Exhibit B

JOINT FILING AGREEMENT

            The  undersigned  hereby agree that the  Statement on Schedule  13G  with  respect  to the Common  Stock of  Numerex Corp. and any further  amendments  thereto executed by each and any of us shall be filed on behalf of each of us  pursuant to and in  accordance  with the provisions of Rule  13d-1(k)(1)  under the  Securities  Exchange Act of 1934, as amended.

            This  Agreement  may be executed in separate  counterparts,  each of which shall be deemed an original, but all of which shall constitute one and the same instrument.

Dated: September 17, 2007

POTOMAC CAPITAL MANAGEMENT LLC

By: /s/   Paul J. Solit­­­

Paul J. Solit, Managing Member

POTOMAC CAPITAL MANAGEMENT INC.

By:   /s/     Paul J. Solit

Paul J. Solit, President

PAUL J. SOLIT

By:  /s/ Paul J. Solit

Paul J. Solit

POTOMAC CAPITAL PARTNERS, LP

By:  /s/ Paul J. Solit

Paul J. Solit, Managing Member of the   General Partner