# code switching EPROM circuit

For this article, we will use the 128K EPROM (27C128 used in the 1986 to 1989 IROC-Zs with the 1227165 ECMs) as the basis for our discussion. This purpose of this article is to educate those on a little bit of binary and hexadecimal background, in order to understand what`s going on. Binary numbers consist of 1`s and 0`s only. 1 = 1, 2=10, 3=11, 4

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=100, 5=101 and so on. Here`s a simple charted example of one byte (8 bits) of data to explain better: The binary number 10101011 equals 171 in decimal. How did we do this We took the number 10101011 and entered the bits into the numbered chart (Going from right to left, start from 1 and double the value as you go left. 1, 2, 4, 8, etc. as shown on the top row. ) Then, wherever this is a 1 bit, add that number represented, then the next 1 bit and so on. So we added 128+32+8+2+1 to equal 171. What is 171 in hexadecimal Well, let`s leave it as binary. Hexadecimal is a 16 number base system. You would count 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, 10, 11, 12, etc. Take the right side of the chart above. Add the 1`s together (8+2+1) = 11 in decimal, but B in hexadecimal. Remember 9, A, B, C represents 11 in decimal. We would then do the same on the left side, but here`s the trick. Use the SAME numbering convention as if you`re starting over on this section. You would not use 128, 64, 32, 16. Use 8, 4, 2, 1 again. Add the 1`s together (8+2) = 10 in decimal, but A in hexadecimal. So 171 = 10101011 in binary = AB in hexadecimal. Yes, we could always use the scientific calculator in Windows, sure, but in order to understand how this code switching works, take the time and convert some numbers to binary and hexadecimal. Then VERIFY with calculator to see if you`re correct. Let`s discuss the 128K EPROM chip, otherwise known as...

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