The HRD+ upgrade design is validated

The final piggyback board arrived today; this one eliminates the open-collector 74xx156 with a 74xx155 and a 74xx08 to handle what the pullup resistor used to do. Everything works well, with one caveat: I never could get reliable operation out of the low-power CMOS 6264 handling the DSR. The old [HN]MOS 6264 that came with […]