PC resistance meter (CB20544E)

We found this circuit in English documentation for PCs from 1996. The ZN448 is a very uncommon integrated circuit today. It is an interface circuit.

Multilayer Varistors for Surge Protection - TDK
TDK provides a wide range of EPCOS varistors for surge protection through Mouser Electronics....
Voltage Controlled Gain Amplifier (CB4898E)
This circuit provides a linear attenuation of a signal up to 100 dB. The circuit operates with...
Gain 100 Amplifier LM3900 (CB5043E)
As in the previous circuit (CB4052E), the relationship of values between the feedback and input...