Got some revised plugs. The Expander Plug now works:
Here’s a sketch which blinks all pins on the expander:
I forgot to check the A0 solder jumper orientation, so I accidentally wired my first plug up for address 0×21 instead of 0×20. Each Expander Plug can be configured for I2C addresses 0×20..0×23, i.e. up to 4 expander plugs can be daisy-chained on each port.
Expander plugs work nicely with the Breadboard Connector, to hook up a mini-breadboard like this:
The width of a mini-breadboard is slightly less than an Expander Plug + sideways headers, so daisy-chained plugs can each have their own breadboard.
In summary: the Blink, Expander, Memory, RTC, and UART Plugs all work. So do the unpopulated Proto Board and Breadboard Connector, of course.
More news about prices and availability in the shop tomorrow… once I finish my homework and figure out a few remaining pesky little details and logistics issues.