slot unusable. The card was capable of a 12‑ and 24‑hour clock format, was both ProDOS and DOS 3.3 compatible, and had on-screen time and date setting built into its ROM, eliminating the need to run a program in order to set the time. The battery was a GE DataSentry rechargeable Ni-cad battery which had a lifespan rating of 20 years. The card retailed for $139 during the late 1980s. For more on the Serial Pro's communication capabilities, see its entry in Apple II serial cards. Thunderclock Plus (Thunderware Incorporated) When the Thunderware Thunderclock Plus was released in 1980, 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. From that point on, all new Apple II system clocks strived to emulate the Thunderclock. The card itself was more compact than the earlier "The Clock" from Mountain Computers and contained two battery holders for off the shelf alkaline batteries which were easily replaceable. Time Master H.O. (Applied Engineering) The Time Master H.O. clock card from Applied Engineering was possibly the most advanced system clock ever designed for any Apple II. The card utilized an onboard VIA 6522 and was capable of emulating all other system clocks which preceded it. The Timemaster H.O. was powered by a GE Datasentry rechargeable Ni-cad battery which had a lifespan rating of 20 years. It was capable of 24‑hour format or 12‑hour with AM/PM format, millisecond timekeeping with an accuracy of 0.00005%, and an onboard timer which could time down any interval up to 48 days. It also maintained an internal calendar, separate of the 7‑year cycle which ProDOS mapped. The Timemaster H.O. was 100% ProDOS and DOS 3.3 compatible. The "H.O." in Timemaster H.O. stood for "High Output". This referred to the 8-pin Digital I/O port on the card for advanced applications. Through this port, one could hook up Applied Engineering's BSR X-10 interface and "command console" to remotely control lights and electrical appliances. The BSR system could send signals over existing 120‑volt wiring, eliminating the need for additional wires. The system could also be used for low‑voltage implementations. The Timemaster H.O. retailed for $99 during the late 1980s while the BSR option cost an additional $29. The command console cost $39. Other system clocks AppleClock (Mountain Computer) California Computer Systems Clock (California Computer