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...

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: August 28, 2008

                                            POTOMAC CAPITAL MANAGEMENT LLC

                                            By:   /s/ Xxxx X. Xxxxx
                                                  Xxxx X. Xxxxx, Managing Member

                                            POTOMAC CAPITAL MANAGEMENT INC.

                                            By:   /s/ Xxxx X. Xxxxx
                                                  Xxxx Xxxxx, President

                                            XXXX X. XXXXX

                                            By:   /s/ Xxxx X. Xxxxx
                                                  Xxxx X. Xxxxx