Interior of HP 9825A with Top Cover and Keyboard Assembly Removed

The Back Side of the Keyboard Assembly

The 9825 also provides a built-in 16 column thermal printer, a magnetic cartridge tape drive, a built-in 32 column LED dot matrix alphanumeric display, and a full "QWERTY" layout keyboard . All of these characteristics combine to make a machine that really blurs the line between a calculator and a computer. Remember, though, that this was all done at a time when microcomputers were just becoming available. While the 9825 was generally more capable than many of the early microcomputer systems such as the MITS Altair, Processor Technology SOL, Apple II, and Commodore PET, it was considerably more expensive, at a base price of nearly $6000, putting it out of reach of most early computer hobbyists. However, compared to the cost of a minicomputer system, a 9825 system was significantly less expensive, and really only a little less capable.

The 16-Bit Hybrid CPU Module

Even though early single-chip microprocessors like the Intel 4004 and 8008 were available in 1976 when the 9825A was introduced, Hewlett Packard kept on with using their own technology to provide calculating equipment that was pretty much "Best of Show" when it came down to performance, reliability, and capability. The 9825 uses a multi-chip hybrid module as the core of its computing capability. This hybrid module consists of three chips combined on a ceramic substrate along with a number of support chips. The whole assembly is packaged together with an integrated heat sink. This combination of chips works together to form a full 16-bit minicomputer-class CPU, with an instruction set similar to that of HP's 211x-series minicomputers. HP also continued its tradition of innovation by building the 9825 in a modular fashion, for ease of service, and also to make the machine easily field-expandable. Most of the main circuit boards in the machine are interconnected by a ribbon cable 'backplane'. The boards are mounted on a clever 'hinge' arrangement, that, by virtue of the flexible ribbon cable backplane, allows the boards to be swung up and away from each other while still allowing the machine to operate. This made service and troubleshooting much easier. The main ROM code that makes up the 9825's operating system is located in a plug-in module, allowing easy field-updates should they be necessary.

The 32-Character 5X7 Dot Matrix Display

As a calculator, the 9825 operates in a similar manner to its predecessor, the 9820. The machine operates in full algebraic mode, meaning that problems are typed into the machine as pretty much as they would be written on paper. The keyboard includes a numeric keypad area with dedicated keys for the four basic math functions, along with parenthesis keys. The calculator follows the standard mathematical order of precedence, with division and multiplication performed before addition and subtraction, with parentheses used to override the precedence rules. Parentheses can be nested to nearly any level, though any expression must fall within the 80-character line length limit that the machine imposes. The 9825 has a comprehensive set of mathematical functions, most of which are accessed by including a keyword representing the function with the argument to the function following the keyword, surrounded by parentheses; For example, to calculate the cosine of 45 degrees, the user would type in SIN(45) and press the "EXECUTE" key to have the result calculated and displayed. The printer can be used to keep a printed log of calculations when enabled via a keyboard press-on/press-off key. The machine calculates results to between 13 and 14 digits of accuracy. The reason that the accuracy varies is that the 9825 represents numbers internally as binary floating-point numbers. This representation of numbers is much more like the way that computers handle numbers than calculators. Calculators generally represent numbers in BCD(Binary Coded Decimal), which is a way of coercing binary numbers to behave more like decimal numbers. BCD has limitations, though. BCD representation is more suited to the 'hard-wired' electronics of early calculators, being easier to translate from internal form to a displayed human-readable number, and also requires less complex logic to manipulate the numbers internally. Being as the core of the 9825 is a true general purpose computer, the floating-point binary representation is much easier to handle, however, the translation between binary floating point and a human-readable number results in a dynamic range that doesn't relate directly to a fixed number of digits of accuracy. In BCD-based calculators, the numbers involved in a problem can determine the time it takes to solve the problem. In many of my other pages, you'll note that I mention the time it takes to solve the 'all-nines' divided by 1 problem. The reason I do this is that, on a BCD-based calculator, such a problem gives a notion of the speed of the machine. BCD calculators perform math similar to the way we do on paper, just faster. As with humans performing math, bigger numbers in a problem take longer for us to figure out. In the 'all-nines' case, the result is obtained on a BCD calculator by repeatedly subtracting one from each digit position (starting at the most significant digit) until an overdraft occurs, keeping track of how many subtractions occur, until all digits of the divisor have been dealt with. The larger the digits of the divisor, the more subtractions are required, and the longer the calculation takes. In the case of floating-point binary math, the numbers are represented in a much more computer-friendly form, and most basic math operations take a relatively consistent amount of time no matter the magnitude of the operands.

Profile view of HP9825A

When it comes to programming, the 9825 is significantly easier to program than earlier HP programmables. With the HPL programming language, it is possible to write much more complex programs without getting bogged down in the details of RPN stack management and memory register allocation, just a few of the tedious things that programmers of earlier calculators had to deal with. On the 9825, variables are stated by name (IE: A through Z), expressions are evaluated in algebraic form, and memory is allocated dynamically and transparently. Entering a program is simply a matter of typing in the program statements followed by the "STORE" key. Once a statement is entered and the STORE key is pressed, the syntax of the statement is checked (and if an error found the machines beeper chirps and an error code is displayed), the statment is translated into an internal representation , and then the 'internal' form of the statement is stored in program memory. The 9825 provides a nice set of functions for editing programs, including listing out the program on the printer (or scrolling through a program on the display), insert and delete line functions, and character editing within a given statement.