A different DPST switch
Posted on November 25, 2011
Today the students use a dip switch which is one that I have not seen before. It is modified from a traditional dip switch which has 8 SPST switches:
When I tested the modified dip switch using the first set of switch which has 4 contacts or 4 pins, I found out the schematic of the switch which is a DPST switch but it is not a normal one. The normal DPST schematic looks like this:
The modified one looks like this:
To implement a single SPDT switch using this kind of DPST switch, you can wire in the following way:
The specification of the dip switch can be found at:
http://www.ctscorp.com/components/Datasheets/206-208.pdf
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Active HIGH or Active LOW
Posted on November 23, 2011
These two terms confused a lot of people. What is it?
Any logical point has a level HIGH or LOW. Adding the word “active” implies some meaning. From my experience is that if we use it for positive logic, that is, use the HIGH, we called this point or line is active HIGH, and if we use it for negative logic, that is, using the LOW, we called this active LOW.
Let’s say, an input or output line called A has a level HIGH or LOW. If we use this line to enable other digital system, then we called this active HIGH and labelled it as EN, which stands for ENABLE logic.
If A is inverted and it is the LOW level that is used to do enabling, then we called the line active LOW and we labelled the line EN.
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Some tricks I learned when designing digital circuit
Posted on November 23, 2011
I learned in the past that you can turn a NAND gate into an inverter:
Say NAND gate is (A*B)’ and when you make or force B to be 1 or HIGH, then we have (A*1)’ which is simply A’ so A is inverted.
Same idea is for NOR gate. NOR is (A+B)’. When B is forced to be grounded or 0, then (A+B)’=(A+0)’=A’. Similarly A is inverted.
So when you do not have inverter or you do not want to use another inverter, you can use this tricks to turn NAND gate or NOR gate to an inverter.
It is funny that in one of the test bank that the question ask how do you say about input B for an NAND gate such that the output follows the input A. It shocked me because you can make output the inversion of the input A but not following it. Well if the gate is AND gate, it is all right.
A*B=A*1=A, so the ouput will be identical to A, if A is high, output is high, if A is low, output is low. Change AND gate to NAND gate is absolutely ridiculous.
Similar problem happened to another question using NOR gate. I said, use OR gate is correct.
There is another way to make NAND gate an inverter or AND gate a follower:
(A*B)’=(A*A)’=A’ or
(A*B)=(A*A)=A
This is the trick to make A the same as B, that is, tie the pin A and pin B together, you will get inversion of A using NAND gate or a follower of A using AND gate.
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Logical Error
Posted on November 21, 2011
Here is the image that I corrected from the lecture:
It is so obvious because NAND is NOT AND, NOR is NOT OR, NOT is NOT, NOT is represented by a single bar, a double bar is NOT NOT which will cancel each other.
If we have NOT NOT A , it will be just simply A. Two bars just confuse the learner.
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Using Programmer’s calculator with PC calculator
Posted on November 16, 2011
Type calc to run the PC calculator and then choose from VIEW the programmer or using Alt+3 key, you can use the programmer’s calculator which help in number representation and conversions.
By default, the number entered is in decimals and you can see the interface will allow you to see the number in various format such as hex (hexadecimal), oct (octal) and bin (binary) representation. You can choose to view the size or number of digits, bytes (8 bits), word (16 bits), DWORD (32 bits) etc.
You can change the number to positive and negative with the + or - button:
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