[Coco] CoCoNet status

[Roger Taylor]

At 05:47 PM 12/8/2008, you wrote:
>On 12/8/08, <Torsten at dittel.info> wrote:
> >
> > I darkly remember, when my father once tried to run the bit banger MIDI
> > OUT of LYRA on his CoCo1 (first board, Serial#338), it didn't work
> > (unlike my CoCo 2B). He then figured out with a scope, that single bits
> > at the bit banger port (e.g. the "1" in "00001000") were "triangles"
> > with lower amplitude (than the expected "rectangle" should have),
> > because the slopes weren't rapid enough. He exchanged a little chip (8
> > pin DIP?) by a compatible faster one (port driver? Would have to look
> > that up in the schematics) and it worked! (Well, there is a mistake in
> > the LYRA MIDI driver which led to some errors, but that's another story).
> >
>
>That sounds very much like the problem I have been seeing. I don't
>have a scope, but after looking at the CoCo 1 schematic and seeing
>that it uses a 741C opamp (the little 8 pin chip) for the RS-232 level
>translation, seeing "triangles" sure would make sense.  The 741C opamp
>has a slew rate of 0.5 volts per microsecond, so a transition from a
>'1' to a '0' (or vise-vera) should appear as a slope over a period of
>several microseconds. This slew rate is not fast enough to support
>56kbps through the bit banger output. The input circuit is capable of
>operating at 56kbps.
>
>Darren

If this is the case, I see no software solution and I'll have to 
limit CoCoNet to CoCo 2's and 3's unless I decide to put in a 38400 
bps mode which seems to me would have slight problems with those 
triangular signals.  Btw, I think I know of a software solution for 
doing speeds much faster than 115.2 kbps but that project is for another day.

-- 
Roger Taylor

http://www.wordofthedayonline.com