Woodstock HP-21 HP-25

Description: The machine utilizes a 12-digit display consisting of 8 LED anodes controlled by an anode driver, and 12 digits managed by a cathode driver, which also scans the keyboard's 7 columns. The bus is simplified to 4 lines: two clock phases, a sync line, and a new line named ISA, which multiplexes addresses and data on the system time word. Additionally, the power-on logic is integrated into the ACT, requiring only a few analog components (4 resistors—only 2 are shown in the HP sketch, 2 capacitors, and 1 inductor) along with 5 chips: ACT (PMOS), ROM1 (PMOS), RAM (PMOS), ROM0, and anode and cathode drivers (the latter being bipolar). There is no substantial assistance available, unlike US Patent 4,001,5692 for the HP-45, which included an object and source ROM listing keyed in by Eric Smith in 1995. Two patents related to the HP-25 exist: #3,987,290, granted to Peter D. Dickinson on October 19, 1976, for a "Calculator apparatus for displaying data in engineering notation," and #4,177,520, granted to Randall B. Neff for a "Calculator apparatus having a single-step key for displaying and executing program steps and displaying the result." Previous studies have proven invaluable; Tom Napier described the machine time word of the HP-67 in 1978 and provided commentary on part of the ROM, while Eric Smith developed "NonPareil," a ROM image emulator for the Classic and Woodstock models. The instruction set for the HP-25 and Woodstock is not entirely new, as it extends the HP-35's set. This instruction set has been completely deciphered, and an emulator for the HP-67 accurately runs the ROM image code, including operations of the card reader. However, significant differences arise due to rapid technological advancements. The integration level improvements have led to a single chip for the processor, allowing for more ROM states to accommodate more complex firmware. The demand for additional space necessitated a larger address space, thus resulting in a 12-bit architecture instead of the 10-bit used in the Classic series, along with a new instruction set designed to facilitate programmability. With the Woodstock series, HP introduced a computed goto feature, represented as the key of a programmable calculator, where a partial content of register A is used as the next microprogram address. In a memory address, a Woodstock disassembler cannot statically determine which ROM the code belongs to, as a "delayed ROM X" could be encountered earlier. Only through a test run can all information be gathered. The ACT serves as the core of the system, functioning as a complete microcalculator on a chip, equipped with a sequencer (similar to the C&T circuit) and an ALU. It requires two voltage levels: Vgg (-12V) and Vss (6V).

The electronic schematic of the described machine features a 12-digit LED display, with the anode driver managing 8 LED anodes and the cathode driver handling 12 digits. The anode driver is responsible for illuminating the appropriate segments of the display, while the cathode driver scans the digits and keyboard columns. The simplified bus architecture consists of four lines, facilitating efficient communication between various components. The clock lines provide timing signals, while the sync line ensures that data is synchronized across the system. The ISA line is pivotal for multiplexing addresses and data, enhancing the overall data handling capabilities of the system.

The power-on logic is integrated into the ACT chip, minimizing the need for external components. The inclusion of only a few resistors, capacitors, and an inductor indicates a streamlined design focused on efficiency and reliability. The five essential chips—ACT, ROM1, RAM, ROM0, and the anode and cathode drivers—are strategically chosen for their PMOS and bipolar characteristics, ensuring optimal performance.

The historical context provided by the patents and previous studies highlights the evolution of the technology, emphasizing the incremental advancements that have led to the current design. The transition from the Classic series to the Woodstock series reflects the ongoing development in microprocessor technology, with the introduction of a 12-bit architecture enabling more complex firmware and enhanced programmability.

The ACT chip, as the central processing unit of the machine, integrates a sequencer and ALU, allowing for sophisticated calculations and operations. The requirement for dual voltage levels (Vgg and Vss) indicates the need for careful power management within the circuit. Overall, the schematic presents a well-thought-out design that balances functionality, efficiency, and technological advancement, reflecting the legacy of innovation in calculator development.The machine used a 12 digit display (8 LED anodes scanned by an Anode driver, and 12 `digits` scanned by a Cathode driver, the latter being used also to scan the keyboard`s 7 columns. The bus is also simplified : 4 lines : the clock`s 2 phases, the Sync line and a new line names Isa multiplexing addresses and data on the system time word.

The Power on logic is also included in the ACT and only a few analog elements (4 resistors - only 2 are shown on the HP sketch, 2 capacitors, 1 inductor) are needed along with the 5 chips : ACT (PMOS), ROM1 (PMOS), RAM (PMOS) ROM0 and Anode driver (PMOS) and Cathode driver (bipolar). Note that there is -this time- no big help like the US Patent 4, 001, 5692, for the HP-45 (with an object and source ROM listing, keyed in by Eric Smith in 1995).

There are 2 patents related to the HP-25, one is #3, 987, 290, 0ct 19, 1976, granted to Peter D. Dickinson for a "Calculator apparatus for displaying data in engineering notation", the other is #4, 177, 520 for a `Calculator apparatus having a single-step key for displaying and executing program steps and displaying the result`, granted to Randall B. Neff. Two previous study were priceless. Tom NAPIER described back in 1978 the machine time word of the HP-67 (the Woodstock Masterpiece) and commented part of the ROM, and Eric SMITH wrote `NonPareil` a ROM image emulator for the Classic and Woodstock.

The HP 25 and Woodstock instruction set is not completely new. In fact, it is an extension of the HP-35`s set. It is now completly deciphered and I wrote emulator of the HP-67 is running faithfully the ROM image code (with the operations of the card reader). But sometimes big differences appear driven by the fast pace of technology evolution. As a result of the integration level progress, we have here one chip for the processor and much more ROM states to store a much complex firmware.

The need for more room implied a larger address `space`. That`s why we have here 12bits instead of 10 in the Classic series and a new instruction set (an extension to be exact) to make `programmability` easier. But with the Woodstocks, HP introduced a real computed goto  presented as the key of a programmable calculator (partial content of register A is taken as the next microprogram address).

At a memory address, a Woodstock disassembler does not know statically to which rom the code belongs since a "delayed rom X" could be encountered earlier. Only a `test run` can gather all the information. The ACT is the heart of the system. It is a complete micro calculator on a chip, with a sequencer (as in the C&T circuit) and an ALU. It needs 2 voltages : Vgg (-12V) and Vss (6V).