Quote:
Originally Posted by audiokit
Hi David, thank you so much for saving me the keying. I thought no one had the bin files as the Elektor SC/MP lost it's appeal when the Junior project was started. Your computer looks super, also the keycaps really look nice. Did you follow the original elektor schematic? I still have some problems with the 7 segment led's showing some shadow from nearby digits. What transistors are used to switch the led's? Did you add/design the single step mode yourself?
Benny
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I followed the schematics only as required to make it Elbug compatible (just page 0):
$0000-05FF Elbug 1.5K (EPROM)
[$0600-06FF INS8154 PIO/RAM]
$0700-07FF Keyboard (A3=1), Display (A3=0)
[$0800-BFFF 1K spare EPROM]
$0C00-0FFF RAM (2114)
[ ] = added in unused Elbug space
I used a 16K EPROM and two 8K SRAMs split into 4K pages patched into whatever page required via wire straps on IC header (see photo in last post).
I used the same Elektor design for keyboard and display (see image below). Only differences are for keyboard I used 74LS244 in place of two 74125. On the display I didn't use inverters on the input and output of 7489 SRAM chips, but drove inputs direct from data bus and buffered outputs with 7417 ICs. I used BC557 PNP transistors (plastic version of BC177 used in original design). All other components are same values. The only thing I changed was to clock driving the display strobing, original design used fixed rate (gate N36, 330ohm, 100nF). I replaced 330ohm resistor with 100ohm in series with 1Kohm preset to alter rate. It's a long time ago but I think it was an attempt at brightness control. I don't recall having any shadowing problem, though I did use a green filter over the displays to improve contrast. Perhaps altering strobe rate similarly may help?
The single step mode was something from breadboard minimal system (see book extract image). I used same design as Elektor for HALT/CONT key mechanism (useful for quick debugging).