TI 990/189 debugging

9.3V might actually work fine for a TMS9980, even though it's below spec. It's not going to damage the part, so it may be worth a try before modifying the board for 12V to the CPU socket. In NMOS digital parts that predate depletion loads, Vdd needs to be significantly higher than the most positive logic level in order to bias the enhancement nFET used for the loads (pull-ups). The Vdd voltage doesn't have to have a precise value, but it needs to be somewhat more than the FET gate threshold above the most positive logic level, and below the breakdown voltage. The higher the Vdd voltage (below breakdown), the faster the pullup will operate, so running below spec will reduce the maximum speed at which the part will operate. This is also dependent on temperature. The part is spec'd for operation over a fairly wide temperature range (even if only "commercial" rated). Since the logic high level is no more than 5.0V, and generally somewhat less, a Vdd of 9.3V is probably more than adequate at room temperature, but may fail at temperature extremes. The MP9529 is a "selected" TMS9980. In most contexts, a "selected" IC is one that has been tested and found to meet specifications more stringent than the normal specifications. However, in this case I think the MP9529 might actually be "selected" in the sense of being tested to *lower* specifications than a standard TMS9980. It's unclear why they would want to use the lower Vdd, except possibly to reduce power consumption. With the introduction of depletion loads in later NMOS ICs, generally starting around 1976, and becoming ubiquitous by 1980, the requirement for a supply above +5V was eliminated. Similarly, by adding an on-chip substrate bias generator, the need for an externally supplied substrate bias voltage (Vbb, typically -5V) was removed.