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

[RETRO Innovations]

On 6/2/2019 9:59 PM, Allen Huffman wrote:
>> On Jun 2, 2019, at 9:57 PM, RETRO Innovations <go4retro at go4retro.com> wrote:
>>
>> Still working out portions of the design.
>>
>> I do have a single port 6551 RS232 Pak unit of which I've sent a few testers out.  Ladd and Michael Brant have them.  They have the 230K mod installed and as much of the Sockmaster mods I could figure out how to implement thus far.
> What is the 230K mod? Being away is exciting. So much new stuff!

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.  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.

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.

Jim


> I’d even be happy with one that was just TTL signals, if you had the option of not populating the rest of the board with the level converters and DB9 port.

I did one of the PCBs and wired it to a esp12e to test Wifi.  I can 
probably make another one.

Jim