We were having a Digital Electronics hardware practical class. One of my friend was given the task to store and retrieve data from the 7489 IC, a 16×4 = 64-bit ram chip. The circuit assembly was very easy and was done very fast. Now just to enter data and take out and check if they are the same with the stored data. Practical complete. But the practical class ended other people finished their assigned works. But the friend disassembled the circuit, and reassembled it and double checked the connections. I asked “what’s the problem” . Answer “I cannot get the proper output, 0s are stored okay, but the 1s are not. Whatever I store in the RAM I get 1111 as output” (The output of 7489 is inverted). Some more time passed. At last I suddenly noticed he was switching the power on taking some readings and putting the power off.
In the practical lab, before changing any wire connection we shut the power down for safety. But when I had a close look of what he was actually doing is described as follows: He fed some data in the first byte of the RAM chip, shut the power change the wire connections to feed data in the second byte, turn on the power, then turn off the power, change the wire connection for new set of data in the next byte, turn on the power and so on. And when he was finished feeding all 64bits, tried reading the data.
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I told “what is going on what are you doing”.
He told what he was doing.
I told again “No, what are you doing”.
Friend : “What do you mean”,
Me : “Why are you turning off the power”.
Friend : “I am turning off the power when changing the wire connections” .
Now i hit straight “How can you get back the fed data if you turn off the power”.
The reply was shocking : “Why?” .
I told “It’s a RAM dammit!”.
Next reply was electrifying: “So what?” .
I told “Did you read Computer Organization books ? Did you know what is a RAM? Or have you used a computer? They come with some RAM installed?”
He told “Yes, RAM can be randomly accessed generally made with semiconductors and volatile”
I told “What do you mean by volatile?”
He answered “The data goes when power is disconnected”
I told “So why were you turning off the power?!!!”
He said “Oh, so the data will go here also?”
I said “…”
At last he realized “How would i know that this RAM is the same as the RAM i read in Computer Organization? This is not right”.
At last he finished the practical. Thank God.
And oh! in the practical exam he did another blunder. He failed to display anything with the 7-segment display unit, because forgot to ground it.
About me, I swapped the B and D input terminals of a BCD to 7-segment display driver resulting erroneous output for some inputs. What ever i detected it and successfully completed the practical.
Phoxis 