SN75374: questions related to power supplies

Hello Takahashi-san,

Due to this device's age, I will have limited access to internal information of the device, so it may be harder for me to tell you exact resistances or structure of the device. With that out of the way, here are my answers to the questions:

1. Is it true that current can flow as the figure?

I believe so, this appears to be a pullup structure that will bring IN and EN high if no signal is applied. Since it is a NAND gate, the driver will default to low in this state. The purpose of the diode, is likely to prevent reverse conduction, otherwise, you could supply current through the device into the VCC1 pin and possible pull it high in an off state. 

2. How much current can flow at the condition 1 & 2?

Current can only flow when IN or EN is low (0V). You would have to define this voltage in a DC state. The datasheet gives this in the electrical table:

You should be able to calculate the current requirements from VCC1 using this data. I don't see an easy way to account for the different VCC2 and 3, but you can compare the row where VCC3 is 28V to the one where it is 24V under the same condition to see the change. I think they may have little effect, but it is hard for me to say for certain. They should change the switching loss and rms current draw, but that can be very small at low frequencies. 

3. Are VIH and VIL are not changed even if power supplies are below?

Usually TTL has little dependence on supply voltage compared to CMOS. If anything, it would depend most on VCC1, so I think it should be the same for you. Unfortunately this one is hard to confirm because of the reason I mentioned earlier.

4. VIH and VIL condition can be applied to Input A, Enable E1 and Enable E2?

I see no mention of different thresholds in the datasheet, so  would assume this applies to all of them. 

Thanks,

Alex M.