Constant transmitter (ENIAC)
The way into the fastest calculating machine on earth ran through a card reader doing about two cards a second.
ENIAC had three ways of receiving numbers, and the constant transmitter was the one you could change without a screwdriver. It consisted of banks of switches where an operator dialled values in by hand, plus an interface to an IBM card reader running at roughly 125 cards a minute. Between those two worlds sat relays acting as a buffer: the reader was mechanical and slow, the electronics behind it moved on a 200-microsecond beat, and something had to hold the digits still in between.
That mismatch is the whole point of the unit. The arithmetic could manage five thousand additions a second while its input arrived at the pace of an electric motor and sheets of card stock. A run needing many values from the deck was therefore limited not by ENIAC but by the IBM machinery at either end — the same bottleneck that half a century later acquired the name I/O-bound.