switch registers are connected to the Central Processor
(see Manual Controls).

Test Address

The fifteen Test Address Switches are used to indicate start points and
to select memory registers for manual examination or change.

Test Word

The thirty-six Test Word Switches indicate a new number for manual
deposit into memory. They may also be used for insertion of constants
while a program is operating by means of the operate instruction.

Sense Switches

The six Sense Switches allow the operator to manually select program
options or cause a jump to another program in memory. The program can
sense individual switches or the state "All Switches ZERO."

ELECTRICAL DESCRIPTION

The PDP-3 circuitry is the static type using saturating transistor
flip-flops and, for the most part, transistor switch elements. The
primary active elements are Micro-Alloy and Micro-Alloy-Diffused
transistors. The flip-flops have built-in delay so that a logic net may
be sampled and changed simultaneously.

Machine timing is performed by a delay line chain. Auxiliary delay line
chains time the step counter instructions (multiply, divide, etc.). The
machine is thus internally synchronous with step counter instructions
being asynchronous. The machine is asynchronous for in-out operations,
that is, the completion of an in-out operation initiates the following
instruction.

MECHANICAL DESCRIPTION

The PDP-3 consists of two mechanical assemblies, the Console and the
Equipment Frame. Fig. 3 is a photograph of PDP-1 which is an 18 bit
version of PDP-3.

Console

The Console is a desk approximately seven feet long. It contains the
controls and indicators necessary for operation and maintenance of the
machine. A cable connects the Console to the Equipment Frame.

Equipment Frame

The Equipment Frame is approximately six feet high and two feet deep.
The length is a function of the amount of optional features included.
The Central Processor requires a length of five and one half feet. The
power cabinet is twenty-two inches long. A memory cabinet is thirty-two
inches long and will hold three memory modules (12,288 words per
cabinet). Memory cabinets may be added at any time.

Magnetic tape units require twenty-two inches per transport. A tape unit
cabinet may be connected as an extension of the Equipment Frame or may
be a free-standing frame.

ENVIRONMENTAL REQUIREMENTS

The PDP-3 requires no special room preparation. The comput