The Hewlett-Packard HP-87, introduced in 1982 as part of the Series 80 lineup, was designed to offer enhanced display capabilities compared to its predecessors. It featured a larger 9-inch screen capable of displaying 80 characters per line and 16 lines of text, with a graphics resolution of 256×128 pixels. Unlike the HP-85, the HP-87 did not include an integrated printer or tape drive, but it came standard with an HP-IB (Hewlett-Packard Interface Bus) interface, facilitating connections to external peripherals such as disk drives and printers. The base model was equipped with 32 KB of RAM, expandable up to 544 KB through additional memory modules. Wikipedia

The enhanced HP-87XM

In 1983, HP released the HP-87XM, an enhanced version of the original model. The HP-87XM came standard with 128 KB of RAM, providing users with greater capacity for complex applications. This model retained the larger display and built-in HP-IB interface, maintaining compatibility with a wide range of external devices. The HP-87XM was discontinued in October 1984, marking the end of its production run. HP Computer Museum

The HP-87 series maintained the use of HP's proprietary processor architecture, running at a clock speed of 625 kHz. Despite the comparatively low processor clock frequency, the machines were quite advanced for their time, offering robust performance for various professional applications.

The Capricorn CPU, designed by Hewlett-Packard for the HP-85 and related Series 80 desktop machines, was a 16-bit architecture implemented largely with bit-serial logic. Internally, the arithmetic logic unit processed one bit at a time, reducing the required hardware complexity at the cost of throughput. The instruction set was tailored for efficient execution of HP BASIC and control tasks, with rich support for decimal arithmetic and string manipulation. Its addressing modes were limited compared to general-purpose CPUs, reflecting its orientation toward high-level language execution and instrument control. The processor typically operated at about 625 kHz, but because of its bit-serial ALU, the effective performance per instruction was considerably lower than that figure might suggest. In the HP Model 85, the CPU ran at a modest 625 kHZ.

When compared to mainstream CPUs of the late 1970s and early 1980s, the Capricorn occupied a unique niche. An Intel 8085 or MOS 6502 at 1 MHz could generally outperform it in raw instruction throughput, since those processors used parallel 8-bit ALUs and had well-tuned instruction pipelines for typical control tasks. The Zilog Z80, also an 8-bit design, offered richer instruction decoding and faster execution of complex operations than Capricorn. On the other end of the spectrum, the Motorola 68000 family, appearing in 1979–1980, delivered true 16-bit external data paths, 32-bit registers, and significantly higher performance at clock rates of 4–8 MHz. Compared to these, Capricorn traded raw speed for simplicity and integration into a tightly controlled system.

HP’s design philosophy with Capricorn paralleled other proprietary CPU efforts in scientific and industrial contexts, where compatibility with existing software ecosystems was less important than providing deterministic performance for specialized applications. While slower than contemporaries like the 6502, Z80, or even the 8086, the Capricorn CPU could efficiently run HP BASIC in ROM and interface smoothly with the HP-IB (GPIB/IEEE-488) bus for instrumentation. Its focus on bit-serial execution and high-level language support made it unusual but well-matched to the HP-85’s role as an engineering desktop machine. In contrast to the general-purpose microprocessors that enabled the personal computer revolution, Capricorn illustrates how proprietary architectures persisted in vertical markets where integration and reliability outweighed the need for compatibility or maximum speed.