Quick answer on the HP 8419A/B/C: This was an early computer interface from the 8410 to the HP 2116 computer. It features an early A/D successive approximation converter (as I recall) and strange logic levels on the output. The big change from the 8542 system that used the 8419 was to use the newer A/D converters in the HPIB based 59313A in the 8408/9 systems.