Apparently the most popular of the HP-8X series, this model added a thermal printer and a tape drive to the features of the HP-83. A touch of a key could copy anything from the HP-85's display to its printer. The built-in tape cartridge drive held up to 210K bytes and had a search speed of 7800 bytes/second and a data transfer rate of 650 bytes/second.

When the machine was turned on, it looked for a program called "Autost" on the tape and executed it if found. For faster access, an electronic disc was also included in some versions. This disc held 32K bytes as shipped but could be expanded up to 544K bytes with plug-in modules.

The 8X Series allowed subroutines to be executed from mass storage devices so the electronic disk made it possible to write large programs that ran quickly. The HP-85 came in versions with 16K or 32K of user program RAM.