this list can tell me without
> > cracking a book or downloading a PDF whether or not I can just plop a
> > much more readily available 6821 in there instead.
> > 
> > If you're interested in following my progress on this project, I've
> > been
> > blogging about it:  http://8littlebits.wordpress.com/category/swtpc/
> > 
> > The CoCo gets mentioned as well, and I used a CoCo emulator to make a
> > boot disk image I'll eventually use when I'm ready to try and boot this
> > thing up.
> > 
> > JCE
> > 
> > 
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> 
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