Binary Logic Diagrams

Diagrams are key components of a process documentation
portfolio. The complexity of a modern production process
has increased the process engineer’s use and reliance on
symbolic representations of a process and its unit operations.
This section reviews diagram symbols and their role in binary
logic diagrams. The complete description of a production
process demands the use of several types of descriptive diagrams.
The three important drawings that make up a specific
process documentation package are the process function diagram
(a site-specific binary logic diagram), the process flow
sheet (a piping and instrument diagram [P&ID]), and a hardware
diagram (a detailed equipment wiring diagram).
LOGIC DIAGRAMS
Consider a section of a chemical vapor deposition process that
includes only a backup vacuum roughing pump. The process
function diagram, Figure 1.12a, along with its accompanying
process flow sheet, Figure 1.12b, are expected to provide all
of the information required to understand the control scheme
and general operation of this process subsystem. For the example
illustrated, the process flow sheet indicates the equipment
and instrumentation associated with the operation of the
backup vacuum pump. However, a P&ID has its limitations.
For example, this P&ID indicates that the vacuum pump is
water-cooled but does not provide any plumbing details. In

addition, the diagram makes little or no attempt to indicate the
specific details of the reactor’s backup vacuum control scheme.
In fact, key control components, start and stop pushbuttons for
example, are commonly omitted in P&IDs.
The role of the process function diagram and the binary
logic diagram is to illustrate the overall control flow of the
process. The diagram is a combination of logic and instrumentation
symbols that reflects the overall control strategy.
It also includes the details for the successful operation of the
specified process equipment. For the example under consideration,
the binary logic diagram indicates the operation
requirements for the auxiliary vacuum pump. Note that the
symbol for this pump doesn’t even appear in the diagram;
however, its operational characteristics with respect to the
cooling water system are clearly indicated. In addition, the

diagram does not provide for the reader or require the reader
to possess knowledge of the complex and specialized circuit
information associated with the assembly of the instrumentation
presented. Its function is to provide an informed view

In addition to this detailed labeling of each sheet of the
diagram, care must be taken to identify lines that come from
or go to other sheets of the drawing. There are no standards
for accomplishing this, but one good practice is to repeat the
label on a line that leaves one sheet as that line begins again
on another sheet. Another popular approach to the task is to
identify each sheet entrance and exit line with its source and
destination. Numbering all exit lines from one sheet from top
to bottom and then identifying each line that enters a new
sheet in the same manner is the usual approach. With this
numbering system in hand, each line entering or leaving a
sheet can be identified with its source and destination address.
For example, suppose a diagram line that leaves sheet 5
of a binary logic diagram was labeled 5–7/8–3. This label
would identify that logic signal as exit signal 7 from sheet 5
and indicate that it was going to enter sheet 8 as the third
external input from the top of the page. To complete the
redundancy for this numbering system, 8–3/5–7 would be the
label on the third external input line from the top of sheet 8.
This tag would confirm that the signal in question indeed was
the third external sheet source input signal for sheet 8 and
that this particular signal was also the seventh off-the-sheet
output from sheet 5. Letters and numbers are often interlaced
in this identification scheme, but the source, destination, and
redundancy aspects of the system are always maintained.
In summary, the creation of a binary logic diagram requires
specific knowledge of how the process is controlled. In addition,
this type of process function diagram must be clearly

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