STANDARD INPUT-OUTPUT

The PDP-3 is designed to accommodate a variety of input-output equipment. Standard input-output units include a Paper Tape Reader, Paper Tape Punch and an Electric Typewriter.

A single instruction, In-Out Transfer (see Central Processor), performs all in-out operations through the 36 bit In-Out Register. The address portion of this instruction specifies the in-out function. One bit of the instruction selects an in-out halt as required.

PAPER TAPE READER

The Paper Tape Reader of the PDP-3 is a photoelectric device capable of reading 300 lines per second. Six lines form the standard 36 bit word when reading binary punched eight hole tape. Five, six and seven hole tape may also be read.

The reader will operate in one of two basic modes or in a third special mode.

Alphanumeric Mode rpa iot 1

In this mode, one line of tape is read for each In-Out Transfer. All eight holes of the line are read. The information is left in the right eight bits of the In-Out Register, the remainder of the register being left clear. The standard PDP alphanumeric paper tape code includes an odd parity bit which may be checked by the program. Tape of non-standard width would be read in this mode.

Binary Mode rpb iot 2

For each In-Out Transfer instruction, six lines of paper tape are read and assembled in the In-Out Register to form a full computer word. For a line to be recognized in this mode, the eighth hole must be punched; i.e., lines with no eighth hole will be skipped over. The seventh hole is ignored. The pattern of holes in the binary tape is arranged so as to be easily interpreted visually in terms of machine instruction.

Read-In Mode

This is a special mode activated by the "Read-In" Switch on the Console. It provides a means of entering programs which neither rely on read-in programs in memory nor require a plug board. Pushing the "Read-In" Switch starts the reader in the binary mode. The first group of six lines and alternate succeeding groups of six lines are interpreted as "Read-In" mode instructions. Even-numbered groups of 6 lines are data. The "Read-In" mode instructions must be either "deposit in-out" (dio Y) or "jump" (jmp Y). If the instruction is dio Y, the next group of six binary lines will be stored in memory location Y and the reader continues moving. If the instruction is jmp Y, the "Read-In" mode is terminated and the computer will commence operation at the address of the jump instruction.

PAPER TAPE PUNCH

The standard PDP-3 Paper Tape Punch has a nominal speed of 20 lines per second. It can operate in either the alphanumeric mode or the binary mode.

Alphanumeric Mode ppa iot 5

For each In-Out Transfer instruction one line of tape is punched. In-Out Register bit 35 conditions hole #1. Bit 34 conditions hole #2, etc. Bit 28 conditions hole #8.

Binary Mode ppb iot 6

For each In-Out Transfer instruction one line of tape is punched. In-Out Register bit five conditions hole #1. Bit four conditions hole #2, etc. Bit zero conditions hole #6. Hole #7 is left blank. The #8 hole is always punched in this mode.

TYPEWRITER

The Typewriter will operate in the input mode or the output mode.

Output Mode tyo iot 3

For each In-Out Transfer instruction one character is typed. The character is specified by the right six bits of the In-Out Register.

Input Mode tyi iot 4

This operation is completely asynchronous and is therefore handled differently than any of the preceding in-out operations.

When a Typewriter key is struck, Program Flag Number One is set. At the same time the code for the struck key is presented to gates connected to the right six bits of the In-Out Register. This information will remain at the gate for a relatively long time by virtue of the slow mechanical action. A program designed to accept typed-in data would periodically check the status of Program Flag One. If at any time Program Flag One is found to be set, an In-Out Transfer instruction with address four must be executed for information to be transferred. This In-Out Transfer normally should not use the optional in-out halt. The information contained in the Typewriter's coder is then read into the right six bits of the In-Out Register.