I am trying to use the octal bus transceiver SN74LS245N and it is not working. I have power going into pin 20 using 4 1.5v AA batteries, Ground on pin 10, output enable low on pin 19, the direction pin high on pin 1, A1 on high pin 2, and B1 on the voltage side of a red LED. As far as I can tell it is supposed to (when output enable is low) send A1 to B1... This is not the result I am receiving. When I put A1 high it turns on the LED, when it's low it turns it off. Here's the weird part, when I turn off pin A1, B1 stays on for a moment, if I only unplug the input side of the wire it turns on the led, and when I have the cord completely unplugged it will sometimes randomly send power to the LED. Am I doing something wrong? If so, how can I fix this?
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1\$\begingroup\$ For bipolar TTL parts such as the 74LS family, an unconnected input will (usually) act as a logic High - you must connect an input directly to Ground for it to be seen as a logic Low. \$\endgroup\$– Peter BennettCommented Jun 9, 2017 at 0:36
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1\$\begingroup\$ And do not, repeat not, drive 7400/74LS ICs with a nominal 6 volts. If you must use batteries, go to the CD4000 series parts. 74LS is only rated for power supplies in the 4.5 to 5.5 volt range. \$\endgroup\$– WhatRoughBeastCommented Jun 9, 2017 at 0:40
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\$\begingroup\$ @PeterBennett could you post this as an answer so I could mark it as the answer \$\endgroup\$– Aaron oliverCommented Jun 9, 2017 at 0:42
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\$\begingroup\$ Show your schematic. \$\endgroup\$– Brian CarltonCommented Jun 9, 2017 at 2:25
2 Answers
For bipolar TTL parts such as the 74LS family, an unconnected input will (usually) act as a logic High - you must connect an input directly to Ground (or drive it Low with another gate) for it to be seen as a logic Low.
For CMOS parts (74HC, 74AC...), inputs must always have a connection to either Vcc or Ground - with these parts, a 5K - 10K resistor will be an adequate connection. Unconnected CMOS inputs will wander randomly between High and Low, and an intermediate(or "Maybe") state may cause excessive current consumption.
The way that part works is as a bidirectional driver. There is an active low output enable. If that is high, A and B are effectively disconnected from one another. If that pin is low the behavior depends on the direction pin. If direction is high then A is re input and B is the output. If directing is low then B is the input and A is three output.
If you want more feedback, post a schematic of your circuit with the part in context.
When the input is floating and output is enabled, behavior is undefined. Really it's based on parasitic capacitance. The delay you describe could be the hysteresis built into the part as the inputs are technically Schmidt-triggered.
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\$\begingroup\$ @bruceabbot typo fixed, sorry \$\endgroup\$– vicatcuCommented Jun 9, 2017 at 22:00