[Coco] Prototype cartridge board?

[Don Barber]

Hi folks,

I recently soldered together my own Altair 8800 
(https://barberd.medium.com if you're interested). Part of that involved 
building a Serial IO board 
(http://s100computers.com/My%20System%20Pages/Serial%20IO%20Board/Serial%20IO%20Board.htm) 
that includes a DLP-USB245R (https://www.dlpdesign.com/usb245r-ds-v10.pdf).

Using this USB module on the Altair is working so well for me that now 
I'm considering building a CoCo cartridge with one. The USB protocol 
takes care of all the speed/parity/stop settings so no need to worry 
about that anymore, and the module itself can buffer quite a bit of 
data, freeing up the CPU from needing to constantly poll the serial port 
to not lose data. And one doesn't need to deal with usb-rs232 adapters 
for the modern PC side of things; it just a straight cable and shows up 
as a new COM device in Windows or ttyUSBX device in Linux.

I think I can make it mostly-compatible with the Deluxe RS232 Pak with 
not much effort. I say mostly-compatible as the module does not have an 
equivalent to the local echo configuration available on the RS232 Pak's 
6551 chip. One could recreate that behavior with a bunch more logic 
chips and a register chip but it doesn't seem worth it.

As such, i'm wondering if there is any source for a prototyping board 
for the coco 40 pin cartridge port? What do you hardware hackers use 
when playing with a design for a new cartridge-based peripheral for the 
coco?

I was thinking something like this: 
http://s100computers.com/My%20System%20Pages/Prototype%20Board/Prototype%20Board.htm 
(scroll all the way to the bottom for the latest design) of course with 
the right 40-pin connector instead of the S-100 connector.

If it turns out such is not available, and I'm not informed of a better 
way to go about things: were I to design such a prototyping board and 
order a run, would others be interested in getting in on the order?

Don

PS: Listened to the recent CocoTalk segment talking about my RSA 
program. I suppose I should be proud that it ended with an 'ASPARAGUS!!!' ?