140418-1000 EDT
fifty60:
Some of the characteristics of one TTL gate, a single inverter (NOT function), of a standard power 7404 are:
Plus 5 V on pin 14, Vcc, and 5 V common (ground) on pin 7. Pin 1 of one of the six inverters is floating at about 1.6 V. High enough to be a logic 1. Pin 2 the inverter output is about 0.09 V. Low enough to be a logic 0.
Some of the versions are low power (L series), somewhat low power (LS series), standard power (no letter), and higher power (H series). The input and output characteristics vary with the versions and whether 54 or 74 series. Also there are CMOS versions.
Unless there is shunting circuitry inside the power supply an open input to the TTL input should cause the power supply to turn on based on your statement that 0 input turns the supply off. Thus, no voltage source is required at the input. But this input is then quite susceptible to noise changing the input to a logic 0. Thus, one wants a low impedance source (a TTL output) or a pullup resistor at the TTL input. The smaller the pullup resistor the less sensitive the gate input is to noise. Further shunt capacitance can help reduce noise problems.
If the supply voltage to the 74 series package is +5 V and the pullup is also supplied from the same +5 V, then the pullup can be any resistance including 0. If the pullup is supplied from +12 V, then a voltage clamp (such as a Zener diode) must be used to limit the peak input voltage to the TTL gate.
Also a negative voltage should not be applied to the TTL gate input.
On a sample of one 7404 at room temperature the output is a solid high output (3.7 V no load) up to about 0.6 V input, and a solid low output for an input above 1.6 V. The logical output of the TTL gate is indeterminate for inputs between 0.6 and 1.6 V.
You have not made it clear where the TTL input is on the power supply. That is what connector. Is it the 25 pin connector? Are +12 and common available on the same connector as the TTL input? If this connector is the 25 pin RS232 connector, then why is a TTL input mixed up into an RS232 connector?
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