Power users often had their Apple II's peripheral slots completely filled with expansion cards, so third party vendors came up with alternative approaches with products like the Serial Pro and No-Slot Clock.
Dallas Semiconductor produced the device as an easy implementation for a real-time clock for a variety of applications.
In an Apple II, the No-Slot Clock resided under any 28-pin ROM chip, including one on a peripheral card.
By combining the functions of two cards into one, the Serial Pro freed up an extra slot for those with highly populated machines.
When the Thunderware Thunderclock Plus was released in 1980,[4] it quickly became the de facto standard for an Apple II system clock.
When Apple Computer released its new ProDOS operating system in 1984, a Thunderclock software driver came built-in.
was powered by a GE Datasentry rechargeable Ni-cad battery which had a lifespan rating of 20 years.
Through this port, one could hook up Applied Engineering's BSR X-10 interface and "command console" to remotely control lights and electrical appliances.