ThunderClock Plus Installation & Operating Manual

The ThunderClock Plus, manufactured by Thunderware Incorporated of Orinda, California, was the best-selling real-time clock card for the Apple II through the early 1980s. Its distinguishing feature over every competing clock card was automatic ProDOS recognition: the kernel scans slots 7–1 at boot, reads four bytes from each ROM, and if it finds the ThunderClock’s signature bytes ($08, $28, $58, $70), installs the firmware’s built-in clock driver without requiring a separate CLOCK file on the boot volume.

The card uses a NEC uPD1990AC serial calendar clock chip, a 40-bit shift-register device that maintains month, day-of-week, date, hour, minute, and second as ten 4-bit BCD nibbles. The 6502 communicates with it through a 6-bit control register, shifting time data in and out serially. Two alkaline N-cells keep the clock running when the Apple is powered off, typically lasting three to four years.

What This Manual Covers

This is the complete factory user’s guide, covering installation through advanced interrupt programming. Key sections include:

• Hardware installation — slot selection (works in any slot 1–7), the SET/PROTECT switch, battery handling and replacement procedure
• BASIC programming — read mode control characters (%, >, &, <, , #) for all time formats; write mode for setting the time; both Applesoft and Integer BASIC examples throughout
• Assembly language interface — the two firmware entry points ($CN08 READ, $CN0B WRITE), reading the time without firmware via the RDTIME subroutine, the Mountain Computer Apple Clock compatible format
• Interrupt system — enabling and disabling hardware interrupts, three selectable rates (64 Hz, 256 Hz, 2048 Hz), reading time from an interrupt-level routine without clobbering the C8 shared I/O space, the TCPUTIL utility library
• BSR/X-10 interface — the optional ultrasonic transducer for home automation control; command characters, duration codes, and the SCHEDULER background program
• Appendices — TCPUTIL full assembly listing, RDTIME subroutine listing, battery testing and replacement, calibration procedure, complete PC card diagram

Relationship to Site Articles

The A2FPGA series article on this site documents emulating the ThunderClock Plus on an FPGA:

• The Clock Card That Just Works — the uPD1990AC serial protocol, ProDOS auto-detection, and how USB-C power rails substitute for the on-card batteries

The manual’s Appendix E (RDTIME subroutine listing) and Appendix F (PC card diagram) were the primary references for understanding the serial shift-register protocol implemented in SystemVerilog. The interrupt chapter explains the C8-space ownership rules that also appear in the Videx VideoTerm implementation — reading the clock from an interrupt routine requires bypassing the firmware to avoid evicting a foreground peripheral from shared I/O space.