Basic ALD Logic Blocks

One of the outputs from the Solid Logic Design Automation system used in the design of the S/360 family were Automated Logic Diagrams, or ALDs. These ALDs documented the logic circuits of the machines, and once they were in production were mainly used by the Customer Engineers during fault-finding. An example of a typical ALD page is shown below (click on the image to open a larger version). It is followed by descriptions of the basic ALD logic blocks.

IBM used rectangular blocks to represent logic gates at least in part because they could easily be printed on the 1403 high speed line printer of the time (c.1962). This was in contrast to the conventional symbology already in use and still used today, as defined in MIL-STD-806 Graphic Symbols for Logic Diagrams. However, the logic itself is exactly the same and it is easy to see the equivalence. An equivalence chart is at the bottom of this page.

LINE POTENTIAL

A small open triangle (or “wedge”) placed on the input line where it joins the block symbol indicates that a less positive potential is required to satisfy the definition of the logic function identified by the block symbol. A wedge on the output line where it leaves a logic block symbol indicates that a less positive potential condition exists when the logic function is satisfied.

Omission of the wedge from an input line to a block symbol indicates that the more positive potential is required to satisfy the definition of the logic function. Absence of the wedge from an output line indicates that a more positive potential results when the logic function is satisfied.

The wedge is equivalent to the small circle used with conventional symbols.

Positive AND

The output of the Positive AND is in its more positive condition when and only when all of the inputs are in their more positive condition.

It is logically equivalent to a Negative OR Invert (Negative NOR).

Positive AND INVERT

The output of the Positive AND INVERT is in its more negative condition when and only when all of the inputs are in their more positive condition.

It is logically equivalent to a Negative OR.

Positive OR

The output of the Positive OR is in its more positive condition when and only when one or more of the inputs are in their more positive condition.

It is logically equivalent to Negative AND Invert (Negative NAND).

Positive OR INVERT

The output of the positive OR INVERT (or Not OR - NOR) is in its more negative condition when and only when one or more of the inputs are in their more positive condition.

It is logically equivalent to Negative AND.

EXCLUSIVE OR

The output of an EXCLUSIVE OR will be at its indicated polarity when one and only one of its inputs is at its indicated polarity.

Two variations are shown.

INVERTER

This is a device whose output is in the more positive condition as a result of its input being in the more negative condition and vice-versa. It shall be shown in one of two ways only: with negative polarity indicated at the input and positive at the output, or vice-versa. An inverter can have no more than one logic input.

AMPLIFIER

This is a device whose fundamental purpose is to provide adequate driving energy and appropriate impedance matching to other devices. Its output will be at its indicated polarity when and only when its input is at its indicated polarity. An AMPLIFIER can have no more than one logic input. (More than one physical input may be necessary, for example a return lead or the other lead of a differential input.)

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