Expandability
The functions of the calculator could be expanded by adding modules at the back of the machine. Four slots were available to add more memory, pre-programmed solution packs containing programs covering engineering, surveying, physics, math, finance, games, etc. As such, an HP-41 could in fact be tailored to the personal needs of the user. Hardware extensions included a thermal printer, a magnetic card reader (HP-67 compatible via converter software), and a barcode "wand" (reader).
Extension modules could also add new instructions to the machine. The standard set of mathematical functions of the 41-series was somewhat limited when compared to the functionality of some of contemporary models among the HP-line of calculators (notably the HP-34C and the HP-15C). Among others, the standard function set offered no integration or root-finding capabilities and lacked support of matrices and complex numbers, which could be added by an extension module.
Another module, known as the Interface Loop allowed for connection of more peripherals: larger printers, microcassette tape recorders, 3½" floppy disk drives, RS-232 communication interfaces, video display interfaces, etc. The Interface Loop could also be used with the HP-71B, HP-75 and HP-110 computers.
Read more about this topic: HP-41C