EXHIBIT 8.1

                   LEGAL OPINION OF GARDERE WYNNE SEWELL LLP

Mark R. Martin
Direct dial: (713) 276-5391
Direct fax: (713) 276-6391
Email: mmartin@gardere.com
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                               November 2, 2001


Petrocon Engineering, Inc.
3155 Executive Boulevard
Beaumont, Texas 77705

     Re:  Registration Statement on Form S-4

Dear Sirs:

     As counsel for Petrocon Engineering, Inc., a Texas corporation
("Petrocon"), we have been asked to render our opinion with respect to the
Federal income tax treatment of the merger of PEI Acquisition, Inc., a Texas
corporation (the "Sub"), with and into Petrocon pursuant to the Agreement and
Plan of Merger between Industrial Data Systems Corporation, a Nevada
corporation, IDS Engineering Management, LC, a Texas limited liability company,
the Sub and Petrocon.

     We hereby confirm that the opinions described in the Registration Statement
on Form S-4 under the caption "Material U.S. Federal Income Tax Consequences of
the Merger - Tax Opinion" constitute our opinions with respect to certain
material federal income tax consequences of the merger and, subject to the
assumptions described therein and assuming that we receive the representations
letters referred to therein, we will deliver an opinion to that effect, which
opinion is a condition to the consummation of the merger of the Sub with and
into Petrocon. This opinion is not intended to be the opinion that satisfies a
condition to the consummation of the merger.

     We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement on Form S-4 and to the use of our name under the caption
"Material U.S. Federal Income Tax Consequences of the Merger--Tax Opinion" in
such Registration Statement.

                                        GARDERE WYNNE SEWELL LLP

                                        /s/ Mark R. Martin
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                                        Mark R. Martin, Partner