Siemens Step5 (for maintaining an installed base of S5 PLCs - the most indestructible piece of computing hardware ever designed, IMHO.)

Also, some work in MPASM (Assembly for the Microchip PIC series of microcontrollers - same reason)

Edit: I am not sure just when S5 got introduced, but I've got installation media - on 5.25" floppies - for CP/M in my office. Made in West Germany.

That old.

Luckily, the IDE has also been ported to more contemporary operating systems.