1. Select the address of the latch you want to read with keys A (wire A1) and B (wire A0).
2. Press key 'D' to make the 'EN' wire have value 1. The lights G and H will indicate the values of the data bits stored in that latch.
3. Release the enable key D.
4. Release the address keys, A and B.
For example, to read latch 10, first press key A (and not key B) to select latch 10 (as indicated in the following diagram).
Selecting the Address to Read
Second, press key D to make the enable (EN) wire have value 1. Then light H comes on indicating that bit 0 of latch 10 has value 1 and light G stays off indicating that latch 10 has value 0 in bit 1. This is shown in the following diagram.
Notice that making wire EN have value 1 connects the loops of the selected latch to the data wires D1 and D0.
Enabling (EN) the Output
Third, release key D when done reading latch 10 (as indicated in the following diagram). That's all there is to reading a latch in memory.
Releasing Enable (EN)
To erase a value from a latch and make all of the latch's loops have value 0, do the following.
1. Select the latch with keys A (A1) and B (A0).
2. Temporarily press key C to make the 'clear' (CL) wire temporarily have value 1.
The following diagram shows latch 10 selected by pressing A and not pressing key B. It also shows key C being pressed to clear both of latch 10's data bits to 0. Don't press C until after A is pressed (so that no other latch is accidentally erased).
Notice that pressing C makes the selected 'H' (for 'Hold') wire, 'H10', have value 0.
Selecting the Address and Clearing (with CL)
Release key CL after clearing latch 10 as indicated in the following diagram. Don't release key A until after CL is released so that you don't accidentally erase another latch.
Writing to a latch will not clear any bits that were previously 1, so always clear a latch before copying (writing) data to it. Therefore, to write to a latch, do the following.
1. Press the correct address keys (A and B) and data keys (E and F).
2. Press the clear key, C, to clear the latch.
3. Release the clear key, C.
4. Press the enable key, D, to send data from the data wires (D1 and D0) to the latch.
5. Release the enable key, D.
6. Release the address keys (A and B) and the data keys (E and F).
To read data, just do the following.
1. Press the correct address keys, A and B, to select the latch to read.
2. Press the enable key, D, to send the latch's values to the lights, G and H.
3. Release the enable key D.
4. Release the address keys, A and B.
Releasing CL
The memory in the drawings is very small. There are only two address wires and two data wires. Because there are two address wires, there are four possible addresses: 00, 01, 10, and 11, and, so, four latches. Because there are two data wires, each latch has two loops. Each loop holds one 'bit' of information, a 0 or a 1. Four latches with two loops each means 8 (= 4 x 2) loops total.
The table below shows all bit values in each latch when data 01 has been written to the loops of latch 10.
Latch Bit
Address Values
00 00
01 00
10 01
11 00
A larger memory with four address bits and four data bits with 0 in all the loops can be represented as below.
latch bit
address values
0000 0000
0001 0000
0010 0000
0011 0000
0100 0000
0101 0000
0110 0000
0111 0000
1000 0000
1001 0000
1010 0000
1011 0000
1100 0000
1101 0000
1110 0000
1111 0000