I/O Module Control And Interrupt Registers - Fluke 9100 Series Service Manual

2/Theory of Operation
Threshold Voltage Generation
Threshold Voltage Generation produces the threshold voltages necessary
for the custom chips to classify input logic levels. Data bit 7 of the
command register (U14) determines the level of the threshold, with a 1
selecting TTL, and a 0 for CMOS.
Figure 2-7. I/O Module Control and Interrupt Registers
The threshold (THRSH) signal output of U14-19 controls the circuitry
that produces a low voltage level (VLO) and a high voltage level (VHI).
These voltage levels are used by the custom chip pins 39 and 45 to
define the logic low, invalid, and logic high voltage ranges. A logic
high out U14-19 designates a TTL logic level and a logic low a CMOS
logic level. The THRSH signal controls resistor dividers that create the
VHI and VLO signals. Two parts of op amp U2 and transistors Q1 and Q2
2-14