I bought CT4520 that won't be recognized by the system. When I start the PC the RESET line (pin 46 / CT8920-NBQ) goes high from the ISA signal, in half a second goes to 2,7V and stays at this voltage (never low). I was wondering where this voltage can come from and the only source that I thought of is 3,3V supply. I measured the resistance between pin 46 and 3,3V voltage rail and found out that there is 200 ohm resistance. I removed R23 and after I started the PC the voltage in pin 46 rose to ~3,27V. I suppose that 2,7V voltage does not allow the processor to start and keeps it constantly in RESET mode. As workaround I was wondering if it would be a good idea to add a resistor (some low valued) from pin 46 to ground so that I could lower the voltage in case the problem is in CT8920-NBQ itself. But I really prefer to check all other possibilities first.
I measured the voltages on P1 (5V), P2 (3,3V) and U11 (-5V) and they are all as expected. All the connections to/from ISA bus, U9, U3, U2, U4 and U6 seems to be fine. Also read the EEPROM and the data is as expected.
Do you have any idea what can cause this behavior? Thanks in advance!