I used SN74LVC4245A and SN74LV4T125 as signal buffers.
Signal transferring direction is fixed (DIR pin is connected to GND in case of SN74LVC4245A) and I intended to disconnect input signals by applying high signal on /OE pin.
I thought when /OE is driven high, output pins would be literally isolated. So I placed pull-down resistors(10k Ohm) on each output port. The output port should be logic low when signal is disconnected in my system's configuration.
However, when I applied logic high on /OE pin, the voltage level of output pins are about 1.5V and unfortunately the other part of system where the output of SN74LVC4245A and SN74LV4T125 are connected as input recognized that level as logic high.
I suspect a couple of causes for this problem:
1. The /OE pin is driven by 3.3V, when supply of the IC's are 5V. Can this be a problem?
2. In case of high-impedance or isolated state, relatively 'low' impedance is connected from VCC or GND to output pin. I tried to change the pull-down resistor on output of SN74LVC4245A to 1.1k Ohm, but the output voltage level about 0.96V, still too high.
Please check this question.
Myonugho Kim
