Page 216 -

243_MasterPieces_04b.qxd 4/18/03 7:07 PM Page 188

188 Masterpiece 4 • PneumADDic II

The Carry Memory

The original PneumADDic had only half the pneumatic adding circuitry that
PneumADDic II does.To perform the addition for a given binary column, we need to
add a bit from Value 1, a bit from Value 2 and the Carry.This is easily done in a two-stage
addition, where we add a bit from Value 1 and a bit from Value 2 to get a partial-sum.We
then add the partial-sum to the Carry to get the final sum.
The original PneumADDic had one two-bit adder, so the complete addition for a
given binary place had to be done as two consecutive pneumatic additions.The original
PneumADDic also had to electronically remember the carry from binary place to binary
place. PneumADDic II contains a complete two-stage pneumatic adding circuit, so a bit
from Value 1 and a bit from Value 2 and the Carry can be collectively added together in a
single calculation. PneumADDic II also pneumatically remembers the Carry from one
binary place to the next. It uses a pneumatic piston that controls a pneumatic switch as a
Carry Memory. In computers, a memory device has a value input, a value output, and an
input that tells the memory device when to disregard the stored value, and when to store
the current input.After the memory device is instructed to store a current input, the
value input can change without the output value changing.
A medium-sized LEGO pneumatic piston creates a substantial amount of friction
between the outside case (sometimes referred to as the cylinder), and the piston head (the
component inside that causes the whole pneumatic piston to expand and contract).The
piston head will only move if there is a strong force to push or pull it.The small LEGO
pistons have less friction and the force of gravity is sometimes enough to move the small
piston head (small pistons are also not strong enough to flip a pneumatic switch reliably).
For a medium-sized piston, once the piston head is moved, friction makes it “remember”
where it is, by not letting it move.