Re: Elevator interface
Differential drivers and receivers such as RS-422 and RS-485 have greater noise immunity than single-ended interfaces such as RS-232. But they won't do diddly for you if there are severe ground loops, or (heaven forbid) a lightning strike. In the '80s I was an engineer at a supplier of wagering systems to horse and dog tracks. The RS-422 line driver and receiver chips were specced at 25 volts common-mode voltage, max. The computer room was typically in the basement of the building. If lightning struck (say) an upper floor of the grandstand, it would let the magic smoke out of serial driver and receiver chips in the terminals and communication processors. The first time it happened, the chip soldered-in at the factory would be replaced in the field by a socket to ease future repairs.
An even more hostile electrical environment was the communication between the infield display board and the computer room. Typically the infield had an artificial lake, so the ground conductivity was very high -- a lightning magnet. A lightning strike to the infield, or to the grandstand, could cause different "ground" voltages between one end of a cable and the other.
Each succeeding generation of display hardware used lower voltage / higher impedance logic than previous ones, so was less and less reliable in hostile weather. 48-volt telephone relays could stand a fair bit of abuse, but the transistors in the computer room that drove them would fail first. Shift registers made of SCRs with 18-volt logic, not as good. TTL logic with RS-422-style line drivers and receivers would fail if there was a storm cloud anywhere in sight. When the Z80 came along and we wanted to respin the display systems, the naysayers in the company said that there was no way we would get a computer to survive in the infield. But fiber optics had become an option. Even though it was pricey, you didn't need many cores. Our stuff Just.Kept.Working in storms that would have taken the older hardware down for days. Fiber saved our bacon; it won't propagate a lightning strike.