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.