Bob Glusick
August 6, 2004
EL-3 Single Board Computer
October, 1973
John Irwin devised a couple of MSI single board computers called the EL-1& EL-2 about the time that MSI ALU chips became available. Remember the old '181? Jim Fawcett needed a digital controller for his control work and I got involved with morphing John's original design into the form Jim needed. In particular, a hardware divider, a more sophisticated program control counter, and specialized A/D input / multiplexor and D/A output / demultiplexor was required. I don't remember if we added and data index register or not, but I think we could save & restore the PC to give a rudimentary subroutine call capability. This was dubbed, what else, EL-3 (E-Lab 3).
Jim Fawcett at the EL-3
The chassis in front with 2 circuit cards, was the actual EL-3 which was intended to run stand alone with ROM program memory. I believe the original EL-1 had a shift register, which did the job of program counter and program memory. The processor is the hidden board, and the visible circuit board was the PROM (empty sockets) and the A/D, D/A, and I/O multiplexors. The second chassis (behind) with the tape reader was a support tool, which served as a ROM simulator, loadable from paper tape, or modifiable with the key pad which Jim is operating. It ran as a "Havard" machine with RAM in the data memory space, addressed by an index register, and ROM (or ROM simulator) in a separate memory space, addressed by the program counter.
At a later point in time, an I2L ( I squared L) version was built using the higher level of integration all on one circuit board.