IBM Multistation5550 keyboard protocol is very similar to IBM PC/XT keyboard , and easy to analyze.
This keyboard has fatal defect to use with Windows or Mac OS.
This keyboard only suitable for dumb terminal.
This keynboard doesn't send SHIFT key signal.
This keynboard doesn't send key release signal. (break signal of make/break)
The protocol converter I created was almost unusable gadget and it must be distcontinued....
Fujitsu OASYS/FMR keyboard protocol is 5V TTL UART High-Low invert.
It's very easy to create protocol converter if you have a skill of Arduino or Microchip PIC controller.