Tracing the PCB is relatively easy, it's a single-sided non-smd unit. Quite compact, very fine tracks in places, but very easy to trace too.

According to the datasheet the M3802 CPU has two serial interfaces built in:

1. Serial 1

   Pin 20 - RDY
   Pin 21 - CLK
   Pin 22 - TX
   Pin 23 - RX

    This serial can support a UART (no cts/rts needed)
2. Serial 2

   Pin 16 - RDY
   Pin 17 - CLK
   Pin 18 - OUT 
   Pin 19 - IN

    This serial needs to use cts/rts

All pins refer to the 64 pin DIP package as used in the AX-100

Tracing through on the B48J283-2 PCB I see:

Serial 1

Pins 20 through 23 go into the 16-wire parallel bus which in turn connects to the keyboard; like all keyboard bus pins they have a resistor array to pull them to VCC. There does not appear to be anything 'special' about them at all. I'd need to do a more detailed trace to determine exactly which kbd lines they go to,

Serial 2

Pins 16 and 17 (rdy/sck) go through R27 and R28 (value not yet known) and then connect together and go to the cathode of the 'power' LED on the board (the anode goes to VCC)
- Look like these pins are used to control the LED, presumably they use two pins to provide sufficient current.

Pins 18 and 19 (out/in) are pulled to VCC by R29 and R30 (no values yet) , each has an (open) solder jumper to 0V and empty positions for a jumper wire (JW5 and JW6) to 0V too.

Solder Jumpers

There are 10 solder jumpers on the copper side of the board, 8 are under the CPU, two are on the Serial2 in/out lines near the JW5/JW6 points on the board. All go (eventually) via a resistor array to VCC and are pulled to ground by the jumper.

On my board the 6th jumper on the row under the CPU is shorted (counting from the 'top' of the cpu) . All others are open, there are no markings for them on the board.

The repair manual lists these jumpers along with an optional extra switch connector (CN3, keyboardswitch) and two resistors (R10 and R11) in a matrix for setting the language for the board. I do not have CN3 populated, but R10 and R11 are present. This conflicts with the matrix since in that R10,R11 and CN3 are always present or absent as a set.

However, having the 6'th jumper on the row of 8 closed corresponds to the GBR.ENGLISH line in the matrix, which is the correct language for the typewriter.

I believe CN3 is a switch on some international keyboards to switch between between two languages/character sets.

I've uploaded the relevant bit of the manual to the project files