This is a simple converter to allow use of a PS/2 style keyboard with a Wyse 60 serial terminal.
Pro-Mini Pin Connection
------------ ----------
3 --> PS/2 Clock
5 --> PS/2 Data
9 --> Pro-Mini 10 (SPI SS)
12 --> Wyse Data (Pin 1)
13 --> Wyse Clock (Pin 3)
+5v --> Wyse +5v (Pin 2), PS/2 +5v
GND --> Wyse Gnd (Pin 4), PS/2 GND
The converter uses the SPI interface in slave mode to shift keyboard data out. The Wyse protocol is a simple bit array that is shifted into the terminal while the terminal sends the Clock signal.
To ensure syncronization, the SPI interface is reset if the Clock signal is low for a number of calls to loop().
See https://terminals-wiki.org/wiki/index.php/Wyse_WY-60
The PS2Advanced Library (https://github.com/techpaul/PS2KeyAdvanced) is needed for the PS2 side of things.