Here are the lines I need to define (23 in all!):
Note that the popular Arduino Uno and Arduino Nano boards only have 20 IO pins...
I2C_Clk and I2C_Dat - obviously, this would be for the LCD (4 rows / 20 cols)
Use the Arduino "Wire" library (and the hardware I2C pins) Or perhaps just an existing LCD library.
Sig_PIA is a serial data input that is sampled via an interrupt
PAI is the corresponding serial data output (manipulated by the same interrupt process)
PAE (also controlled within the interrupt process) controls a relay that puts the unit in transmit or receive mode.
Is this UART-compatible serial data, or something custom? The Arduino chips all have a lot more peripherals built-in than a 6801, so it's hard to tell which features of your old board should be moved to hardware, and which you want to reproduce the old software-driven behavior.
Init_En is an output to control one of the features (to drive a small signal relay)KB_R0...R3 and KB_C0...C3 are for the 4x4 matrix keyboard (8 lines total)RlyA... RlyD are four lines that are read as a nybble to determine the status of a set of relay outputsSW0 / SW1 are a pair of lines to determine the test switch setting (yielding a value between 0 and 3)
Likewise, Jmp0 / JMP1 are a pair of lines that check a pair of configuration jumpers (again, yielding a value between 0 and 3)
pinMode(x, INPUT) or pinMode(x, OUTPUT)
There are also existing keypad libraries.
The Arduino API only defines functions for reading/writing single bits; to replace code that reads multiple bits, you'll need to either convert the logic, assemble the multiple-bit fields from single bits, or step out of the Arduino APIs.