Recently, I have been putting my full focus of the project on designing and refining the SBC V2.1. I have some great new features on it while keeping the PCB just about the same size. The largest of the additions is likely the new expansion bus, which has been designed to be very similar to RC2014 (albeit with some slight variations to make the design easier to manage) and allows for nearly endless amounts of freedom to expand the board. It has been implemented with a 40-pin header just like on V2.0. Next is the addition of a TTL serial port that is connected to channel B of the Z80 DART. The port is using an FTDI cable pinout, so cables are easy to find. With a little work, the port could be made to communicate with a device such as an Arduino or Raspberry Pi to add something like a USB keyboard. 

Along with these major changes, I have added some small fixes and edits. One of these changes is leaving no inputs floating, so the board should be reliable for CMOS chips. I've always used a CMOS Z80 and peripherals without problems, but now it's guaranteed to work. I've also added two pins to the PIO headers. One pin (pin 10) is a ground pin so that it's easier to ground your connections, and the other (pin 9, only on port B) is a common cathode pin for the flyback diodes on the ULN2803A. No more fried transistors from your motors! Pin 9 is left unconnected on port A as it doesn't have a transistor array.

While working on this design, I had the idea to make a smaller version of the board that has the PIO, CTC, SIO + serial ports, and power jack removed, allowing it to be very compact and easily usable in a backplane-based environment. It would also be cost effective, as it would be under 100x100mm. Of course, the downside to this design is that it would have to be used along with a backplane and at least a serial board. Do you think I should create it? My current board is too large to be reliably used on a backplane, so I'll probably use a ribbon cable to connect it temporarily.

I should be placing an order for the new boards within the next few weeks.