[Coco] Modern RS-232 solutions for the CoCo?

[Allen Huffman]

> On Jun 2, 2019, at 10:28 PM, RETRO Innovations <go4retro at go4retro.com> wrote:
> 
> Most of the home computer brands knew about this, but I learned about it from the CBM camp.
> 
> the 6551 normally offers bps rates up to 19200, but can be clocked at a faster RS232 base frequency (I think the normal crystal is a 1.8432Mhz one), which the unit divides down to generate the bops rates.  So, lots of 6551 devices used a double clocked crystal to get 38400 bps.

I first learned about that (or something similar) from Roger Taylor. Wish I had known that ‘back in the day!"

>   Then, folks realized that there is a "special" bps rate that will allow F/16 bps rate.  For a 3.6864MHz xtal, that's 230.4Kbps.  In the CBM space, Creative Micro Designs took the idea close to it's conclusion, pairing a 6551 with an oscillator and a small PAL IC.  Thus, by adding a few more registers, one could enjoy all of the normal bps rates, 115200bps, and 230400 bps.

That’s cool. It’s an updated 6551 variant? Still being produced???

> I added the same functionality, but found a way to re-use some of the existing 6551 register bits to enable the higher bps rates, so the unit can still live within the same IO region as the existing RS232 and modem paks.
> 
> CMD also rewired DSR to the DCD input pin on the 6551, as I recall, to get around the issue that the 6551 will not transmit when DSR is low.  I think they also fixed an issue with CTS as well.

That, alone, would make me want one!

		— Allen