[Coco] CoCoNet status

[Darren A]

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