Auto Ranging for ICL7107 had to be done with window comparators. In ICL7135 Digital Outputs are available for autoranging. Here in this circuit is a close approximation of what i may have designed once.
Auto ranging 4-1/2 Digit Digital Voltmeter
The below Details from Datasheet - These pins are used in this circuit to Autorange the DPM with some peripheral CMOS Logic.
"OVERRANGE (Pin 27) - This pin goes positive when the input signal exceeds the range (20,000) of the converter. The output F/F is set at the end of BUSY and is reset to zero at the beginning of reference integrate in the next measurement cycle.
UNDERRANGE (Pin 28) - This pin goes positive when the reading is 9% of range or less. The output F/F is set at the end of BUSY (if the new reading is 1800 or less) and is reset at the beginning of signal integrate of the next reading."
ICL7135 4 and Half A to D converter with BCD Output
The Range is Defined by QB and QC of 4029, there are four ranges. The two bit Code from this can control the decadic range of an Attenuator or Amplifier.
The Overrange and Underrange are in a OR formation with R26-27. Clock from either will Inc/Dec 4029. The Underrange Pulses are used by D2 -C8 to form a Steady Logic signal on Up/Down Control of 4029. This works in a closed loop manner, to Change the Range to match input Signal. When there are no Pulses from Either UR or OR, then that range is held.