Electronic – Can a TTL LS device (low-power Schottky) serve as an input to a basic TTL device

schottkyttl

A 74121 (One-shot) worked well as a reset to a counter device. There is an enable line using a 7400 NAND gate to allow or disallow the trigger to pass.

While duplicating the circuit, I found that all I have at the moment is a 74LS00. Are the outputs from the low-power Schottky devices compatible with a non-LS input?

schematic

simulate this circuit – Schematic created using CircuitLab

Best Answer

For the circuit as shown, yes. According to my late-70's databook, 74LS parts have an output current of 400\$\mu\$A at 2.5V out, while 7400-series logic has an input current of 40\$\mu\$A (the similar rating for 74LS logic is 20\$\mu\$A). You certainly can't drive as many 74xx inputs from a 74LSxx output as you can from a 74xx output, but just as certainly you can drive one.

I was going to check the fan-out ratings of 74LSxx logic, but as I was turning to the paper book I realized that it didn't have a search bar, and desisted.

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