Cerial-X2

 I now show three styles of a RS-232 PCB.
Each one has TTL decoding and one type of UART.
First one has a single 16550 (not shown)
Second one has a choice of 16550 or 6551A but not both.
Third one eliminates the patch area and includes a MAX232 type IC that would need to be hand-wired in.

Hand-wired PCB, below. 2020/09/26
2020/09/29: Both UARTs now functioning on the hand-wired prototype PCB.
2020/10/22: A dual PCB, where both can function.
2020/11/16: Dual 1550's with full address selection.

Second One.
You choose which UART to install.
Both are wired the same for the FTDI-232-5V connector.
The double row of jumpers sets the address for each UART.

Third One.

2020/09/26
Hand-wired decoding works. Bicycle spokes give support.
Decoding TTL IC's are installed, but not in this pix.

Top View, murdered Teensy PCB, added to a holey-pcb.
Allows either 16550 or 6551A but not both.

And not finished as yet, needs the FTDI connector, xtals, etc.

Here it is with all parts stuffed (16550 and it's 14.3mhz xtal).
That xtal gives 406800 baud.
List of jumper settings on the edge of the PCB.

Jumper is set to 16550 at $FF60-7
TTL end of that FTDI-232R-5V USB2 cable is showing, the ftdi chip is inside of the usb end.

This one has both UART's installed and are addressed as follows:
16550: $FF30-7 or $FF38-F (CoCo3 only)
6551A: $FF60-3, etc.
    Where the default is $FF68-B for the non-modem version.

Two each 16550's.
With a full address selection setup.
One (header-3) jumper selects high or low range,
while the other jumper set (header-2x8) selects which address to use.

IdeZilla

Comments Welcomed