Those of you who visited my SCELBI/Scopewriter exhibit at VCF east may have noticed that I was dealing with some stability problems with the Scopewriter display. I finally found some time to resolve the issue.

The Scopewriter uses a TMS3112 Static Shift Register to hold the output. The design uses a clock to circulate the text through the shift register in order to refresh the display. This is somewhat similar to the Apple 1 or TV Typewriter design. It turns out that the clock feeding from IC1, pin 10, to the TMS3112, pin 7, doesn’t meet the pulse width, clock low specification of 150ns. A quick fix that seems to work is to add a .1uF capacitor in parallel to C2, which couples these two pins.

There is another change that I made a while back in order to get the Scopewriter to work. That is adding a 470 ohm resistor in parallel to the 100K resistor, R4. I can’t remember exactly why that change was necessary, but last week, as an experiment, I removed the hacked on resistor and I couldn’t get a good display on the scope. That may be documented in one of my earlier Scopewriter posts, but I don’t have the time to figure it out. I ended up putting the resister back in place.

At this point, Scopewriter seems to be working very well with the SCELBI.