Notes from a Conversation with Tim Nelson

Tim designed the EM189, along with several other versions. He's an intuitive engineer, which comes across in the design and function of the EM189, as well as conversation with him. He gave me a few interesting tidbits on the emulators.

First, on the EM-180 Z80 machine, he hijacked a few pins on the RS-232 connector. When connected to an amplifier via RC filters, the emulator was able to play music in four-part harmony. Somehow, I suspect that this never saw use outside of marketing.

On the EM189, he really likes the orthogonal nature of the 6809, along with its dual stack pointer and indirect addressing modes. These features are what took Applied Microsystems to use the 6809 in their 68000 emulator (ES-1800). Plus, of course, the EM189 could debug the ES-1800 code.

Much of the design was about managing the TLZ - twilight zone. This is the place between console and target operation, where bus and state integrity must be carefully preserved. The manual hints about this, but defers the reader to a presumably unpublished applications note.

In retrospect, he would have liked to use more grounds in the system, in particular between the keyboard board and the main board.

Tim claims packrat credentials, and may have some documentation on the unit. We chatted about code. Of course, 1983 is the data archival dark ages. The code was apparently written on a big old VAX, and archived off to magnetic tape. At this stage, printouts will be about all we can hope for.