External Signal; Direct Data Feature; Magnetic Drum Storage - IBM 709 General Information Manual

A schematic of two data channels operating asyn-
chronously with processing in the central processing
unit is shown in Figure 78.
Each data channel may have as many as ten inter-
mixed
IBM
729 II and 729 IV Magnetic Tape Units
in addition to a card reader, card punch, and a printer.
External Signal
A standard feature of the computer system is its abil-
ity to accept a signal from an external source. This
signal causes the computer to execute a trapping
operation when it is received. The instruction being
executed is completed and the location of the next
instruction in sequence is placed in the address part
of core location 0003. The computer then takes its
next instruction from location 0004.
Direct Data Feature
As an optional feature, the computer may be equipped
with the direct data feature. This allows the trans-
mission of data between the computer and an external
data device. With this feature, an external signal
initiates the trapping operation and the stored pro-
gram may then take whatever action is required to
introduce data into the system or supply data from
the system to the direct data device.
Thus the main office of a company using the com-
puter could receive data from branch offices by direct
wire. This direct data path, as is shown in Figure 79,
could be an
IBM
65-66 Data Transceiver or anyone of
many types of transmi tting devices.
Figure 79. Schematic, External Signal and Direct Data Features
Magnetic Drum Storage
The magnetic drum is another storage device used in
electronic computers. It uses the same principle as mag-
netic tape: A magnetic material can be rapidly mag-
netized and de-magnetized, and will, like magnetic
tape, remain magnetized until deliberately erased.
Magnetic drum storage has the advantage that, once
information is recorded, it is retained even if the
power to the computer is interrupted.
An important additional advantage possessed by a
magnetic drum is the ability to read or alter stored
information selectively, without reading or rewriting
the entire contents of the drum. In the case of mag-
netic tape, the entire file must be searched or re-
written. This ability to read or write selectively is a
valuable asset when large tables are stored and search
operations are to be performed or when information
stored in the table must be modified. Therefore, the
drum is used primarily for the storage of large blocks
of related information such as subprograms, rate
tables, or supplementary data needed for the solution
of a problem.
A comparison between magnetic tape and drum
storage shows that tape is a long, thin, storage device
using one recording head while the drum is a short,
wide, storage device using many recording heads. This
similarity is shown in Figure 80.
Figure 80. Magnetic Tape and Drum as Storage Devices
The magnetic material used on the drum is a wire
made of an alloy containing copper, nickel, cobalt,
and iron. This wire is wound and fused to the surface
of the drum and the surface is then ground to an ex-
tremely fine tolerance, providing a smooth even mag-
netic surface.
The writing or recording process is similar to mag-
netic tape in that a coated surface passes under a re-
cording head. Information is written on this mag-
netic surface by an electrical pulse that causes the
head to become active. Whenever the head is active,
the area directly under it becomes magnetized as
shown in Figure 81. This area is called a bit or a one.
Thus, information is stored on the surface of the
drum in a pattern represented by combinations of
magnetized areas.
Input-Output Components 37