Functional Diagrams and Function Symbols

The purpose of this section is to help the reader establish a
uniform means of depicting and identifying mainly digitalbased
application software functions used for measurement,
monitoring, and control. It is done by presenting a designation
system including graphic symbols and identification codes
as well as functional diagramming techniques that were
formerly known as the Scientific Apparatus Manufacturers’
Association (SAMA) system.
It must be noted that a significant part of this section has
been extracted from the revision work of the ISA SP5.1
subcommittee, and much of it has been based on draft working
documents being utilized at the time of this writing,
documents with which one of the authors has been actively
involved. Other portions of this section are based on the
author’s experience in the industry and are not any part of
the SP5.1 subcommittee proposed forthcoming revision.
A disclaimer to any future ISA standards documents is
hereby stated: The reader is cautioned that the draft ISA
document that provided much of the information in this
section has not been approved as of the time of this writing.
It cannot be presumed to reflect the position of ISA or any
other committee, society, or group.
The intent is to pass along to the reader the best and latest
thinking on this subject at this point in time.
ISA FUNCTIONAL DIAGRAMMING (EX-SAMA)*
Instrument and Control Systems Functional Diagramming
Symbol tables are given for use in preparing instrument and
control loop functional diagrams, which are not normally
shown on process flow diagrams (PFDs) and piping and instrument
diagrams (P&IDs). They are used to depict monitoring
and control loops in functional instrument diagrams, functional
logic diagrams, application software diagrams, sketches, and
text. They shall be prepared from the following:
a) Instrument line symbols
b) Instrument functional diagramming symbols
c) Mathematical function block symbols
Equivalent P&ID Loop, Functional Instrument
and Electrical Diagrams
See statement of permission in the footnote below.

Note: There is no equivalent electrical schematic for the
process control instrumentation.
Functional Diagramming Symbol Tables*
The symbols used in Table 1.2a are not normally used on
P&IDs but are used to diagram control systems at the
hardware and function level for configuration and other
purposes.
The symbols in Table 1.2b are never used in P&IDs and
are used to help document and diagram logic control designs
and narratives. The present standard ISA S5.2 (ANSI/ISAS5.2
–1976 [R1992]) is now being revised and rolled into the
new ANSI/ISA-5.01.01 standard as proposed in the current
(as of this writing) Draft 4. Symbols, truth tables, definitions,
and graphs used in this section are in accordance with Draft
4 and are very different from S5.2. They are given here to
illustrate the latest thinking in this area, including expanded
*The symbols have been extracted by ISA, with permission, from Scientific
Apparatus Manufacturers’ Association SAMA Standard PMC 22.1–1981,
Functional Diagramming of Instrument and Control Systems, which is no
longer supported by SAMA.

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