An expansion method was an important feature of the design and provisions were put in place to allow data a parallel data path in and out of the system. The current design is aligned to support the RCA 1802 (COSMAC) with up to eight input and output registers, four external flag inputs (EF), and a single flip-flop output (Q).
Two 16-pin headers are installed on the main PCB to expose the two data busses and a minimal set of control signals. From here only two 3-to-8 decoders and a 4-bit buffer is needed to complete the expansion interface. These components are not included on the main PCB since the expansion is optional and not needed for normal operation.
A simple expander card has been designed in order to test this interface and will be manufactured alongside Rev. 2 main board. The expander is typically 10cm x 10cm in size (the threshold for the lowest price tier for most PCBs) and mounts over the main PCB like a shield.
The design includes a socket for a single 8-bit expansion register. Two sets of headers with jumpers allow this register to appear as any one of the eight possible input and/or output registers. An additional set of four flip-flops is included in the 7th register position and can be used to scan a 4x4 keypad matrix with the return 4-bits going to the expansion flags (EF).
The keypad is just a temporary measure for testing and will not be needed once the PS/2 keyboard serial code is built and working. That could be up to 6 months away though.