Woodstock HP-21 HP-25

Posted on Feb 7, 2014

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.

Woodstock HP-21 HP-25
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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,...

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