The Heathkit/Zenith MicroComputer Learning System model ET-3400 was a very popular item designed to teach principles of computers and programming at Universities in the 1970s., and to educate the students of internal computer hardware and software components by self-assembly and programming the machine in pure Hexadecimal language.
It was delivered in assembled or Kit form. It also featured a prototype area and could be used as a design aid for developing special interface circuitry with common 6820 parallel interface or 6850 asynchronous chips.
Several software in ROM were also available. Among them an Assembler and a Tiny BASIC.
Mendel Cooper's memories:
I built one of these from a kit back in about 1984, I guess. Price was approx. $200 in kit form.
Heathkit used to sell three educational courses based on the ET-3400:
1) Microprocessor Course, EE-3401
2) Microprocessor Interfacing Course, EE-3402
3) Microprocessor Applications Course, EE-3405
Using the specs in the manual, it was pretty easy to build homebrew memory expansion. For a couple of dollars worth of parts, I built a 2K module, using a 6116 (2K x 8) static RAM chip.