# 7400 series problem.

Discussion in 'Electronics Resources' started by bushrat, Feb 11, 2015.

1. ### bushrat Thread Starter Member

Nov 29, 2014
97
23
Hello guys,

I was trying to build something for school and I noticed that my circuit is completely dead. Found that all of my IC's are probably dead. For example:
7404
5V to pin 14
GND to pin 7
I'm measuring 1.6 V on pin 1 (input) and 40 mV on pin 2 (output). Applying Vcc through 1k resistor to pin 1 still measures 40 mV on output (pin 2).

Seems like 7 of my 7404's have same problem. Am I missing something? Am I doing something wrong?

2. ### ScottWang Moderator

Aug 23, 2012
5,476
864
When the pin 1 is floating, it means that the input of pin 1 is high, and the output will be low as you measured, so you should connecting pin 1 to ground and then measuring the voltage of pin 2.

3. ### bushrat Thread Starter Member

Nov 29, 2014
97
23
I have no idea how I missed that, Thank You ScottWang. My mind to too fried to thing straight. I need rest.... plenty of it.

I was about to throw away those IC's..

4. ### ScottWang Moderator

Aug 23, 2012
5,476
864
Never left any input of any logic IC floating, and never connected any output of any logic IC to ground directly, these are the basic rules and knowledges.

5. ### bushrat Thread Starter Member

Nov 29, 2014
97
23
this example just gave me an idea... once I get my rest, I will like to build arduino IC tester. I can do hardware, but i will defensively need help with software. dunno how this will work once I get some sleep. maybe it will turn out to be a bad idea, i don't know..

6. ### ScottWang Moderator

Aug 23, 2012
5,476
864
Using arduino to build an IC tester, that is the software training, or you can using some switches and ne555 and some resistors and leds to build an easy function hardware IC tester.

7. ### absf Senior Member

Dec 29, 2010
1,658
444
You can start with simple gates like 7400 (nand), 7404 (not), 7408 (and) 7432 (or) and 7486(xor). SInce the Vcc and Gnd are the same, you can hardwire then on the breadboard.

Use bi-directional buffers like 74245 between the "device under test" and the Arduino ports. Once you have successfully tested all the chips mentioned above. You can try to identify them by plugging one of them and have your program to call out their part numbers.

A cool project to do for a beginner....

I have done that with a 6502 mpu many years ago in assembly.

Allen

8. ### MrChips Moderator

Oct 2, 2009
14,498
4,268
Very doable with Arduino. I would use 5V relays to select Vcc and GND pins. Then connect all pins to bidirectional I/O pins.
I have done this before on a DG Nova minicomputer. I even got mine to identify unknown chips.

absf likes this.
9. ### MrChips Moderator

Oct 2, 2009
14,498
4,268
Now that I think more about, it would be useful to use an MCU chip that has both weak pull-up and pull-down capabilities on the I/O pins.
This would allow you to test for input pins that are stuck low or high.
Atmel ATmega328 used on the Arduino has pull-ups only.
TI MSP430 has both pull-up and pull-down.

absf likes this.
10. ### bushrat Thread Starter Member

Nov 29, 2014
97
23
I'll be using Arduino Mega (ATMega328). bottom row has 32 digital pins, and that's going to be my I/O.. If I have time, I'll post schematics soon on different thread.

11. ### absf Senior Member

Dec 29, 2010
1,658
444
In my project I used MC6821 as programmable ports to connect to the 'DUT'. Using 74245 buffer does not accomplish what I have intended, as they are not individually addressable. I am not aware of any such buffer that allow each IO pin to be controlled individually.

The reason is that the Atmega328 in the Arduino is the single most expensive IC in the project. And if I make a mistake in the port direction config, it might ended up with a toasted IO pin. But a buffer is much cheaper than the mcu chip.

Until MrChips brought up the weak pullups and pulldowns in the MSP430, this problems is solve completely.

Allen